Papers - OTSUKA, Yuichi

Was the Unseasonal Development of Post-Sunset Equatorial Plasma Bubbles in Southeast Asia Driven by Quasi-2-Day Planetary Waves?

Dai, GF; Li, GZ; Otsuka, Y; Hu, LH; Sun, WJ; Ning, BQ; Zhang, DH; Lei, JH; Xie, HY; Zhao, XK; Nishioka, M; Perwitasari, S; Jamjareegulgarn, P; Li, Y; Yan, CX

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 130 ( 3 )   2025.3

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DOI： 10.1029/2024JA033280

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Thank You to Our 2024 Reviewers

Karnauskas, K; Aiyyer, A; Camargo, S; Capitanio, F; Feakins, S; Fulweiler, R; Ganju, N; Giannini, A; Gu, Y; Huber, C; Ivanov, V; Korte, M; Kumar, S; Lewis, K; Magnusdottir, G; Morlighem, M; Otsuka, Y; Prieto, GA; Qiu, B; Russell, L; Seki, K; Su, H; Sun, DY; Viswanathan, H; Wang, GL; Wang, KC; White, AE; Williams, Q; Zhang, L

GEOPHYSICAL RESEARCH LETTERS   Vol. 52 ( 4 )   2025.2

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DOI： 10.1029/2025GL115360

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Medium-scale traveling ionospheric disturbances created by primary gravity waves generated by a winter storm

Kogure, M; Chou, MY; Yue, J; Otsuka, Y; Liu, HX; Sassi, F; Pedatella, N; Randall, CE; Harvey, VL

JOURNAL OF SPACE WEATHER AND SPACE CLIMATE   Vol. 14   2024.12

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DOI： 10.1051/swsc/2024036

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Machine learning approach for ionospheric scintillation prediction on ROTI parameter over the African region during solar cycle 24

Tete, S; Otsuka, Y; Zahra, WK; Mahrous, A

ADVANCES IN SPACE RESEARCH   Vol. 74 ( 12 ) page： 6325 - 6342   2024.12

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DOI： 10.1016/j.asr.2023.12.026

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X-Raying Neutral Density Disturbances in the Mesosphere and Lower Thermosphere Induced by the 2022 Hunga-Tonga Volcano Eruption-Explosion

Katsuda, S; Shinagawa, H; Fujiwara, H; Jin, HDKT; Miyoshi, Y; Miyoshi, Y; Motizuki, Y; Nakajima, M; Nakazawa, K; Nobukawa, KK; Otsuka, Y; Shinbori, A; Sori, T; Tao, CHR; Tashiro, MS; Wada, Y; Yamawaki, T

GEOPHYSICAL RESEARCH LETTERS   Vol. 51 ( 20 )   2024.10

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DOI： 10.1029/2024GL112025

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Longitudinal Range of the Eastward-Traveling Equatorial Plasma Bubble Inducing Ionospheric Scintillation Reviewed

Abadi, P; Otsuka, Y; Saito, S; Yamamoto, M; Perwitasari, S; Muafiry, IN; Putra, AY; Faturahman, A

SPACE WEATHER-THE INTERNATIONAL JOURNAL OF RESEARCH AND APPLICATIONS   Vol. 22 ( 8 )   2024.8

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DOI： 10.1029/2024SW003908

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High-resolution 3-D imaging of electron density perturbations using ultra-dense GNSS observation networks in Japan: an example of medium-scale traveling ionospheric disturbances Reviewed International coauthorship

Fu, WZ; Otsuka, Y; Ssessanga, N

EARTH PLANETS AND SPACE   Vol. 76 ( 1 )   2024.7

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DOI： 10.1186/s40623-024-02051-2

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Leveraging machine learning techniques and GPS measurements for precise TEC rate predictions Reviewed International coauthorship

Tete, S; Otsuka, Y; Zahra, WK; Mahrous, A

GPS SOLUTIONS   Vol. 28 ( 3 )   2024.7

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DOI： 10.1007/s10291-024-01652-4

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Thank You to Our 2023 Peer Reviewers Reviewed International coauthorship

Rajaram, H; Aiyyer, A; Camargo, S; Cappa, CD; Dombard, AJ; Donohue, KA; Feakins, S; Flesch, L; Fulweiler, R; Ganju, N; Giannini, A; Gu, Y; Huber, C; Ivanov, V; Karnauskas, K; Korte, M; Lewis, K; Lu, G; Magnusdottir, G; Morlighem, M; Oieroset, M; Otsuka, Y; Prieto, GA; Qiu, B; Russell, L; Su, H; Sun, DY; Wang, GL; Wang, KC; Whalen, C; White, AE; Williams, Q; Yau, A

GEOPHYSICAL RESEARCH LETTERS   Vol. 51 ( 9 )   2024.5

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DOI： 10.1029/2024GL109626

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Permulaan Gelembung Plasma Khatulistiwa Semasa Angin Neutral Kuat dan Lemah di Asia Tenggara

Sarudin, I; Hamid, NSA; Abdullah, M; Buhari, SM; Otsuka, Y; Perwitasari, S; Yatini, C

JURNAL KEJURUTERAAN   Vol. 36 ( 3 ) page： 1281 - 1289   2024.5

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DOI： 10.17576/jkukm-2024-36(3)-36

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Assessing the potential of ionosonde for forecasting post-sunset equatorial spread F: an observational experiment in Southeast Asia Reviewed International coauthorship

Abadi, P; Ahmad, UA; Otsuka, Y; Jamjareegulgarn, P; Almahi, A; Perwitasari, S; Supriadi, S; Harjupa, W; Septiawan, RR

EARTH PLANETS AND SPACE   Vol. 75 ( 1 )   2023.12

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DOI： 10.1186/s40623-023-01941-1

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Equinoctial asymmetry of plasma bubble occurrence and electric field at evening: GPS and ionosonde measurements in Southeast Asia Reviewed

Otsuka, Y; Abadi, P; Hozumi, K; Almahi, A

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 252   2023.11

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DOI： 10.1016/j.jastp.2023.106136

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Daytime Medium Scale Traveling Ionospheric Disturbances (MSTIDS) Over the Andes Mountains at Equatorial and Low Magnetic Latitudes Reviewed International coauthorship

Figueiredo, CAOB; Wrasse, CM; Vadas, S; Takahashi, H; Otsuka, Y; Nyassor, PK; Shiokawa, K; Paulino, I; Barros, D

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 128 ( 10 )   2023.10

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DOI： 10.1029/2023JA031477

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Responses of the African-European equatorial-, low-, mid-, and high-latitude ionosphere to geomagnetic storms of 2013, 2015 St Patrick's Days, 1 June 2013, and 7 October 2015 Reviewed International coauthorship

  Vol. 72 ( 3 ) page： 775 - 789   2023.8

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DOI： 10.1016/j.asr.2022.10.029

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Equatorial plasma bubble intensities across longitudinal sectors of the globe using GNSS observations Reviewed International coauthorship

  Vol. 249   2023.8

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DOI： 10.1016/j.jastp.2023.106097

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Ionospheric disturbances over South America related to Tonga volcanic eruption Reviewed International coauthorship

  Vol. 75 ( 1 )   2023.5

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DOI： 10.1186/s40623-023-01844-1

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Generation of equatorial plasma bubble after the 2022 Tonga volcanic eruption Reviewed

A. Shinbori, T. Sori, Y. Otsuka, M. Nishioka, S. Perwitasari, T. T. Tsuda, A. Kumamoto, F. Tsuchiya, S. Matsuda, Y. Kasahara, A. Matsuoka, S. Nakamura, Y. Miyoshi, I. Shinohara

Sci. Rep.   Vol. 13 ( 1 ) page： 6450   2023.5

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Equatorial plasma bubbles are a phenomenon of plasma density depletion with small-scale density irregularities, normally observed in the equatorial ionosphere. This phenomenon, which impacts satellite-based communications, was observed in the Asia-Pacific region after the largest-on-record January 15, 2022 eruption of the Tonga volcano. We used satellite and ground-based ionospheric observations to demonstrate that an air pressure wave triggered by the Tonga volcanic eruption could cause the emergence of an equatorial plasma bubble. The most prominent observation result shows a sudden increase of electron density and height of the ionosphere several ten minutes to hours before the initial arrival of the air pressure wave in the lower atmosphere. The propagation speed of ionospheric electron density variations was ~ 480–540 m/s, whose speed was higher than that of a Lamb wave (~315 m/s) in the troposphere. The electron density variations started larger in the Northern Hemisphere than in the Southern Hemisphere. The fast response of the ionosphere could be caused by an instantaneous transmission of the electric field to the magnetic conjugate ionosphere along the magnetic field lines. After the ionospheric perturbations, electron density depletion appeared in the equatorial and low-latitude ionosphere and extended at least up to ±25° in geomagnetic latitude.

DOI： 10.1038/s41598-023-33603-3

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Day-to-day variability of the equatorial ionosphere in Asian sector during August-October 2019 Reviewed International coauthorship

  Vol. 10   2023.5

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DOI： 10.3389/fspas.2023.1198739

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Multi-Event Conjugate Measurements of the SAR Arc Detachment From the Auroral Oval Using DMSP Satellites and an All-Sky Camera at Athabasca, Canada Reviewed International coauthorship

  Vol. 128 ( 4 )   2023.4

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DOI： 10.1029/2022JA030544

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Solved and unsolved riddles about low-latitude daytime valley region plasma waves and 150-km echoes Reviewed International coauthorship

  Vol. 10   2023.2

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DOI： 10.3389/fspas.2023.1091319

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Tandem Observations of Nighttime Mid-Latitude Topside Ionospheric Perturbations Reviewed International coauthorship

  Vol. 21 ( 2 )   2023.2

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DOI： 10.1029/2022SW003312

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Ionospheric Modulation by EMIC Wave-Driven Proton Precipitation: Observations and Simulations Reviewed International coauthorship

Tian, XB; Yu, YQ; Gong, F; Ma, LX; Cao, JB; Solomon, SC; Shreedevi, PR; Shiokawa, K; Otsuka, Y; Oyama, S; Miyoshi, Y

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 128 ( 1 )   2023.1

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DOI： 10.1029/2022JA030983

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Relationship between geomagnetic storms and occurrence of ionospheric irregularities in the west sector of Africa during the peak of the 24th solar cycle

Ondede, GO; Rabiu, AB; Okoh, D; Baki, P; Olwendo, J; Shiokawa, K; Otsuka, Y

FRONTIERS IN ASTRONOMY AND SPACE SCIENCES   Vol. 9   2022.11

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DOI： 10.3389/fspas.2022.969235

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Localized mesospheric ozone destruction corresponding to isolated proton aurora coming from Earth's radiation belt

Ozaki Mitsunori, Shiokawa Kazuo, Kataoka Ryuho, Mlynczak Martin, Paxton Larry, Connors Martin, Yagitani Satoshi, Hashimoto Shion, Otsuka Yuichi, Nakahira Satoshi, Mann Ian

SCIENTIFIC REPORTS   Vol. 12 ( 1 )   2022.10

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DOI： 10.1038/s41598-022-20548-2

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Three-Dimensional Fourier Analysis of Atmospheric Gravity Waves and Medium-Scale Traveling Ionospheric Disturbances Observed in Airglow Images in Hawaii Over Three Years

Naito Hideto, Shiokawa Kazuo, Otsuka Yuichi, Fujinami Hatsuki, Tsuboi Takuma, Sakanoi Takeshi, Saito Akinori, Nakamura Takuji

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 127 ( 10 )   2022.10

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DOI： 10.1029/2022JA030346

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Statistical Study of Subauroral Arc Detachment at Athabasca, Canada: New Insights on STEVE

Yadav, S; Shiokawa, K; Otsuka, Y; Connors, M

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 127 ( 9 )   2022.9

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DOI： 10.1029/2021JA029856

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Electromagnetic conjugacy of ionospheric disturbances after the 2022 Hunga Tonga-Hunga Ha'apai volcanic eruption as seen in GNSS-TEC and SuperDARN Hokkaido pair of radars observations

Shinbori Atsuki, Otsuka Yuichi, Sori Takuya, Nishioka Michi, Perwitasari Septi, Tsuda Takuo, Nishitani Nozomu

EARTH PLANETS AND SPACE   Vol. 74 ( 1 )   2022.7

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DOI： 10.1186/s40623-022-01665-8

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Modeling Post-Sunset Equatorial Spread-F Occurrence as a Function of Evening Upward Plasma Drift Using Logistic Regression, Deduced from Ionosondes in Southeast Asia

Abadi, P; Ahmad, UA; Otsuka, Y; Jamjareegulgarn, P; Martiningrum, DR; Faturahman, A; Perwitasari, S; Saputra, RE; Septiawan, RR

REMOTE SENSING   Vol. 14 ( 8 )   2022.4

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DOI： 10.3390/rs14081896

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Variations of Zonal Wind Velocity in the Thermosphere Observed at Southeast Asian Sector During Quiet and Active Geomagnetic Days

Sarudin, I; Hamid, NSA; Abdullah, M; Rusli, FNSM; Otsuka, Y; Shiokawa, K; Yatini, C; Komonjinda, S; Somboon, E

JURNAL FIZIK MALAYSIA   Vol. 43 ( 1 ) page： 10025 - 10035   2022

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Xtend, the Soft X-ray Imaging Telescope for the X-ray Imaging and Spectroscopy Mission (XRISM)

Mori, K; Tomida, H; Nakajima, H; Okajima, T; Noda, H; Tanaka, T; Uchida, H; Hagino, K; Kobayashi, SB; Suzuki, H; Yoshida, T; Murakami, H; Uchiyama, H; Nobukawa, M; Nobukawa, KK; Yoneyama, T; Matsumoto, H; Tsuru, TG; Yamauchi, M; Hatsukade, I; Ishida, M; Maeda, Y; Hayashi, T; Tamura, K; Boissay-Malaquin, R; Sato, T; Hiraga, J; Kohmura, T; Yamaoka, K; Dotani, T; Ozaki, M; Tsunemi, H; Kanemaru, Y; Sato, J; Takaki, T; Terada, Y; Miyazaki, K; Kusunoki, K; Otsuka, Y; Yokosu, H; Yonemaru, W; Asahina, Y; Asakura, K; Yoshimoto, M; Ode, Y; Sato, J; Hakamata, T; Aoyagi, M; Aoki, Y; Tsunomachi, S; Doi, T; Aoki, D; Fujisawa, K; Kitajima, M; Hayashida, K

SPACE TELESCOPES AND INSTRUMENTATION 2022: ULTRAVIOLET TO GAMMA RAY   Vol. 12181   2022

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DOI： 10.1117/12.2626894

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ROLES OF EVENING EASTWARD NEUTRAL WIND AND EQUATORIAL ELECTROJET ON PRE-REVERSAL ENHANCEMENT INFERRED FROM GOCE SATELLITE AND GROUND-BASED OBSERVATIONS

Abadi, P; Otsuka, Y; Liu, HX; Hozumi, K; Martiningrum, DR

18TH ANNUAL MEETING OF THE ASIA OCEANIA GEOSCIENCES SOCIETY, AOGS 2021     page： 218 - 220   2022

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DOI： 10.1142/9789811260100_0074

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An experimental investigation into the possible connections between the zonal neutral wind speeds and equatorial plasma bubble drift velocities over the African equatorial region

Okoh, DI; Rabiu, AB; Shiokawa, K; Otsuka, Y; Wu, Q; Seemala, GK; Katamzi-Joseph, ZT

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 220   2021.9

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DOI： 10.1016/j.jastp.2021.105663

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Preface to the Special Issue on recent advances in the study of Equatorial Plasma Bubbles and Ionospheric Scintillation

Otsuka, Y; Spogli, L; Ram, ST; Li, GZ

EARTH AND PLANETARY PHYSICS   Vol. 5 ( 5 ) page： 365 - 367   2021.9

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DOI： 10.26464/epp2021050

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L-Band Synthetic Aperture Radar Observation of Ionospheric Density Irregularities at Equatorial Plasma Depletion Region

Sato, H; Kim, JS; Otsuka, Y; Wrasse, CM; de Paula, ER; de Souza, JR

GEOPHYSICAL RESEARCH LETTERS   Vol. 48 ( 16 )   2021.8

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DOI： 10.1029/2021GL093541

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Characteristics of Medium-Scale Traveling Ionospheric Disturbances and Ionospheric Irregularities at Mid-Latitudes Revealed by the Total Electron Content Associated With the Beidou Geostationary Satellite

Huang, F; Lei, J; Otsuka, Y; Luan, X; Liu, Y; Zhong, J; Dou, X

IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING   Vol. 59 ( 8 ) page： 6424 - 6430   2021.8

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DOI： 10.1109/TGRS.2020.3032741

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Statistical study of medium-scale traveling ionospheric disturbances in low-latitude ionosphere using an automatic algorithm

Cheng, PH; Lin, C; Otsuka, Y; Liu, HL; Rajesh, PK; Chen, CH; Lin, JT; Chang, MT

EARTH PLANETS AND SPACE   Vol. 73 ( 1 )   2021.5

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DOI： 10.1186/s40623-021-01432-1

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Medium-Scale Traveling Ionospheric Disturbances

Otsuka, Y

IONOSPHERE DYNAMICS AND APPLICATIONS   Vol. 260   page： 421 - 437   2021

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DOI： 10.1002/9781119815617.ch18

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Equatorial Plasma Bubble Occurrence Under Propagation of MSTID and MLT Gravity Waves

Takahashi, H; Wrasse, CM; Figueiredo, CAOB; Barros, D; Paulino, I; Essien, P; Abdu, MA; Otsuka, Y; Shiokawa, K

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 125 ( 9 )   2020.9

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DOI： 10.1029/2019JA027566

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Temporal and Spatial Variations of Total Electron Content Enhancements During a Geomagnetic Storm on 27 and 28 September 2017

Shinbori, A; Otsuka, Y; Sori, T; Tsugawa, T; Nishioka, M

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 125 ( 7 )   2020.7

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DOI： 10.1029/2019JA026873

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Equatorial Plasma Bubble Zonal Drift Velocity Variations in Response to Season, Local Time, and Solar Activity across Southeast Asia

Sarudin, I; Hamid, NSA; Abdullah, M; Buhari, SM; Shiokawa, K; Otsuka, Y; Yatini, CY

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 125 ( 3 )   2020.3

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DOI： 10.1029/2019JA027521

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Wavenumber Spectra of Atmospheric Gravity Waves and Medium-Scale Traveling Ionospheric Disturbances Based on More Than 10-Year Airglow Images in Japan, Russia, and Canada

Satoshi Tsuchiya, Kazuo Shiokawa, Yuichi Otsuka, Takuji Nakamura, Mamoru Yamamoto, Martin Connors, Ian Schofield, Boris Shevtsov, Igor Poddelsky

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 125 ( 3 )   2020.3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We have studied atmospheric gravity waves (AGWs) and nighttime medium-scale traveling ionospheric disturbances (MSTIDs) by applying three-dimensional spectral analysis technique to 557.7- and 630.0-nm airglow images at Shigaraki (SGK) (35 degrees N, 136 degrees E, 1999-2017) and Rikubetsu (RIK) (44 degrees N, 144 degrees E, 1999-2017), Japan, Athabasca (ATH), Canada, (55 degrees N, 247 degrees E, 2005-2017), and Magadan (MGD), Russia (60 degrees N, 151 degrees E, 2008-2017), focusing on their horizontal wavenumber spectra. For the AGWs in 557.7-nm images, the power spectra in summer are stronger than in other seasons, probably due to stronger tropospheric convection. The highest energy content of the waves are mostly at wavelengths between 20 and 300 km at MGD, ATH, and RIK, while it is above 200 km at SGK. The largest power spectral density is obtained at RIK at wavelengths of 30-100 km and then ATH. The slopes of the horizontal wavenumber spectra varies from -2.77 to -3.22. From the MSTIDs in 630.0-nm images, the power spectra in summer at RIK and SGK are stronger than those in other seasons regardless of solar activity. The power spectra in solar quiet time are stronger than those in solar active time at all four stations. These features can be explained by the Perkins instability with coupling between sporadic E and F layers. The spectral slope decreases with increasing latitudes. Weak positive correlations were obtained between the daily wave power of AGWs in 557.7-nm images and MSTIDs in 630.0-nm images, suggesting that the MSTIDs in the thermosphere may be partially generated by the AGWs from the mesopause region.Plain Language Summary In this paper we study atmospheric gravity waves (AGWs) and nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed at four stations in Japan, Canada, and Russia, in 557.7- and 630.0-nm airglow images over more than 10 years. The 557.7-nm airglow has an emission layer at altitudes of 90-100 km (mesopause region). The waves seen in the 557.7-nm airglow images mainly indicates AGWs. The 630.0-nm airglow has an emission layer at altitudes of 200-300 km (bottomside ionosphere). The waves seen in the 630.0-nm airglow images mainly indicates MSTIDs in the ionosphere. The AGWs in the mesopause region are the main driver of global atmospheric circulation in the middle atmosphere. The MSTIDs in the bottomside ionosphere are one of the causes of the satellite positioning error. We show typical energy content, propagation direction, and wavelengths of these waves at these two altitudes and discuss possible reason of the observed characteristics. These results contribute to our understanding of generation and propagation of AGWs and MSTIDs in the upper atmosphere.

DOI： 10.1029/2019JA026807

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Multievent Analysis of Oscillatory Motion of Medium-Scale Traveling Ionospheric Disturbances Observed by a 630-nm Airglow Imager Over Tromso

Yadav, S; Shiokawa, K; Oyama, S; Otsuka, Y

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 125 ( 3 )   2020.3

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DOI： 10.1029/2019JA027598

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Characteristics of GNSS Total Electron Content Enhancements Over the Midlatitudes During a Geomagnetic Storm on 7 and 8 November 2004

Sori, T; Shinbori, A; Otsuka, Y; Tsugawa, T; Nishioka, M

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 124 ( 12 ) page： 10376 - 10394   2019.12

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DOI： 10.1029/2019JA026713

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Adjuvant therapy with nivolumab versus ipilimumab after complete resection of stage III/IV melanoma: Japanese subgroup analysis from the phase 3 CheckMate 238 study

Yokota, K; Uchi, H; Uhara, H; Yoshikawa, S; Takenouchi, T; Inozume, T; Ozawa, K; Ihn, H; Fujisawa, Y; Qureshi, A; de Pril, V; Otsuka, Y; Weber, J; Yamazaki, N

JOURNAL OF DERMATOLOGY   Vol. 46 ( 12 ) page： 1197 - 1201   2019.12

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DOI： 10.1111/1346-8138.15103

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IpsDst of Dst Storms Applied to Ionosphere-Thermosphere Storms and Low-Latitude Aurora

Balan, N; Zhang, QH; Shiokawa, K; Skoug, R; Xing, ZY; Ram, ST; Otsuka, Y

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 124 ( 11 ) page： 9552 - 9565   2019.11

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DOI： 10.1029/2019JA027080

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Statistical Study of Auroral/Resonant-Scattering 427.8-nm Emission Observed at Subauroral Latitudes Over 14 Years

Shiokawa, K; Otsuka, Y; Connors, M

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 124 ( 11 ) page： 9293 - 9301   2019.11

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DOI： 10.1029/2019JA026704

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Japanese Surveillance of Neuroendovascular Therapy in JR-NET - Part II. Japanese Registry of NeuroEndovascular Treatment 3. Main Report

Sakai, N; Uchida, K; Iihara, K; Satow, T; Ezura, M; Hyodo, A; Miyachi, S; Miyamoto, S; Nagai, Y; Nishimura, K; Toyoda, K; Yoshimura, S; Imamura, H; Sakai, C; Matsumaru, Y; Sakai, N; Iihara, K; Satow, T; Ezura, M; Hyodo, A; Miyachi, S; Miyamoto, S; Nagai, Y; Nishimura, K; Toyoda, K; Fujinaka, T; Higashi, T; Hirohata, M; Ishii, A; Imamura, H; Ito, Y; Kuwayama, N; Oishi, H; Matsumaru, Y; Matsumoto, Y; Nakahara, I; Sakai, C; Sugiu, K; Terada, T; Yoshimura, S; Sakai, N; Adachi, H; Ueno, Y; Yamagami, H; Imamura, H; Kunieda, T; Koyanagi, M; Kuramoto, Y; Todo, K; Ishikawa, T; Shigematsu, T; Yamamoto, S; Sato, S; Asai, K; Arimura, K; Tani, S; Mineharu, Y; Narumi, O; Ishii, A; Sakai, C; Ikeda, H; Kono, T; Matsumoto, Y; Kondo, R; Furui, E; Suzuki, I; Takahashi, T; Akiyama, T; Endo, H; Sato, K; Kageyama, Y; Ssawa, S; Niizuma, K; Yazawa, Y; Itabashi, R; Ezura, M; Kimura, N; Tsuboi, K; Sato, K; Nishimura, S; Sato, K; Osawa, S; Suzuki, I; Sugiu, K; Terai, Y; Tokunaga, K; Katsumata, A; Kusaka, N; Nishida, A; Kawada, S; Watanabe, K; Hishikawa, T; Hirashita, K; Itami, H; Ota, S; Sekihara, Y; Shimizu, N; Maeda, K; Matsumaru, Y; Izumoto, H; Tsuruta, W; Nakane, Y; Okumura, H; Hayakawa, M; Hirota, Y; Sato, M; Kamiya, Y; Watanabe, A; Amano, T; Aoki, M; Hyuga, T; Watanabe, D; Mori, K; Seida, M; Osanai, T; Hirohata, M; Takeuchi, M; Tsutsumi, M; Nii, K; Eto, H; Kazekawa, K; Aikawa, H; Onizuka, M; Yoshida, H; Sakamoto, K; Mizokami, T; Oishi, H; Iko, M; Nakai, K; Kin, S; Nonaka, T; Yonemasu, Y; Takahashi, A; Onda, T; Kogure, S; Ueda, R; Shimizu, T; Nomura, T; Yamamura, A; Hayashi, S; Nakahara, I; Nakamura, M; Furuichi, S; Iwamuro, Y; Ohta, T; Toyota, S; Furui, E; Watanabe, Y; Matsumoto, S; Ishibashi, R; Urabe, Y; Gomi, M; Fukushima, Y; Saka, M; Nakazawa, T; Matsuda, Y; Hyodo, A; Hori, Y; Shirato, M; Suzuki, K; Suzuki, S; Takigawa, T; Shimizu, N; Tanaka, Y; Takano, I; Satow, T; Iihara, K; Okazaki, T; Morita, K; Yamagami, H; Ito, K; Sugata, S; Hayakawa, M; Matsushige, T; Ishii, D; Ishikawa, T; Fukuda, K; Masuda, K; Hashimoto, T; Miyazaki, Y; Hamano, E; Maruyama, D; Nakajima, N; Okumura, H; Funatsu, N; Yoshimura, S; Kokuzawa, J; Enomoto, Y; Yoshida, Y; Egashira, Y; Takagi, T; Yamada, K; Ishiguro, M; Tsujimoto, M; Watarai, H; Yamauchi, K; Kitajima, H; Ishihara, H; Oka, F; Oku, T; Kumasaka, A; Harada, K; Morioka, J; Hattori, I; Morimoto, M; Ozaki, S; Kobanawa, S; Sasaki, M; Matsumaru, Y; Kawaguchi, K; Kawanishi, M; Shindo, A; Hayashi, N; Kawakita, K; Yano, T; Inukai, T; Okauchi, M; Kuroiwa, T; Shimizu, F; Shirakawa, M; Oonishi, H; Nakahara, T; Yamashita, K; Ohta, H; Kamata, I; Araki, H; Ogami, R; Hatano, T; Ishii, A; Hasegawa, H; Kunieda, T; Loyanagi, M; Yakenobu, Y; Yoshida, K; Ogino, E; Kikuchi, T; Takemoto, K; Yamana, N; Goto, M; Munemitsu, T; Yokoyama, Y; Chihara, H; Mineharu, Y; Ando, M; Saiki, M; Sekihara, Y; Murai, N; Yamao, Y; Ikeda, H; Hayashi, H; Waro, T; Sateshima, S; Arai, D; Sakai, N; Nakamura, M; Mizobe, T; Miyachi, S; Ikushima, I; Izumi, T; Matsubara, N; Haraguchi, K; Makiuchi, T; Hashimoto, K; Asai, T; Hyogo, T; Kataoka, T; Ogino, T; Imai, K; Takegami, T; Hamanaka, M; Yamada, T; Ikeda, N; Okamoto, T; Kano, T; Toi, H; Matsubara, S; Murao, K; Nakazawa, K; Takahashi, A; Nakajima, N; Iwamuro, Y; Ohta, T; Aketa, S; Takabatake, N; Kunieda, T; Fukuda, H; Takemoto, K; Miyake, K; Morioka, J; Yukawa, H; Akiyama, T; Konno, H; Nakahara, K; Nishio, M; Hiramatsu, H; Shibanai, K; Nishio, A; Yamauchi, S; Kondo, K; Hayasaki, K; Mitsuhashi, Y; Kawakami, T; Yamauchi, S; Mizuguchi, A; Demura, K; Satoh, K; Hanaoka, M; Manabe, S; Tamura, T; Kinouchi, T; Tsurukiri, J; Horie, N; Morikawa, M; Hayashi, K; Higashi, T; Morofuji, Y; Nakai, Y; Irie, S; Tsuruta, W; Takigawa, T; Matsumaru, Y; Sato, M; Ikeda, G; Nakazawa, K; Ayabe, J; Akaji, K; Iwai, T; Naito, I; Dembo, T; Ueda, R; Ishimori, H; Kimura, H; Suzuki, K; Fujinaka, T; Kajikawa, R; Nishida, T; Ohara, N; Sakaguchi, M; Nakamura, H; Tone, O; Shigeta, K; Yatsushige, H; Mohri, M; Uchiyama, N; Hamada, J; Misaki, K; Toma, N; Shibata, M; Sato, Y; 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Oonishi, H; Hiramatsu, R; Shoda, D; Tamakawa, N; Tomokiyo, M; Manabe, S; Moroi, J; Yoshioka, S; Matsumoto, Y; Kitajima, H; Tenjin, H; Nanto, M; Osaka, Y; Higa, T; Niimi, Y; Onozuka, S; Katayama, M; Kagami, H; Iwabuchi, S; Hayashi, M; Yokouchi, T; Yamana, N; Toyota, S; Ohara, N; Nakamura, H; Nishida, T; Iito, O; Yamaguchi, S; Mase, M; Hashimoto, N; Ohno, T; Oomura, M; Yamada, K; Ushikoshi, S; Miyamoto, M; Yamaguchi, S; Ito, O; Uda, K; Yasumori, K; Ueno, Y; Kuramoto, Y; Shinoda, N; Tateshima, S; Sakai, C; Sakai, N; Momoji, J; Shimamura, N; Owada, K; Inoue, Y; Ebihara, K; Nakahara, I; Oishi, H; Hayasaka, M; Matsumoto, H; Takemoto, H; Hirohata, Y; Naito, I; Asakura, K; Shimaguchi, H; Wakabayashi, K; Sato, K; Sakamoto, M; Takeuchi, H; Uno, T; Mase, M; Aihara, N; Nishikawa, Y; Akiyama, Y; Ogino, E; Mitsuhashi, Y; Nishio, A; Yamauchi, S; Takahashi, T; Matsumoto, Y; Morishima, H; Ooshima, K; Maekawa, M; Morioka, J; Watanabe, S; Sakata, Y; Ohashi, M; Hayashi, N; Kawanishi, M; Shindo, A; Yano, T; Ogata, A; Takase, Y; Takada, T; Otsuka, Y; Ueda, T; Nogoshi, S; Takeuchi, H; Arakawa, H; Sugawara, T; Ozaki, S; Ichikawa, T; Okamoto, T; Munemitsu, T; Saiki, M; Ueba, Y; Yoshida, Y; Kominami, S; Suzuki, M; Watanabe, A; Kikuchi, O; Wada, H; Sakurai, J; Takata, H; Shiraga, S; Terada, T; Morishima, H; Koyama, S; Wakui, D; Onodera, H; Ooshima, K; Tsuji, A; Tanaka, T; Higuchi, K; Uchida, K; Shirakawa, M; Maeno, K; Tanaka, Y; Kojima, A; Suzuki, M; Watanabe, A; Srivatanakul, K; Tanoue, S; Okahara, M; Kashiwagi, J; Kiyosue, H; Sagara, Y; Shimada, R; Yoshioka, T; Sambongi, Y; Negoro, M; Miyasaki, A; Hamasaki, O; Takabatake, Y; Handa, A; Higuchi, K; Hamada, K; Ichihashi, T; Fukazawa, S; Nonaka, Y; Shojima, M; Kato, T; Kaku, Y; Sakai, H; Fukui, K; Mina-Mide, H; Suyama, T; Kobayashi, J; Ohara, N; Takeda, N; Ota, S; Arai, M; Uemura, A; Fukui, N; Fujimura, N; Uchiyama, N; Mohri, M; Higashi, R; Misaki, K; Sugiura, Y; Watanabe, K; Yamazaki, S; Munemitsu, T; Murai, N; Miyakoshi, A; Yamao, Y; Yoshida, Y; Yamaura, I; Minami, H; Matsumoto, H; Masuda, A; Hara, Y; Iihara, K; Suyama, Y; Yamasaki, H; Ishii, Y; Wakabayashi, S; Tanoue, S; Kubo, T; Tsutsumi, M; Shimada, A; Oura, Y; Fujita, A; Kimoto, A; Abe, H; Takemoto, H; Okada, H; Yoshimura, R; Obata, Y; Yoshino, Y; Kawano, Y; Matsumoto, K; Mutoh, T; Dehara, M; Kubota, T; Kawarabuki, K; Shirato, M; Asano, T; Sagara, Y; Shibuya, T; Suma, T; Amari, K; Oishi, H; Yamamoto, M; Iihoshi, S; Miyata, K; Fukasaku, K; Tamaki, R; Nishio, M; Kojima, T; Maekawa, H; Yanaka, M; Okawa, M; Iwaasa, M; Goda, M; Kubo, T; Hirai, M; Yamada, M; Yoshino, Y; Nemoto, S; Kuwayama, N; Yamamoto, H; Akioka, N; Nakayama, H; Nakabayashi, K

NEUROLOGIA MEDICO-CHIRURGICA   Vol. 59 ( 3 ) page： 106 - 115   2019.3

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Spatiotemporal development of pulsating auroral patch associated with discrete chorus elements: Arase and PWING observations

Ozaki M., Shiokawa K., Miyoshi Y., Hosokawa K., Oyama S., Yagitani S., Kasahara Y., Kasaba Y., Matsuda S., Kataoka R., Ebihara Y., Ogawa Y., Otsuka Y., Kurita S., Moore R. C., Tanaka Y. -M., Nose M., Nagatsuma T., Connors M., Nishitani N., Hikishima M., Kumamoto A., Tsuchiya F., Kadokura A., Nishiyama T., Inoue T., Imamura K., Matsuoka A., Shinohara I.

2019 URSI ASIA-PACIFIC RADIO SCIENCE CONFERENCE (AP-RASC)     2019

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Spatial and Temporal Characteristics of Ionospheric Total Electron Content over Indian Equatorial and Low-Latitude GNSS Stations

Sivavaraprasad G., Otsuka Yuichi, Tripathi Nitin Kumar, Chowdhary V. Rajesh, Ratnam D. Venkata, Khan Mohammed Afzal

2018 CONFERENCE ON SIGNAL PROCESSING AND COMMUNICATION ENGINEERING SYSTEMS (SPACES)     page： 105 - 108   2018

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Observations of Ultrawideband Signals in GPS TEC Variations over Europe during Solar Eclipse

Panasenko Sergii V., Chernogor Leonid F., Lazorenko Oleg V., Otsuka Yuichi, van de Kamp Max

2018 9TH INTERNATIONAL CONFERENCE ON ULTRAWIDEBAND AND ULTRASHORT IMPULSE SIGNALS (UWBUSIS)     page： 115 - 118   2018

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Temporal and spatial variations of mid-latitude ionospheric trough during a geomagnetic storm based on global GNSS-TEC and Arase satellite observations

Shinbori Atsuki, Otsuka Yuichi, Tsugawa Takuya, Nishioka Michi, Kumamoto Atsushi, Tsuchia Fuminori, Matsuda Shoya, Kasahara Yoshiya

2018 2ND URSI ATLANTIC RADIO SCIENCE MEETING (AT-RASC)     2018

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STATISTICAL ANALYSIS OF THE OCCURRENCES OF MSTIDs OBSERVED BY ALL-SKY IMAGER IN LOW MAGNETIC LATITUDE

Ednofri, Wu Falin, Otsuka Yuichi, Ishii Mamoru, Marlia Dessi, Zhao Yan

2017 IEEE INTERNATIONAL GEOSCIENCE AND REMOTE SENSING SYMPOSIUM (IGARSS)     page： 4425 - 4428   2017

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On the effect of thermospheric neutral winds on post-midnight field-aligned irregularities at low latitudes

Dao Tam, Otsuka Yuichi, Shiokawa Kazuo, Nishioka Michi, Yamamoto Mamoru, Buhari Suhaila M., Abdullah Mardina, Husin Asnawi

2017 XXXIIND GENERAL ASSEMBLY AND SCIENTIFIC SYMPOSIUM OF THE INTERNATIONAL UNION OF RADIO SCIENCE (URSI GASS)     2017

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Early Phase Arterial Reaction Following Drug-Eluting and Bare-Metal Stent Implantation in Patients With ST-Segment Elevation Myocardial Infarction Optical Coherence Tomography Assessment at 2 Weeks

Nishio, R; Shinke, T; Morino, Y; Tanabe, K; Furuya, J; Takizawa, K; Ako, J; Kozuma, K; Hibi, K; Ishii, H; Hirohata, A; Otsuka, Y; Otake, H; Takaya, T; Hirata, K

INTERNATIONAL HEART JOURNAL   Vol. 56 ( 4 ) page： 389 - 394   2015.7

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Micro-source development for XMASS experiment

Kim, NY; Abe, K; Hieda, K; Hiraid, K; Hirano, S; Kishimoto, Y; Kobayashi, K; Moriyama, S; Nakagawa, K; Nakahata, M; Nishiie, H; Ogawa, H; Oka, N; Sekiya, H; Shinozaki, A; Suzuki, Y; Takeda, A; Takachio, O; Ueshima, K; Umemoto, D; Yamashita, M; Yang, BS; Tasaka, S; Liu, J; Martens, K; Hosokawa, K; Miuchi, K; Murata, A; Onishi, Y; Otsuka, Y; Takeuchi, Y; Kim, YH; Lee, KB; Lee, MK; Lee, JS; Fukuda, Y; Itow, Y; Nishitani, Y; Masuda, K; Takiya, H; Uchida, H; Kim, YD; Kusaba, F; Motoki, D; Nishijima, K; Fujii, K; Murayama, I; Nakamura, S

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT   Vol. 784   page： 499 - 503   2015.6

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Climatology of Equatorial Plasma Bubble Observed by MyRTKnet over the Years 2008-2013

Buhari S. M., Abdullah M., Yokoyama T., Hasbi A. M., Otsuka Y., Nishioka M., Bahari S. A., Tsugawa T.

2015 INTERNATIONAL CONFERENCE ON SPACE SCIENCE AND COMMUNICATION (ICONSPACE)     page： 101 - 105   2015

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GAST-D flight experiment results with disturbed and quiet ionospheric conditions

Saito Susumu, Yoshihara Takayuki, Kezuka Atsushi, Saitoh Shinji, Fukushima Sonosuke, Otsuka Yuichi

PROCEEDINGS OF THE 28TH INTERNATIONAL TECHNICAL MEETING OF THE SATELLITE DIVISION OF THE INSTITUTE OF NAVIGATION (ION GNSS+ 2015)     page： 1494 - 1499   2015

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Direct observations of blob deformation during a substorm

Ishida T., Ogawa Y., Kadokura A., Hosokawa K., Otsuka Y.

ANNALES GEOPHYSICAE   Vol. 33 ( 5 ) page： 525 - 530   2015

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Search for Bosonic Superweakly Interacting Massive Dark Matter Particles with the XMASS-I Detector

Abe, K; Hieda, K; Hiraide, K; Hirano, S; Kishimoto, Y; Ichimura, K; Kobayashi, K; Moriyama, S; Nakagawa, K; Nakahata, M; Ogawa, H; Oka, N; Sekiya, H; Shinozaki, A; Suzuki, Y; Takeda, A; Takachio, O; Umemoto, D; Yamashita, M; Yang, BS; Tasaka, S; Liu, J; Martens, K; Hosokawa, K; Miuchi, K; Murata, A; Onishi, Y; Otsuka, Y; Takeuchi, Y; Kim, YH; Lee, KB; Lee, MK; Lee, JS; Fukuda, Y; Itow, Y; Masuda, K; Takiya, H; Uchida, H; Kim, NY; Kim, YD; Kusaba, F; Nishijima, K; Fujii, K; Murayama, I; Nakamura, S

PHYSICAL REVIEW LETTERS   Vol. 113 ( 12 )   2014.9

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Search for inelastic WIMP nucleus scattering on Xe-129 in data from the XMASS-I experiment

Uchida H., Abe K., Hieda K., Hiraide K., Hirano S., Ichimura K., Kishimoto Y., Kobayashi K., Moriyama S., Nakagawa K., Nakahata M., Ogawa H., Oka N., Sekiya H., Shinozaki A., Suzuki Y., Takeda A., Takachio O., Umemoto D., Yamashita M., Yang B. S., Tasaka S., Liu J., Martens K., Hosokawa K., Miuchi K., Murata A., Onishi Y., Otsuka Y., Takeuchi Y., Kim Y. H., Lee K. B., Lee M. K., Lee J. S., Fukuda Y., Itow Y., Masuda K., Nishitani Y., Takiya H., Kim N. Y., Kim Y. D., Kusaba F., Nishijima K., Fujii K., Murayama I., Nakamura S.

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   Vol. 2014 ( 6 )   2014.6

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The observation of equatorial plasma bubble using all sky imager and GPS TEC measurement

Buhari Suhaila M., Abdullah Mardina, Hasbi Alina Marie, Otsuka Yuichi, Bahari Siti Aminah, Mokhtar Mohd Hezri, Nishioka Michi, Tsugawa Takuya

2014 XXXITH URSI GENERAL ASSEMBLY AND SCIENTIFIC SYMPOSIUM (URSI GASS)     2014

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Two-dimensional structure of equatorial plasma bubble observed using GPS networks in South East Asia region

Buhari Suhaila M., Abdullah Mardina, Hasbi Alina Marie, Otsuka Yuichi

2014 INTERNATIONAL CONFERENCE ON ELECTRONICS, INFORMATION AND COMMUNICATIONS (ICEIC)     2014

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Typhoon-induced concentric airglow structures in the mesopause region

Suzuki S., Vadas S. L., Shiokawa K., Otsuka Y., Kawamura S., Murayama Y.

GEOPHYSICAL RESEARCH LETTERS   Vol. 40 ( 22 ) page： 5983 - 5987   2013.11

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XMASS detector

Abe, K; Hieda, K; Hiraide, K; Hirano, S; Kishimoto, Y; Kobayashi, K; Moriyama, S; Nakagawa, K; Nakahata, M; Nishiie, H; Ogawa, H; Oka, N; Sekiya, H; Shinozaki, A; Suzuki, Y; Takeda, A; Takachio, O; Ueshima, K; Umemoto, D; Yamashita, M; Yang, BS; Tasaka, S; Liu, J; Martens, K; Hosokawa, K; Miuchi, K; Murata, A; Onishi, Y; Otsuka, Y; Takeuchi, Y; Kim, YH; Lee, KB; Lee, MK; Lee, JS; Fukuda, Y; Itow, Y; Nishitani, Y; Masuda, K; Takiya, H; Uchida, H; Kim, NY; Kim, YD; Kusaba, F; Motoki, D; Nishijima, K; Fujii, K; Murayama, I; Nakamura, S

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT   Vol. 716   page： 78 - 85   2013.7

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DOI： 10.1016/j.nima.2013.03.059

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Light WIMP search in XMASS

Abe, K; Hieda, K; Hiraide, K; Hirano, S; Kishimoto, Y; Kobayashi, K; Moriyama, S; Nakagawa, K; Nakahata, M; Ogawa, H; Oka, N; Sekiya, H; Shinozaki, A; Suzuki, Y; Takeda, A; Takachio, O; Ueshima, K; Umemoto, D; Yamashita, M; Yang, BS; Tasaka, S; Liu, J; Martens, K; Hosokawa, K; Miuchi, K; Murata, A; Onishi, Y; Otsuka, Y; Takeuchi, Y; Kim, YH; Lee, KB; Lee, MK; Lee, JS; Fukuda, Y; Itow, Y; Masuda, K; Nishitani, Y; Takiya, H; Uchida, H; Kim, NY; Kim, YD; Kusaba, F; Motoki, D; Nishijima, K; Fujii, K; Murayama, I; Nakamura, S

PHYSICS LETTERS B   Vol. 719 ( 1-3 ) page： 78 - 82   2013.2

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Label-free observation of tissues by high-speed stimulated Raman spectral microscopy and independent component analysis

Ozeki, Y; Otsuka, Y; Sato, S; Hashimoto, H; Umemura, W; Sumimura, K; Nishizawa, N; Fukui, K; Itoh, K

MULTIPHOTON MICROSCOPY IN THE BIOMEDICAL SCIENCES XIII   Vol. 8588   2013

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Small-scale ionospheric delay variation associated with plasma bubbles studied with GNSS and optical measurements and its impact on GBAS

Saito S., Yoshihara T., Otsuka Y.

PROCEEDINGS OF THE 26TH INTERNATIONAL TECHNICAL MEETING OF THE SATELLITE DIVISION OF THE INSTITUTE OF NAVIGATION (ION GNSS 2013)     page： 1869 - 1874   2013

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High-speed molecular spectral imaging of tissue with stimulated Raman scattering

Ozeki, Y; Umemura, W; Otsuka, Y; Satoh, S; Hashimoto, H; Sumimura, K; Nishizawa, N; Fukui, K; Itoh, K

NATURE PHOTONICS   Vol. 6 ( 12 ) page： 844 - 850   2012.12

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A comparative study of equatorial daytime vertical E x B drift in the Indian and Indonesian sectors based on 150 km echoes

Patra A. K., Chaitanya P. Pavan, Mizutani N., Otsuka Y., Yokoyama T., Yamamoto M.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.11

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Observation of equatorial nighttime medium-scale traveling ionospheric disturbances in 630-nm airglow images over 7 years

Fukushima D., Shiokawa K., Otsuka Y., Ogawa T.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.10

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Motion of high-latitude nighttime medium-scale traveling ionospheric disturbances associated with auroral brightening

Shiokawa K., Mori M., Otsuka Y., Oyama S., Nozawa S.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.10

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Response of low-latitude ionosphere to medium-term changes of solar and geomagnetic activity

Kutiev Ivan, Otsuka Yuichi, Pancheva Dora, Heelis Rod

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.8

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Significance of C4d deposition in antibody-mediated rejection

Takeda, A; Otsuka, Y; Horike, K; Inaguma, D; Hiramitsu, T; Yamamoto, T; Nanmoku, K; Goto, N; Watarai, Y; Uchida, K; Morozumi, K; Kobayashi, T

CLINICAL TRANSPLANTATION   Vol. 26   page： 43 - 48   2012.7

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GPS total electron content variations associated with poleward moving Sun-aligned arcs

Jayachandran P. T., Hosokawa K., Shiokawa K., Otsuka Y., Watson C., Mushini S. C., MacDougall J. W., Prikryl P., Chadwick R., Kelly T. D.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.5

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Polarization of Pc1/EMIC waves and related proton auroras observed at subauroral latitudes

Nomura R., Shiokawa K., Sakaguchi K., Otsuka Y., Connors M.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.2

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DOI： 10.1029/2011JA017241

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Ionospheric and thermospheric storms at equatorial latitudes observed by CHAMP, ROCSAT, and DMSP

Balan N., Liu J. Y., Otsuka Y., Ram S. Tulasi, Luehr H.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.1

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DOI： 10.1029/2011JA016903

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On postmidnight low-latitude ionospheric irregularities during solar minimum: 2. C/NOFS observations and comparisons with the Equatorial Atmosphere Radar

Yokoyama T., Pfaff R. F., Roddy P. A., Yamamoto M., Otsuka Y.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.11

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DOI： 10.1029/2011JA016798

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New aspects of thermospheric and ionospheric storms revealed by CHAMP

Balan N., Yamamoto M., Liu J. Y., Otsuka Y., Liu H., Luehr H.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.7

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DOI： 10.1029/2010JA016399

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Decay of polar cap patch

Hosokawa K., Moen J. I., Shiokawa K., Otsuka Y.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.5

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DOI： 10.1029/2010JA016297

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Vertical connection from the tropospheric activities to the ionospheric longitudinal structure simulated by a new Earth's whole atmosphere-ionosphere coupled model

Jin H., Miyoshi Y., Fujiwara H., Shinagawa H., Terada K., Terada N., Ishii M., Otsuka Y., Saito A.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.1

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DOI： 10.1029/2010JA015925

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Dynamic temporal evolution of polar cap tongue of ionization during magnetic storm

Hosokawa K., Tsugawa T., Shiokawa K., Otsuka Y., Nishitani N., Ogawa T., Hairston M. R.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.12

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DOI： 10.1029/2010JA015848

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Midnight latitude-altitude distribution of 630 nm airglow in the Asian sector measured with FORMOSAT-2/ISUAL

Adachi Toru, Yamaoka Masashi, Yamamoto Mamoru, Otsuka Yuichi, Liu Huixin, Hsiao Chun-Chieh, Chen Alfred B., Hsu Rue-Ron

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.9

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DOI： 10.1029/2009JA015147

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Finite-Temperature Properties across the Charge Ordering Transition-Combined Bosonization, Renormalization Group, and Numerical Methods-

Yoshioka, H; Tsuchiizu, M; Otsuka, Y; Seo, H

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   Vol. 79 ( 9 )   2010.9

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DOI： 10.1143/JPSJ.79.094714

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Large-scale traveling ionospheric disturbance observed by superDARN Hokkaido HF radar and GPS networks on 15 December 2006

Hayashi H., Nishitani N., Ogawa T., Otsuka Y., Tsugawa T., Hosokawa K., Saito A.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.6

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DOI： 10.1029/2009JA014297

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A physical mechanism of positive ionospheric storms at low latitudes and midlatitudes

Balan N., Shiokawa K., Otsuka Y., Kikuchi T., Lekshmi D. Vijaya, Kawamura S., Yamamoto M., Bailey G. J.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.2

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DOI： 10.1029/2009JA014515

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Reorganization of polar cap patches through shears in the background plasma convection

Hosokawa K., St-Maurice J. -P., Sofko G. J., Shiokawa K., Otsuka Y., Ogawa T.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.1

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DOI： 10.1029/2009JA014599

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Spatial relationship of nighttime medium-scale traveling ionospheric disturbances and F region field-aligned irregularities observed with two spaced all-sky airglow imagers and the middle and upper atmosphere radar

Otsuka Y., Shiokawa K., Ogawa T., Yokoyama T., Yamamoto M.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.5

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DOI： 10.1029/2008JA013902

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Motion of polar cap patches: A statistical study with all-sky airglow imager at Resolute Bay, Canada

Hosokawa K., Kashimoto T., Suzuki S., Shiokawa K., Otsuka Y., Ogawa T.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.4

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DOI： 10.1029/2008JA014020

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Medium-scale traveling ionospheric disturbances observed with the SuperDARN Hokkaido radar, all-sky imager, and GPS network and their relation to concurrent sporadic E irregularities

Ogawa T., Nishitani N., Otsuka Y., Shiokawa K., Tsugawa T., Hosokawa K.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.3

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DOI： 10.1029/2008JA013893

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Super plasma fountain and equatorial ionization anomaly during penetration electric field

Balan N., Shiokawa K., Otsuka Y., Watanabe S., Bailey G. J.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.3

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DOI： 10.1029/2008JA013768

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Relationship between polar cap patches and field-aligned irregularities as observed with an all-sky airglow imager at Resolute Bay and the PolarDARN radar at Rankin Inlet

Hosokawa K., Shiokawa K., Otsuka Y., Ogawa T., St-Maurice J. -P., Sofko G. J., Andre D. A.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.3

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DOI： 10.1029/2008JA013707

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Three-dimensional simulation of the coupled Perkins and E-s-layer instabilities in the nighttime midlatitude ionosphere

Yokoyama Tatsuhiro, Hysell David L., Otsuka Yuichi, Yamamoto Mamoru

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.3

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DOI： 10.1029/2008JA013789

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Characteristics of equatorial gravity waves derived from mesospheric airglow imaging observations

Suzuki S., Shiokawa K., Liu A. Z., Otsuka Y., Ogawa T., Nakamura T.

ANNALES GEOPHYSICAE   Vol. 27 ( 4 ) page： 1625 - 1629   2009

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Equatorial GPS ionospheric scintillations over Kototabang, Indonesia and their relation to atmospheric waves from below

Ogawa Tadahiko, Miyoshi Yasunobu, Otsuka Yuichi, Nakamura Takuji, Shiokawa Kazuo

EARTH PLANETS AND SPACE   Vol. 61 ( 4 ) page： 397 - 410   2009

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Propagation characteristics of nighttime mesospheric and thermospheric waves observed by optical mesosphere thermosphere imagers at middle and low latitudes

Shiokawa K., Otsuka Y., Ogawa T.

EARTH PLANETS AND SPACE   Vol. 61 ( 4 ) page： 479 - 491   2009

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Relative effects of electric field and neutral wind on positive ionospheric storms

Balan N., Alleyne H., Otsuka Y., Lekshmi D. Vijaya, Fejer B. G., McCrea I.

EARTH PLANETS AND SPACE   Vol. 61 ( 4 ) page： 439 - 445   2009

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The Optical Mesosphere Thermosphere Imagers (OMTIs) for network measurements of aurora and airglow

Shiokawa K., Hosokawa K., Sakaguchi K., Ieda A., Otsuka Y., Ogawa T., Connors M.

FUTURE PERSPECTIVES OF SPACE PLASMA AND PARTICLE INSTRUMENTATION AND INTERNATIONAL COLLABORATIONS   Vol. 1144   page： 212 - +   2009

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Zonal asymmetry of daytime 150-km echoes observed by Equatorial Atmosphere Radar in Indonesia

Yokoyama T., Hysell D. L., Patra A. K., Otsuka Y., Yamamoto M.

ANNALES GEOPHYSICAE   Vol. 27 ( 3 ) page： 967 - 974   2009

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VHF radar observations of nighttime F-region field-aligned irregularities over Kototabang, Indonesia

Otsuka Y., Ogawa T., Effendy

EARTH PLANETS AND SPACE   Vol. 61 ( 4 ) page： 431 - 437   2009

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Prospective multicenter trial comparing repeated immunosuppressive therapy with stem-cell transplantation from an alternative donor as second-line treatment for children with severe and very severe aplastic anemia

Kosaka, Y; Yagasaki, H; Sano, K; Kobayashi, R; Ayukawa, H; Kaneko, T; Yabe, H; Tsuchida, M; Mugishima, H; Hara, A; Morimoto, A; Otsuka, Y; Ohga, S; Bessho, F; Nakahata, T; Tsukimoto, I; Kojima, S

BLOOD   Vol. 111 ( 3 ) page： 1054 - 1059   2008.2

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DOI： 10.1182/blood-2007-08-099168

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PubMed

Quantum dot sensitized solar cells

Tachibana, Y; Akiyama, HY; Umekita, K; Otsuka, Y; Torimoto, T; Kuwabata, S

2008 2ND IEEE INTERNATIONAL NANOELECTRONICS CONFERENCE, VOLS 1-3     page： 160 - +   2008

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Efficacy of STI571 for a patient with metastatic gastrointestinal stromal tumor

Sasaki, Y; Niwa, Y; Ando, N; Otsuka, Y; Ohmiya, N; Hirooka, Y; Itoh, A; Furuta, S; Goto, H

HEPATO-GASTROENTEROLOGY   Vol. 52 ( 66 ) page： 1764 - 1767   2005.11

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PubMed

Inhibition of platelet adherence to mononuclear cells by alpha-tocopherol - Role of P-selectin

Murohara T, Ikeda H, Otsuka Y, Aoki M, Haramaki N, Katoh A, Takajo Y, Imaizumi T

CIRCULATION   Vol. 110 ( 2 ) page： 141 - 148   2004.7

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DOI： 10.1161/01.CIR.0000134485.30393.63

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Usefulness of magnifying endoscopy in the diagnosis of early gastric cancer

Otsuka, Y; Niwa, Y; Ohmiya, N; Ando, N; Ohashi, A; Hirooka, Y; Goto, H

ENDOSCOPY   Vol. 36 ( 2 ) page： 165 - 169   2004.2

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DOI： 10.1055/s-2004-814184

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PubMed

Novel gene encoding a Ca2+-binding protein and under hexokinase-dependent sugar regulation

Otsuki S, Ikeda A, Sunako T, Muto S, Yazaki J, Nakamura K, Fujii F, Shimbo K, Otsuka Y, Yamamoto K, Sakata K, Sasaki T, Kishimoto N, Kikuchi S, Yamaguchi J

BIOSCIENCE BIOTECHNOLOGY AND BIOCHEMISTRY   Vol. 67 ( 2 ) page： 347 - 353   2003.2

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Vitamin E inhibits lysophosphatidylcholine-induced endothelial dysfunction and platelet activation

Murohara, T; Ikeda, H; Katoh, A; Takajo, Y; Otsuka, Y; Haramaki, N; Imaizumi, T

ANTIOXIDANTS & REDOX SIGNALING   Vol. 4 ( 5 ) page： 791 - 798   2002.10

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Language：Japanese  

DOI： 10.1089/152308602760598945

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PubMed

Platelet GPIIb/IIIa is activated and platelet-leukocyte coaggregates formed in vivo during hemodialysis

Kawabata, K; Nakai, S; Miwa, M; Sugiura, T; Otsuka, Y; Shinzato, T; Hiki, Y; Tomimatsu, I; Ushida, Y; Hosono, F; Maeda, K

NEPHRON   Vol. 90 ( 4 ) page： 391 - 400   2002.4

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DOI： 10.1159/000054726

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PubMed

Changes in Mac-1 and CD14 expression on monocytes and serum soluble CD14 level during push/pull hemodiafiltration

Kawabata, K; Nakai, S; Miwa, M; Sugiura, T; Otsuka, Y; Shinzato, T; Hiki, Y; Tomimatsu, I; Ushida, Y; Hosono, F; Maeda, K

NEPHRON   Vol. 90 ( 3 ) page： 273 - 281   2002.3

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DOI： 10.1159/000049063

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PubMed

Numerical simulation of gas flow through sand core

Maeda, Y; Nomura, H; Otsuka, Y; Tomishige, H; Mori, Y

INTERNATIONAL JOURNAL OF CAST METALS RESEARCH   Vol. 15 ( 4 ) page： 441 - 444   2002

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Imaging observations of midlatitude ionospheric disturbances during the geomagnetic storm of February 12, 2000

Sahai Y, Shiokawa K, Otsuka Y, Ihara C, Ogawa T, Igarashi K, Miyazaki S, Saito A

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 106 ( A11 ) page： 24481 - 24492   2001.11

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DOI： 10.1029/2000JA900169

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CD31 expression on leukocytes is downregulated in vivo during hemodialysis

Kawabata, K; Nakai, S; Miwa, M; Sugiura, T; Otsuka, Y; Shinzato, T; Hiki, N; Tomimatsu, I; Ushida, Y; Hosono, F; Maeda, K

NEPHRON   Vol. 89 ( 2 ) page： 153 - 160   2001.10

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DOI： 10.1159/000046062

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PubMed

The assessment of vertical and lateral invasion of early gastric cancer by magnifying endoscopy

Otsuka, Y; Goto, H; Niwa, Y; Ohmiya, N; Kamiya, K; Sakata, T; Ando, N; Ito, B; Sasaki, Y; Hayakawa, T

GASTROINTESTINAL ENDOSCOPY   Vol. 53 ( 5 ) page： AB212 - AB212   2001.4

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The assessment of vertical and lateral invasion of early gastric cancer by magnifying endoscopy

Otsuka Y, Goto H, Niwa Y, Ohmiya N, Kamiya K, Sakata T, Ando N, Ito B, Sasaki Y, Hayakawa T

GASTROINTESTINAL ENDOSCOPY   Vol. 53 ( 5 ) page： AB212 - AB212   2001.4

Diagnosis of gastrointestinal stromal tumor using endscopic ultrasonography-guided fine needle aspiration biopsy.

Ando, N; Goto, H; Niwa, Y; Hirooka, Y; Koijima, S; Hosoi, T; Shinkai, M; Sakata, T; Otsuka, Y; Hayakawa, T

GASTROINTESTINAL ENDOSCOPY   Vol. 51 ( 4 ) page： AB162 - AB162   2000.4

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DOI： 10.1016/S0016-5107(00)14387-1

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Multi‐Event Analysis of STEVE, SAR Arc, and Red/Green Arc at Subauroral Latitudes Using Data From Ground Optical and Radio Instruments and the Arase and Van Allen Probes Satellites

Rei Sugimura, Kazuo Shiokawa, Yuichi Otsuka, Shin‐ichiro Oyama, Arto Oksanen, Martin Connors, Akira Kadokura, Igor Poddelsky, Nozomu Nishitani, Simon G. Shepherd, J. Michael Ruohoniemi, Charles Smith, Harlan Spence, Geoff Reeves, Herbert O. Funsten, Yoshizumi Miyoshi, Iku Shinohara, Yoshiya Kasahara, Fuminori Tsuchiya, Atsushi Kumamoto, Atsuki Shinbori, Kazushi Asamura, Shoichiro Yokota, Yoichi Kazama, C.‐W. Jun, Shiang‐Yu Wang, Sunny W. Y. Tam, Tzu‐Fang Chang, Bo‐Jhou Wang, Satoshi Kasahara, Kunihiro Keika, Tomoaki Hori, Ayako Matsuoka

Journal of Geophysical Research: Space Physics   Vol. 130 ( 2 )   2025.2

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Abstract

Strong Thermal Emission Velocity Enhancement (STEVE) is a latitudinally narrow, purple‐band emission observed at subauroral latitudes. Stable Auroral Red (SAR) arcs characterized by major red emission, and red/green arcs with both red and green emissions also occur at subauroral latitudes. Characteristics of magnetospheric source plasma and electromagnetic fields of these three types of arcs have not been fully understood because of the limited conjugate observations between magnetosphere and the ground. In this study, we report 11 conjugate observations (2 STEVEs, 7 SAR arcs, and 2 red/green arcs), using all‐sky images obtained at seven ground stations over more than four years from January 2017 to April 2021 and magnetospheric satellites (Arase and Van Allen Probes). We found that, in the inner magnetosphere, the source region of STEVEs and red/green arcs were located outside the plasmasphere, and that of the SAR arc was in the region of spatial overlap between the plasmasphere and ring current region. Electromagnetic waves at frequencies below 1 Hz were observed for STEVEs and red/green arcs. SuperDARN radar data showed a strong westward plasma flow in the ionosphere, especially during STEVE events, whereas the plasma flows associated with SAR arcs and red/green arcs were generally weaker and variable. The STEVE and SAR arc can appear simultaneously at slightly different latitudes and STEVEs and red/green arcs can transform into SAR arcs. These first comprehensive ground‐satellite measurements of three types of subauroral‐latitude auroras increase our understanding on similarlity, differences, and coupling of these auroras in the ionosphere and the magnetosphere.

DOI： 10.1029/2024JA032793

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Midlatitude Plasma Blob-like Structures Along With Super Equatorial Plasma Bubbles During the May 2024 Great Geomagnetic Storm

Wenjie Sun, Guozhu Li, Biqiang Zhao, Shun Rong Zhang, Yuichi Otsuka, Lianhuan Hu, Guofeng Dai, Xiukuan Zhao, Haiyong Xie, Yi Li, Jianfei Liu, Yu Li, Baiqi Ning, Libo Liu, Atsuki Shinbori, Michi Nishioka, Septi Perwitasari

Geophysical Research Letters   Vol. 51 ( 21 )   2024.11

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Publishing type：Research paper (scientific journal)  

Plasma blob is generally a low-latitude phenomenon occurring at the poleward edge of equatorial plasma bubble (EPB) during post-sunset periods. Here we report a case of midlatitude ionospheric plasma blob-like structures occurring along with super EPBs over East Asia around sunrise during the May 2024 great geomagnetic storm. Interestingly, the blob-like structures appeared at both the poleward and westward edges of EPBs, reached up to 40°N magnetic latitudes, and migrated westward several thousand kilometers together with the bubble. The total electron content (TEC) inside the blob-like structures was enhanced by ∼50 TEC units relative to the ambient ionosphere. The blob-like structure at the EPB poleward edge could be partly linked with field-aligned plasma accumulation due to poleward development of bubble. For the blob-like structure at the EPB west side, one possible mechanism is that it was formed and enhanced accompanying the bubble evolution and westward drift.

DOI： 10.1029/2024GL111638

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Monitoring of Ionospheric Variability Using the Low lAtitude Long Range Ionospheric raDar (LARID): Capabilities, Advantages and Limitations

Wenjie Sun, Guozhu Li, Baiqi Ning, Lianhuan Hu, Yuichi Otsuka, Guofeng Dai, Haiyong Xie, Xiukuan Zhao, Yi Li, Atsuki Shinbori, Michi Nishioka, Septi Perwitasari, Chi Wang

Space Weather   Vol. 22 ( 11 )   2024.11

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Publishing type：Research paper (scientific journal)  

A Low lAtitude long Range Ionospheric raDar (LARID), which was built recently at Dongfang (19.2°N, 108.8°E, magnetic latitude 13.9°), Hainan Is., China, is employed to study the ionospheric variability over a large region. The LARID consists of two high frequency (HF) coherent backscatter radars, looking toward east and west, respectively, with a field-of-view covering a wide longitude from Africa to Pacific. Here we report some results by the LARID, with main focus on its capabilities, advantages and limitations in monitoring ionospheric variability. The results show that the LARID can detect various ionospheric variations at low latitudes, including (a) sequentially generated equatorial plasma bubbles over a longitude span of ∼80°, (b) near-range E region irregularities drifting across more than 1,000 km, (c) ionospheric background information retrieved from ground scatters, (d) traveling ionospheric disturbances, and (e) sporadic E structures reflecting HF radio waves at low elevation angles. It is demonstrated that the LARID provides an important tool for investigating different types of ionospheric variability over a broad region, especially over the Indian Ocean and west Pacific, and will contribute significantly to the regional ionospheric weather forecasting.

DOI： 10.1029/2024SW004134

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Scopus

Solar activity dependence for the relationship between nighttime medium-scale traveling ionospheric disturbance and sporadic E (Es) layer activities in summer during 1998–2019 over Japan Reviewed International coauthorship

Veera Kumar Maheswaran, Yuichi Otsuka, James A. Baskaradas, Venkata Ratnam Devanaboyina, Sriram Subramanian, Atsuki Shinbori, Takuya Sori, Michi Nishioka, Septi Perwitasari

Earth, Planets and Space   Vol. 76 ( 1 )   2024.6

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Publishing type：Research paper (scientific journal)  

To investigate solar activity dependence of the coupling between medium-scale traveling ionosphere disturbance (MSTID) and sporadic E (Es) layer, we analyzed the total electron content (TEC) obtained from a Japanese global positioning system (GPS) receivers and ionosonde at Kokubunji (35.7° N, 139.5° E) in Japan during the summer period of May–August from 1998 to 2019. To obtain perturbation TEC caused by MSTIDs, the detrended TEC is calculated by subtracting 1-h moving averages from the measured TEC for each pair of GPS satellite and receiver. The detrended TEC data are mapped on to the geographical coordinates to make detrended 2-D maps with spatial resolution of 0.15° × 0.15° in longitude and latitude. The MSTID activity is defined as a ratio of the standard deviation to the background TEC over Kokubunji in Japan. Day-to-day variations of the MSTID activity during summer nights was compared to Es layer parameters [critical frequency (foEs) and Δfo-b≡foEs-fbE, where fbEs is blanketing frequency] derived from ionosonde station at Kokubunji. We have found that the correlation coefficient between the MSTID activity and foEs(Δfo-b) between 1998 and 2019 is 0.5 3 (0.46) on average, suggesting that there is an electrodynamical coupling between the Es layer and F region could generate nighttime MSTIDs. We also have found that the correlation coefficient positively correlates with solar activity. This finding indicates that in the high solar activity conditions, when the growth rate of Perkins instability is relatively low, generation of the polarization electric fields in the Es layer could play a more important role to grow MSTIDs than in the low solar activity conditions. Graphical Abstract: (Figure presented.)

DOI： 10.1186/s40623-024-02023-6

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Regional Ionospheric Super Bubble Induced by Significant Upward Plasma Drift During the 1 December 2023 Geomagnetic Storm Reviewed International coauthorship

Wenjie Sun, Guozhu Li, Shun Rong Zhang, Lianhuan Hu, Guofeng Dai, Biqiang Zhao, Yuichi Otsuka, Xiukuan Zhao, Haiyong Xie, Yi Li, Baiqi Ning, Libo Liu, Atsuki Shinbori, Michi Nishioka, Septi Perwitasari

Journal of Geophysical Research: Space Physics   Vol. 129 ( 6 )   2024.6

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Publishing type：Research paper (scientific journal)  

An unseasonal equatorial plasma bubble (EPB) event occurred in the East/Southeast Asian sector during the geomagnetic storm on 1 December 2023, causing strong amplitude scintillations from equatorial to middle latitudes. Based on the observations from multiple instruments over a large latitudinal and longitudinal region, the spatial features of the super EPB were investigated. The EPB developed vertically at a fast rising speed ∼470 m/s over the magnetic equator and extended to a very high middle latitude more than 40°N, despite that the storm intensity was not very strong with the minimum SYM-H index −132 nT. In the zonal direction, the super EPB covered over a specific region ∼95–140°E, where the local sunset roughly coincided with southward turning of interplanetary magnetic field (IMF) Bz component. Before the onset of the super EPB, significant upward plasma drift up to ∼110 m/s was observed over the magnetic equator, which could amplify the growth rate of Rayleigh-Taylor instability and lead to the generation of the super EPB. The significant drift was likely caused by eastward penetration electric field (PEF) due to sharp southward turning of IMF Bz. The local time of storm onset and duration of IMF Bz southward turning during the storm main phase may partly determine the onset region and zonal coverage of the EPB.

DOI： 10.1029/2024JA032430

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Statistical Analysis of Low-Latitude Boundary of Polar-Type Medium-Scale Traveling Ionospheric Disturbances Observed by a 630-nm Airglow Imager at Nyrölä, Finland Reviewed International coauthorship

Masaki Sato, Kazuo Shiokawa, Shin ichiro Oyama, Yuichi Otsuka, Atsuki Shinbori, Arto Oksanen

Journal of Geophysical Research: Space Physics   Vol. 129 ( 4 )   2024.4

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Language：English   Publishing type：Research paper (scientific journal)  

Medium-scale traveling ionospheric disturbances (MSTIDs) are one of the ionospheric plasma density structures and are observable through 630-nm airglow images. Previous studies using airglow images at Tromsø (69.6°N, 19.2°E; magnetic latitude: 66.7°N), Norway, reported high-latitude MSTIDs (here we call them as polar-type MSTIDs) whose propagation direction changes associated with auroral brightening and magnetic field disturbances. However, there has been little statistical analysis on the connection of MSTIDs occurring at high and middle latitudes. In this study, we statistically analyzed the MSTIDs observed by an airglow imager at Nyrölä (62.3°N, 25.5°E; magnetic latitude: 59.4°N), Finland, which is located ∼7° south of Tromsø, corresponding to subauroral latitudes. The period analyzed was from 23 January 2017, to 30 September 2021. We found 11 cases of MSTIDs during this period. Eight cases were found to be the polar-type MSTIDs whose motion changes associated with auroral brightening and magnetic field disturbances. We found that 9 cases of MSTID show the low-latitude boundary at 61° ± 2°N for geographic latitude and 58° ± 2°N for magnetic latitude, indicating disconnection between high- and mid-latitude MSTIDs. We also derived occurrence probability, velocity, wavelength, period, wave front direction, and propagation direction of these MSTIDs. The occurrence probability of MSTIDs at Nyrölä is 1.9%, which is much lower than those at high (Tromsoe, more than 50%) and middle (Japan, ∼30%) latitudes. We discuss these MSTID characteristics at subauroral latitudes based on possible difference of generation mechanisms of nighttime MSTIDs at high and middle latitudes.

DOI： 10.1029/2023JA032077

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Performance of the double-thin-shell approach for studying nighttime medium-scale traveling ionospheric disturbances using two dense GNSS observation networks in Japan Reviewed

Weizheng Fu, Yuichi Otsuka, Atsuki Shinbori, Michi Nishioka, Septi Perwitasari

Earth, Planets and Space   Vol. 76 ( 1 )   2024.1

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Publishing type：Research paper (scientific journal)  

Electrodynamic coupling between the ionospheric E and F regions is widely recognized as the underlying mechanism for generating medium-scale traveling ionospheric disturbances (MSTIDs) during nighttime at midlatitudes. Recently, the double-thin-shell approach has proven to be a useful tool for studying the E–F coupling. By using total electron content (TEC) measurements, this approach enables the simultaneous reconstruction of electron density perturbations in both the E and F regions with broad and continuous coverage. However, the current reconstruction performance is limited when using only GPS-TEC measurements from GEONET, a dense network of ground-based Global Navigation Satellite System (GNSS) receivers over Japan. The expansion of available data sources and the integration of multi-GNSS observation data are considered important to enhance the double-thin-shell model. Fortunately, SoftBank Corp., a Japanese telecommunications provider, has recently developed a dense independent GNSS observation network to improve positioning services. In this paper, we analyze the potential of the improved double-thin-shell approach and emphasize the importance of incorporating multi-GNSS observation data from both GEONET and SoftBank networks. The solvability analysis, simulation, and observation results collectively indicate a substantial improvement in the spatiotemporal resolution. Specifically, the longitudinal and latitudinal resolution is improved from 0.15° to 0.1° in the E region, and from 0.5° to 0.3° in the F region. The temporal resolution is also improved from 2 to 1 min. In addition, significant improvements have been achieved in the reconstruction performance, particularly for the E region under complex background conditions. Based on these assessments, we conclude that the incorporation of GEONET and SoftBank GNSS observation data holds significant potential for improving the double-thin-shell model and advancing our understanding of MSTIDs. Graphical abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-023-01956-8

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Scopus

Statistical Analysis of Global and Regional Ionospheric Total Electron Content (TEC) Using Extreme Value Distributions. Reviewed International coauthorship

Suneetha Emmela, Devanaboyina Venkata Ratnam, Yuichi Otsuka, Atsuki Shinbori, Takuya Sori, Michi Nishioka, Septi Perwitasari

IEEE Trans. Geosci. Remote. Sens.   Vol. 62   page： 1 - 8   2024

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DOI： 10.1109/TGRS.2023.3338513

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Statistical Analysis of the Horizontal Phase Velocity Distribution of Atmospheric Gravity Waves and Medium-Scale Traveling Ionospheric Disturbances in Airglow Images Over Sata (31.0°N, 130.7°E), Japan Reviewed

Takuma Tsuboi, Kazuo Shiokawa, Yuichi Otsuka, Hatsuki Fujinami, Takuji Nakamura

Journal of Geophysical Research: Space Physics   Vol. 128 ( 12 )   2023.12

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DOI： https://doi.org/10.1029/2023JA031600

DOI： https://doi.org/10.1029/2023JA031600

Machine learning approach for ionospheric scintillation prediction on ROTI parameter over the African region during solar cycle 24 Reviewed

Stephen Tete, Yuichi Otsuka, Waheed K. Zahra, Ayman Mahrous

Advances in Space Research     2023.12

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DOI： https://doi.org/10.1016/j.asr.2023.12.026

DOI： https://doi.org/10.1016/j.asr.2023.12.026

New aspects of the upper atmospheric disturbances caused by the explosive eruption of the 2022 Hunga Tonga–Hunga Ha’apai volcano Invited Reviewed

A. Shinbori, Y. Otsuka, T. Sori, M. Nishioka, S. Perwitasari, T. T. Tsuda, N. Nishitani, A. Kumamoto, F. Tsuchiya, S. Matsuda, Y. Kasahara, A. Matsuoka, S. Nakamura, Y. Miyoshi, I. Shinohara

Earth Planets Space   Vol. 75 ( 1 ) page： 175   2023.11

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DOI： 10.1186/s40623-023-01930-4

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Periodic oscillations of Doppler frequency excited by the traveling ionospheric disturbances associated with the Tonga eruption in 2022 Reviewed

Hiroyuki Nakata, Keisuke Hosokawa, Susumu Saito, Yuichi Otsuka, Ichoro Tomizawa

Earth, Planets and Space   Vol. 75 ( 1 )   2023.10

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Abstract

The explosive eruption of the Hunga Tonga-Hunga Ha’apai volcano on 15 January 2022 generated atmospheric waves traveling around the Earth, which caused ionospheric disturbances on various spatio-temporal scales. A HF Doppler sounding system in Japan detected characteristic ionospheric disturbances showing periodic oscillations in the Doppler frequency with a period of ~ 4 min. In this study, such periodic oscillations were examined by comparing Doppler frequency data with Total Electron Content data obtained by Global Navigation Satellite System. The observed periodic oscillations in the Doppler frequency were characterized by a sawtooth or S-letter shaped variation, implying the passage of the traveling ionospheric disturbances through the reflection points of the HF Doppler sounding system. It was also found that the periodic oscillations occurred prior to the arrival of the tropospheric Lamb wave excited by the Tonga eruption. From the total electron content data, the traveling ionospheric disturbances causing the periodic oscillations were excited by the tropospheric Lamb waves at the conjugate point in the southern hemisphere, namely, the electric field perturbations due to the Lamb waves in the southern hemisphere mapped onto the sensing area of the HF Doppler sounding system in the northern hemisphere along the magnetic field lines. The periodic oscillations were observed only in the path between Chofu transmitter and Sarobetsu receiver, whose the radio propagation path is almost aligned in the north–south direction. This suggests that the traveling ionospheric disturbance has a structure elongating in the meridional direction. The variation in the Doppler frequency was reproduced by using a simple model of the propagation of the traveling ionospheric disturbances and the resultant motion of the reflection point. As a result, the vertical motion of the reflection point associated with the periodic oscillations was estimated to be about 1 km. It is known that 4-min period variations are sometimes observed in association with earthquakes, which is due to resonances of acoustic mode waves propagating between the ground and the lower ionosphere. Therefore, a similar resonance structure in the southern hemisphere is a plausible source of the traveling ionospheric disturbances detected in the northern hemisphere.

Graphical Abstract

DOI： 10.1186/s40623-023-01914-4

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Other Link： https://link.springer.com/article/10.1186/s40623-023-01914-4/fulltext.html

Statistical Analysis of the Horizontal Phase Velocity Distribution of Atmospheric Gravity Waves and Medium-Scale Traveling Ionospheric Disturbances in Airglow Images Over Darwin (12.4°S, 131.0°E) Reviewed International coauthorship

Takuma Tsuboi, Kazuo Shiokawa, Yuichi Otsuka, Hatsuki Fujinami, Takuji Nakamura, David Neudegg

Journal of Geophysical Research: Space Physics   Vol. 128 ( 8 )   2023.8

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DOI： https://doi.org/10.1029/2022JA030769

DOI： https://doi.org/10.1029/2022JA030769

Other Link： https://doi.org/10.1029/2022JA030769

Spatio-Temporal Characteristics of IPDP-Type EMIC Waves on April 19, 2017: Implications for Loss of Relativistic Electrons in the Outer Belt Reviewed

Asuka Hirai, Fuminori Tsuchiya, Takahiro Obara, Yuto Katoh, Yoshizumi Miyoshi, Kazuo Shiokawa, Yasumasa Kasaba, Hiroaki Misawa, Chae Woo Jun, Satoshi Kurita, Martin G. Connors, Aaron T. Hendry, Atsuki Shinbori, Yuichi Otsuka, Takuya Tsugawa, Michi Nishioka, Septi Perwitasari, Jerry W. Manweiler

Journal of Geophysical Research: Space Physics   Vol. 128 ( 8 )   2023.8

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To understand the mechanism of the increased frequency of intervals of pulsations of diminishing periods (IPDPs), we analyzed IPDP-type electromagnetic ion cyclotron (EMIC) waves that occurred on 19 April 2017, using ground and satellite observations. Observations by low-altitude satellites and ground-based magnetometers indicate that the increased IPDP frequency is caused by an inward (i.e., Earthward) shift of the EMIC wave source region. The EMIC wave source region moves inward along the mid-latitude trough, which we used as a proxy for the plasmapause location. A statistical analysis shows that increases in the IPDP frequency showed a positive correlation with polar cap potentials. These results suggest an enhanced convection electric field causes an inward shift of the source region. The inward shift of the source region allows EMIC waves to scatter relativistic electrons over a wide range of radial distances during the IPDP event. This mechanism suggests that IPDP-type EMIC waves are more likely to scatter relativistic electrons than other EMIC waves. We also show that the decreased phase-space density of relativistic electrons in the outer radiation belt is consistent with the extent of the source region and the resonant energy of EMIC waves, implying a possible contribution of EMIC waves to outer radiation belt loss during the main phase of geomagnetic storms.

DOI： 10.1029/2023JA031479

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First Detection of Midlatitude Plasma Bubble by SuperDARN During a Geomagnetic Storm on May 27 and 28, 2017 Reviewed

Takuya Sori, Atsuki Shinbori, Yuichi Otsuka, Michi Nishioka, Septi Perwitasari, Nozomu Nishitani

Journal of Geophysical Research: Space Physics   Vol. 128 ( 4 )   2023.4

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We used the global navigation satellite system-total electron content (TEC) and Super Dual Auroral Radar Network (SuperDARN) radar to elucidate the characteristics of the plasma bubble extending to midlatitudes over North America during a geomagnetic storm on 27 and 28 May 2017. To identify plasma bubbles, we analyzed the rate of the TEC index (ROTI), which is a good indicator of the occurrence of plasma bubbles. The enhanced ROTI region expanded up to 50°N (geomagnetic latitude), and the upper limit of this region coincided with the equatorward wall of the midlatitude trough. Two-dimensional ROTI maps show that the enhanced midlatitude ROTI region moved westward at a bulk speed of approximately 310 m/s. The Fort Hays East SuperDARN radar also detected the radar echo, showing the existence of plasma density irregularities at ∼10-m scales within the midlatitude plasma bubble. The radar echo had a westward velocity of ∼300 m/s, which is almost consistent with the westward motion of the enhanced midlatitude ROTI region. We deduced that the westward propagation of the plasma bubble could be caused by a poleward sub-auroral polarization stream electric field. The observed radar echo overlapping with the enhanced ROTI region had a narrower spectral width (<50 m/s) than that of the auroral activity. This feature resembled that of the type 1 echo, although the Doppler velocity was smaller than the ion acoustic speed at the F-region height. This is the first case in which plasma density irregularities within plasma bubbles were captured by the SuperDARN radar.

DOI： 10.1029/2022JA031157

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Statistical analysis for EUV dynamic spectra and their impact on the ionosphere during solar flares Reviewed

Shohei Nishimoto, Kyoko Watanabe, Hidekatsu Jin, Toshiki Kawai, Shinsuke Imada, Tomoko Kawate, Yuichi Otsuka, Atsuki Shinbori, Takuya Tsugawa, Michi Nishioka

Earth, Planets and Space   Vol. 75 ( 1 )   2023.3

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Abstract

The X-rays and extreme ultraviolet (EUV) emitted during solar flares can rapidly change the physical composition of Earth’s ionosphere, causing space weather phenomena. It is important to develop an accurate understanding of solar flare emission spectra to understand how it affects the ionosphere. We reproduced the entire solar flare emission spectrum using an empirical model and physics-based model, and input it into the Earth’s atmospheric model, GAIA to calculate the total electron content (TEC) enhancement due to solar flare emission. We compared the statistics of nine solar flare events and calculated the TEC enhancements with the corresponding observed data. The model used in this study was able to estimate the TEC enhancement due to solar flare emission with a correlation coefficient greater than 0.9. The results of this study indicate that the TEC enhancement due to solar flare emission is determined by soft X-ray and EUV emission with wavelengths shorter than 35 nm. The TEC enhancement is found to be largely due to the change in the soft X-ray emission and EUV line emissions with wavelengths, such as Fe XVII 10.08 nm, Fe XIX 10.85 nm and He II 30.38 nm.

Graphical Abstract

DOI： 10.1186/s40623-023-01788-6

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Other Link： https://link.springer.com/article/10.1186/s40623-023-01788-6/fulltext.html

Dependence of Ionospheric Responses on Solar Wind Dynamic Pressure During Geomagnetic Storms Using Global Long-Term GNSS-TEC Data Reviewed

Takuya Sori, Atsuki Shinbori, Yuichi Otsuka, Michi Nishioka, Septi Perwitasari

Journal of Geophysical Research: Space Physics   Vol. 128 ( 3 )   2023.3

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To elucidate the effect of solar wind dynamic pressure on the activities of ionospheric plasma irregularities in both high-latitude and equatorial regions, we analyzed the long-term global navigation satellite system (GNSS) total electron content (TEC) data from 2000 to 2018 and performed a superposed epoch analysis of the GNSS rate of the TEC index (ROTI), solar wind, interplanetary magnetic field (IMF), and geomagnetic index. We found that during the main phase of geomagnetic storms, the activities of ionospheric plasma irregularities were considerably enhanced in both high-latitude and equatorial regions under high-pressure conditions, and the equatorward edge of the auroral oval moved to lower latitudes. The poleward edge of the enhanced low-latitude ROTI region (occurrence region of the plasma bubbles) in the evening sector extended to higher latitudes under high-pressure conditions. During the recovery phase of geomagnetic storms, the plasma bubble occurrence in the dusk sector was suppressed under high-pressure conditions. Our results suggest that the high-latitude convection electric field and the penetration and disturbance dynamo electric fields at low latitudes become stronger during geomagnetic storms when the solar wind dynamic pressure is enhanced. This is because the conversion from solar wind energy to electromagnetic energy in the magnetosphere is enhanced by the formation of a high plasma pressure area in the high-latitude cusp and mantle region. Therefore, not only the southward IMF but also solar wind dynamic pressure are important factors for varying global ionospheric responses during geomagnetic storms.

DOI： 10.1029/2022JA030913

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Multi-Event Analysis of Magnetosphere-Ionosphere Coupling of Nighttime Medium-Scale Traveling Ionospheric Disturbances From the Ground and the Arase Satellite Reviewed International coauthorship

K. Kawai, K. Shiokawa, Y. Otsuka, S. Oyama, M. G. Connors, Y. Kasahara, Y. Kasaba, S. Nakamura, F. Tsuchiya, A. Kumamoto, A. Shinbori, A. Matsuoka, I. Shinohara, Y. Miyoshi

Journal of Geophysical Research: Space Physics   Vol. 128 ( 2 )   2023.2

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Kawai et al. (2021) reported the first ground-satellite conjugate observation of nighttime medium-scale traveling ionospheric disturbances (MSTIDs), by analyzing measurements from an airglow imager at Gakona (geographic latitude: 62.39°N, geographic longitude: 214.78°E, magnetic latitude: 63.60°N) and the Arase satellite in the magnetosphere on 3 November 2018. The Arase satellite observed variations in both the polarization electric field and the electron density as the Arase footprint passed through the MSTID structures in the ionosphere. In this study, we investigated whether these electric field and density variations associated with MSTIDs at subauroral latitudes are always observed by Arase in the magnetosphere. We used three airglow imagers installed at Gakona, Athabasca (geographic latitude: 54.60°N, geographic longitude: 246.36°E, magnetic latitude: 61.10°N), and Kapuskasing (geographic latitude: 49.39°N, geographic longitude: 277.81°E, magnetic latitude: 58.70°N) and the Arase satellite. We found eight observations of MSTIDs conjugate with Arase. They indicate that electric field and density variations associated with MSTIDs are not always observed in the magnetosphere. These variations tend to be observed in the magnetosphere during geomagnetically quiet times and when the amplitude of the MSTID is large. We categorized the MSTIDs into those caused by plasma instabilities and gravity waves and found that the electric field and density variations can be observed in the magnetosphere for both types of MSTIDs.

DOI： 10.1029/2022JA030542

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A Statistical Study of Longitudinal Extent of Pc1 Pulsations Using Seven PWING Ground Stations at Subauroral Latitudes Reviewed International coauthorship

Jie Liu, Kazuo Shiokawa, Shin‐Ichiro Oyama, Yuichi Otsuka, Chae‐Woo Jun, Masahito Nosé, Tsutomu Nagatsuma, Kaori Sakaguchi, Akira Kadokura, Mitsunori Ozaki, Martin Connors, Dmitry Baishev, Nozomu Nishitani, Alexey Oinats, Vladimir Kurkin, Tero Raita

Journal of Geophysical Research: Space Physics   Vol. 128 ( 1 )   2023.1

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DOI： 10.1029/2021JA029987

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JA029987

An Extreme Value Analysis of Long-Term GNSS Ionospheric Total Electron Content Data Observed at Japan Grid Point Location (34.95° N and 134.05° E). Reviewed International coauthorship

Suneetha Emmela, Devanaboyina Venkata Ratnam, Yuichi Otsuka, Atsuki Shinbori, Takuya Sori, Michi Nishioka, Septi Perwitasari

IEEE Geoscience and Remote Sensing Letters   Vol. 20   page： 1 - 5   2023

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DOI： 10.1109/LGRS.2023.3292593

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Geomagnetic conjugacy of plasma bubbles extending to mid-latitudes during a geomagnetic storm on March 1, 2013

Takuya Sori, Yuichi Otsuka, Atsuki Shinbori, Michi Nishioka, Septi Perwitasari

Earth, Planets and Space   Vol. 74 ( 1 )   2022.8

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This study, for the first time, reports the geomagnetically conjugate structure of a plasma bubble extending to the mid-latitudes and the asymmetrical structure of the decay of the plasma bubble during a geomagnetic storm. We investigated the temporal and spatial variations of plasma bubbles in the Asian sector during a geomagnetic storm on March 1, 2013, using Global Navigation Satellite System-total electron content data with high spatiotemporal resolutions. The first important point of our data analysis results is that the plasma bubble extended from the equator to the mid-latitudes with geomagnetic conjugacy along the magnetic field lines. The total electron content data showed that the plasma bubbles appeared in the equatorial regions near 150° E after sunset during the main phase of the geomagnetic storm. From ionosonde data over both Japan and Australia, they suggest that a large eastward electric field existed in the Asian sector. Finally, the plasma bubbles extended up to the mid-latitudes (~ 43° geomagnetic latitude) in both hemispheres, maintaining geomagnetic conjugacy. The second point is that the mid-latitude plasma bubble disappeared 1–2 h earlier in the northern hemisphere than in the southern hemisphere at close to midnight. In the northern hemisphere, the ionospheric virtual height decreased near midnight, followed by a rapid decrease in the total electron content and a rapid increase in the ionospheric virtual height. These results imply that the mid-latitude plasma bubble disappeared as the background plasma density decreased after midnight due to the recombination resulting from the descent of the F layer. Therefore, we can conclude that mid-latitude plasma bubbles can be asymmetric between the northern and southern hemispheres because of the rapid decay of plasma bubbles in one of the hemispheres. Graphical Abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-022-01682-7

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A confirmation of vertical acoustic resonance and field-aligned current generation just after the 2022 Hunga Tonga Hunga Ha’apai volcanic eruption

Toshihiko Iyemori, Michi Nishioka, Yuichi Otsuka, Atsuki Shinbori

Earth, Planets and Space   Vol. 74 ( 1 )   2022.6

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A strong volcanic eruption caused a clear vertical acoustic resonance between the sea surface and the thermosphere. Its effects are observed as geomagnetic and GPS-TEC oscillations near the volcano and its geomagnetic conjugate area. The geomagnetic oscillations are observed at Apia and Honolulu geomagnetic observatories with amplitude of about 2 nT and 0.2 nT, respectively. The volcanic eruption started around 04:14 UT on January 15, 2022. The oscillations appeared at 04:21UT at Apia, Samoa, only about 7 min after the start of eruption. Because the distance between the volcano and Apia is about 841 km, it takes about 40 min for a sound wave to propagate from the volcano to Apia. Therefore, it is more plausible to assume that the magnetic oscillation observed at Apia about 7 min after the eruption is caused by the sound waves propagated vertically upward to the ionosphere and generated an electric current. The coherent appearance of geomagnetic oscillation at Honolulu located near the geomagnetic conjugate point of the volcano strongly support the idea that the ionospheric current generated over the volcano diverted as a field-aligned current which flew to the opposite hemisphere and caused the geomagnetic oscillation at Honolulu. The earliest start of GPS-TEC oscillation was around 04:15UT near the volcanic eruption, and it was around 04:20 UT at KOKV station in Hawaii. The time-lag of the TEC variations between Samoa and Hawaii obtained by a cross-correlation analysis is 4.5 min or 8.5 min. These time differences are much smaller than the travel time of the seismic waves from the volcano to Hawaii islands. Therefore, it is suggested that the electric field transmitted along geomagnetic field caused the TEC variation observed over Hawaii Islands. A sawtooth waveform of geomagnetic oscillation observed at Apia and Honolulu is analyzed and a possible generation mechanism is discussed. Graphical Abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-022-01653-y

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On the Role of E-F Region Coupling in the Generation of Nighttime MSTIDs During Summer and Equinox: Case Studies Over Northern Germany

Mani Sivakandan, Carlos Martinis, Yuichi Otsuka, Jorge L. Chau, Jessica Norrell, Jens Mielich, J. Federico Conte, Claudia Stolle, J. Rodríguez-Zuluaga, Atsuki Shinbori, Michi Nishioka, Takuya Tsugawa

Journal of Geophysical Research: Space Physics   Vol. 127 ( 5 )   2022.5

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Simultaneous observations from a 630 nm all-sky airglow imager, GNSS-TEC receivers, and an ionosonde are used to investigate the role of E- and F-region coupling on the generation of medium-scale traveling ionospheric disturbances (MSTIDs). The primary observations are OI 630 nm airglow images taken by an all-sky imager in Kühlungsborn (54.07°N; 11.46°E, 53.79°N Mlat.), a site in northern Germany. Out of 226 nights of observations, MSTIDs were found only in 18 nights, demonstrating the low occurrence rate over Kühlungsborn. We focused on four MSTIDs events: two during the vernal equinox and two during summer. Coincident measurements of detrended GNSS-TEC supported the presence of MSTIDs during the selected events, and simultaneous observations from the ionosonde in Juliusruh (54.60°N, 13.4°E, 54.02°N Mlat.) showed sporadic-E (Es) layer and spread-F activity in the E- and F-region, respectively. We observed the onset of the observed MSTIDs to be around the 15°–20°E longitude and 60–45°N latitude belts. Additionally, we found that in each case, the onset of MSTIDs coincides with the presence of an Es layer with sporadic-E trace is observed (foEs) exceeding 4 MHz. This suggests that an Es layer with foEs ≥ 4MHz was a source of the generation of these MSTIDs. Altitude of the Es layer could be another important factor in generating MSTIDs. The Es layer should exist at an altitude where Hall conductivity is large, as happened in the present study.

DOI： 10.1029/2021JA030159

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Generation Mechanisms of Plasma Density Irregularity in the Equatorial Ionosphere During a Geomagnetic Storm on 21–22 December 2014

T. Sori, A. Shinbori, Y. Otsuka, T. Tsugawa, M. Nishioka, A. Yoshikawa

Journal of Geophysical Research: Space Physics   Vol. 127 ( 5 )   2022.5

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The equatorial ionosphere endured plasma density irregularities during a geomagnetic storm on 21–22 December 2014. To understand the underlying mechanism, we analyzed the rate of the total electron content change (ROTI) data obtained from a global navigation satellite system, along with solar wind, interplanetary magnetic field (IMF), geomagnetic indices, Jicamarca incoherent scatter radar, and magnetometer data. The results indicate that the ROTI enhancement related to plasma density irregularities (plasma bubbles) occurred three times in the equatorial and low latitude regions of the American sector during the geomagnetic storm. The first, second, and third enhancements which have a longitudinal extent of ∼20° appeared in the post-sunset, pre-midnight, and post-midnight sectors, respectively. The second enhancement occurred during the recovery phase of the storm-time substorm even though the IMF remained southward. During this period, the direction of the dayside equatorial electrojet (EEJ) changed from eastward to westward, while the nightside upward plasma velocity at Jicamarca increased to 28.8 m/s. The response of the EEJ and upward ion drift implies that the westward and eastward electric fields were intensified on the dayside and nightside, respectively. Therefore, these results suggest that an over-shielding electric field penetrates the dayside/nightside equator simultaneously in association with a substorm recovery phase, and that the electric field generates plasma bubbles by the Rayleigh-Taylor instability mechanism. Plasma bubbles induced by the penetration of an over-shielding electric field due to substorm activity have not previously been reported.

DOI： 10.1029/2021JA030240

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Propagation characteristics of sporadic E and medium-scale traveling ionospheric disturbances (MSTIDs): statistics using HF Doppler and GPS-TEC data in Japan

Ryo Matsushima, Keisuke Hosokawa, Jun Sakai, Yuichi Otsuka, Mitsumu K. Ejiri, Michi Nishioka, Takuya Tsugawa

EARTH PLANETS AND SPACE   Vol. 74 ( 1 )   2022.4

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We carried out a statistical analysis of the propagation characteristics of Es and Medium-Scale Traveling Ionospheric Disturbances (MSTIDs) by combining data of HF Doppler (HFD) sounder and Total Electron Content (TEC) obtained from the GPS receivers of GEONET (GPS-TEC) for 4 years from 2014 to 2017. We made use of Es reflection data from the HFD receivers in Sugito, Saitama (36.0 degrees N, 139.7 degrees E), Fujisawa, Kanagawa (35.3 degrees N, 139.5 degrees E), and Sugadaira, Nagano (36.4 degrees N, 138.3 degrees E) in Japan. By using this triangle observation, we succeeded in deriving the horizontal speed and direction of the motion of Es. In addition, we estimated the phase velocity of MSTIDs observed in the simultaneously obtained maps of GPS-TEC with the same triangle observation procedure. The speeds of Es and MSTIDs were commonly less than 100 m/s in most cases and their propagation direction was predominantly southwestward. This result is consistent with the statistical characteristics of nighttime MSTIDs observed in the previous studies. More importantly, good correspondence between the propagation characteristics of the two phenomena at two different altitudes confirms that Es and MSTIDs move in tandem with each other, further suggesting that Es in the E region plays an important role in the generation and propagation of MSTIDs in the F region.

DOI： 10.1186/s40623-022-01616-3

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Statistical Behavior of Large-Scale Ionospheric Disturbances From High Latitudes to Mid-Latitudes During Geomagnetic Storms Using 20-yr GNSS-TEC Data: Dependence on Season and Storm Intensity

Atsuki Shinbori, Yuichi Otsuka, Takuya Sori, Takuya Tsugawa, Michi Nishioka

Journal of Geophysical Research: Space Physics   Vol. 127 ( 1 )   2022.1

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To establish the statistical behavior of ionospheric TEC variations from high latitudes to mid-latitudes during the main and recovery phases of geomagnetic storms, we conducted a superposed epoch analysis of interplanetary magnetic field, solar wind, geomagnetic indices (AE and SYM-H), and global navigation satellite system (GNSS)-total electron content (TEC) data for 20 yr (2000–2019). In this study, we identify 663 geomagnetic storm events with the minimum SYM-H value of less than −40 nT and investigate the characteristics of the TEC variations for the weak (−60 ≤ SYM-Hmin < −40 nT), moderate (−100 ≤ SYM-Hmin < −60 nT), and strong (−150 ≤ SYM-Hmin < −100 nT) geomagnetic storms. The main results obtained from the present study are as follows: (a) The TEC enhancements related to the tongue of ionization (TOI), auroral oval, and storm-enhanced density (SED) plume are more dominant in winter than in summer during the main phase of geomagnetic storms. (b) The structure of the mid-latitude trough in the nighttime sector becomes unclear in winter. (c) The TEC depletion at auroral and mid-latitudes (40°–70° GMLAT: geomagnetic latitude) starts to appear in the morning sector (8–10 hr GMLT: geomagnetic local time) during the main phase of geomagnetic storms and the decreased region extends in the lower latitude and GMLT directions with time. The negative storm activity tends to be enhanced significantly as the storm intensity becomes larger. The activity of the TEC depletion is dominant in summer than in winter, which is agreement with the classical ionospheric storm scenario.

DOI： 10.1029/2021JA029687

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Propagation direction analyses of medium‐scale traveling ionospheric disturbances observed over North America with GPS‐TEC perturbation maps by three‐dimensional spectral analysis method

Septi Perwitasari, Takuji Nakamura, Takuya Tsugawa, Michi Nishioka, Yoshihiro Tomikawa, Mitsumu K. Ejiri, Masaru Kogure, Yuichi Otsuka, Atsuki Shinbori, Hidekatsu Jin, Chihiro Tao

Journal of Geophysical Research: Space Physics   Vol. 127 ( 1 )   2022.1

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DOI： 10.1029/2020JA028791

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Propagation Mechanism of Medium Wave Broadcasting Waves Observed by the Arase Satellite: Hectometric Line Spectra

Kozo Hashimoto, Atsuki Shinbori, Yuichi Otsuka, Fuminori Tsuchiya, Atsushi Kumamoto, Yoshiya Kasahara, Ayako Matsuoka, Isamu Nagano, Yoshizumi Miyoshi, Tatsuhiro Yokoyama

Journal of Geophysical Research: Space Physics   Vol. 126 ( 11 )   2021.11

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A new type of terrestrial line spectra found by the Arase satellite was reported in 2018. These spectra are called “hectometric line spectra (HLS)”. They primarily consist of constant frequency narrowband components at frequencies between 525 kHz and 1,700 kHz, which originate and are sometimes amplified from AM broadcasting waves. In addition to these, other generated emissions are observed. Entrances and the mode conversion of the AM broadcasting waves into the equatorial and low-latitude ionosphere with plasma density depletion called plasma bubbles are observed. Electron density profiles and equatorial plasma bubbles are examined through Global Positioning System (GPS)-Total electron content (TEC) analyses. As a result, the Arase satellite which observed the HLS passed through the TEC depression region near the equatorial ionosphere associated with plasma bubbles. The scenario based on the mode conversion of the L-O mode to the Z mode and vice versa was confirmed with observations and GPS-TEC analyses. Another entrance when foF2 estimated from TEC is lower than 1,000 kHz instead of a plasma bubble is also found. No mode conversion is necessary, then.

DOI： 10.1029/2021JA029813

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Isolated Proton Aurora Driven by EMIC Pc1 Wave: PWING, Swarm, and NOAA POES Multi-Instrument Observations

Hyangpyo Kim, Kazuo Shiokawa, Jaeheung Park, Yoshizumi Miyoshi, Yukinaga Miyashita, Claudia Stolle, Hyunju Kim Connor, Junga Hwang, Stephan Buchert, Hyuck Jin Kwon, Satoko Nakamura, Kohki Nakamura, Shin Ichiro Oyama, Yuichi Otsuka, Tsutomu Nagatsuma, Kaori Sakaguchi

Geophysical Research Letters   Vol. 48 ( 18 )   2021.9

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We report the concurrent observations of F-region plasma changes and field-aligned currents (FACs) above isolated proton auroras (IPAs) associated with electromagnetic ion cyclotron Pc1 waves. Key events on March 19, 2020 and September 12, 2018 show that ground magnetometers and all-sky imagers detected concurrent Pc1 wave and IPA, during which NOAA POES observed precipitating energetic protons. In the ionospheric F-layer above the IPA zone, the Swarm satellites observed transverse Pc1 waves, which span wider latitudes than IPA. Around IPA, Swarm also detected the bipolar FAC and localized plasma density enhancement, which is occasionally surrounded by wide/shallow depletion. This indicates that wave-induced proton precipitation contributes to the energy transfer from the magnetosphere to the ionosphere.

DOI： 10.1029/2021GL095090

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Plasma depletions lasting into daytime during the recovery phase of a geomagnetic storm in May 2017: Analysis and simulation of GPS total electron content observations

Yuichi Otsuka, Atsuki Shinbori, Takuya Sori, Takuya Tsugawa, Michi Nishioka, Joseph D. Huba

Earth and Planetary Physics   Vol. 5 ( 5 ) page： 427 - 434   2021.9

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This paper reports that plasma density depletions appearing at middle latitudes near sunrise survived until afternoon on 29 May 2017 during the recovery phase of a geomagnetic storm. By analyzing GPS data collected in Japan, we investigate temporal variations in the horizontal two-dimensional distribution of total electron content (TEC) during the geomagnetic storm. The SYM-H index reached −142 nT around 08 UT on 28 May 2017. TEC depletions extending up to approximately 38°N along the meridional direction appeared over Japan around 05 LT (LT = UT + 9 hours) on 29 May 2017, when TEC rapidly increased at sunrise due to the solar extreme ultraviolet (EUV) radiation. The TEC depletions appeared sequentially over Japan for approximately 8 hours in sunlit conditions. At 06 LT on 29 May, when the plasma depletions first appeared over Japan, the background TEC was enhanced to approximately 17 TECU, and then decreased to approximately 80% of the TEC typical of magnetically quiet conditions. We conclude that this temporal variation of background plasma density in the ionosphere was responsible for the persistence of these plasma depletions for so long in daytime. By using the Naval Research Laboratory: Sami2 is Another Model of the Ionosphere (SAMI2), we have evaluated how plasma production and ambipolar diffusion along the magnetic field may affect the rate of plasma depletion disappearance. Simulation shows that the plasma density increases at the time of plasma depletion appearance; subsequent decreases in the plasma density appear to be responsible for the long-lasting persistence of plasma depletions during daytime. The plasma density depletion in the top side ionosphere is not filled by the plasma generated by the solar EUV productions because plasma production occurs mainly at the bottom side of the ionosphere.

DOI： 10.26464/epp2021046

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Multi-instrument study of longitudinal wave structures for plasma bubble seeding in the equatorial ionosphere

H. Takahashi, P. Essien, C. A.O.B. Figueiredo, C. M. Wrasse, D. Barros, M. A. Abdu, Y. Otsuka, K. Shiokawa, GuoZhu Li

Earth and Planetary Physics   Vol. 5 ( 5 ) page： 368 - 377   2021.9

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Large Scale Wave Structures (LSWS) in the equatorial ionospheric F-region were observed by measuring spatial and temporal variations within detrended total electron content (dTEC) data obtained by ground-based GNSS receivers over the South American continent. By using dTEC-maps, we have been able to produce, for the first-time, two-dimensional representations of LSWS. During the period from September to December, the LSWS frequently occurred starting a few hours prior to Equatorial Plasma Bubble (EPB) development. From 17 events of LSWS observed in 2014 and 2015, wave characteristics were obtained: the observed wavelengths, periods, and the phase speeds are respectively, ~900 km, ~41 min and ~399 m/s
the waves propagated from the northeast to southeast. In some cases the front of the oscillation was meridionally aligned, extending to more than 1600 km, the first time such large extension of the wavefront has been reported. From F-layer bottom height oscillation data, measured by ionosonde, LSWS exhibit two different vertical phase propagation modes, in-phase and downward phase. The former mode indicates the presence of a polarization electric field in the F-layer bottom side
the latter suggests propagation of atmospheric gravity waves. The presence of LSWS near the solar terminator, followed by the development of EPBs, suggests that the upwelling of the F-layer bottom height produces a condition favorable to the development of Rayleigh–Taylor instability.

DOI： 10.26464/epp2021047

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First Simultaneous Observation of a Night Time Medium‐Scale Traveling Ionospheric Disturbance From the Ground and a Magnetospheric Satellite

K. Kawai, K. Shiokawa, Y. Otsuka, S. Oyama, Y. Kasaba, Y. Kasahara, F. Tsuchiya, A. Kumamoto, S. Nakamura, A. Matsuoka, S. Imajo, Y. Kazama, S.‐Y. Wang, S. W. Y. Tam, T. F. Chang, B. J. Wang, K. Asamura, S. Kasahara, S. Yokota, K. Keika, T. Hori, Y. Miyoshi, C. Jun, M. Shoji, I. Shinohara

Journal of Geophysical Research: Space Physics   Vol. 126 ( 9 )   2021.9

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DOI： 10.1029/2020JA029086

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Coupled investigations of ionosphere variations over European and Japanese regions: observations, comparative analysis, and validation of models and facilities

Sergii V. Panasenko, Dmytro V. Kotov, Yuichi Otsuka, Mamoru Yamamoto, Hiroyuki Hashiguchi, Philip G. Richards, Vladimir Truhlik, Oleksandr V. Bogomaz, Maryna O. Shulha, Taras G. Zhivolup, Igor F. Domnin

Progress in Earth and Planetary Science   Vol. 8 ( 1 )   2021.8

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

This paper presents the results of a coordinated measurement campaign with ground based and satellite observations over European and Japanese regions during September 5–6, 2017. Two incoherent scatter radars, two satellite missions, International Reference Ionosphere (IRI-2016) empirical model, and Field Line Interhemispheric Plasma (FLIP) physical model were employed to examine the regular behavior of the F2-layer peak height and density and the topside ionosphere electron density, electron, and ion temperatures as well as traveling ionospheric disturbances (TIDs). The daily ionospheric variations over Kharkiv and Shigaraki exhibited similar behavior qualitatively and quantitatively. The results show that none of the empirical IRI-2016 models of F2-layer peak height, topside electron density, and temperature can be preferred for predicting the key qualitative features of variations in ionospheric plasma parameters over Kharkiv and Shigaraki. The likely reason is rapid day to day changes in solar activity and series of moderate enhancements of magnetic activity occurring in the observation period and preceding days. Compared with IRI-2016 model, the FLIP physical model was shown to provide the best agreement with the observations when constrained to follow the observed diurnal variations of F2-layer peak height both over Europe and Japan. This paper presents the first direct comparison of the mid-latitude electron density measured by the Swarm satellite with incoherent scatter radar data and it confirms the high quality of the space-borne data. For the first time, evidence of the possible need to increase the neutral hydrogen density in NRLMSISE-00 model by at least a factor of 2 was obtained for the Asian longitudinal sector. The TIDs, which have predominant periods of about 50 min over Europe and 80 min over Japan, were detected, likely caused by passage of the solar terminator. Such a difference in the periods could indicate regional features and is the topic for further research.

DOI： 10.1186/s40645-021-00441-8

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Other Link： https://link.springer.com/article/10.1186/s40645-021-00441-8/fulltext.html

PSTEP: project for solar–terrestrial environment prediction

Kanya Kusano, Kiyoshi Ichimoto, Mamoru Ishii, Yoshizumi Miyoshi, Shigeo Yoden, Hideharu Akiyoshi, Ayumi Asai, Yusuke Ebihara, Hitoshi Fujiwara, Tada-Nori Goto, Yoichiro Hanaoka, Hisashi Hayakawa, Keisuke Hosokawa, Hideyuki Hotta, Kornyanat Hozumi, Shinsuke Imada, Kazumasa Iwai, Toshihiko Iyemori, Hidekatsu Jin, Ryuho Kataoka, Yuto Katoh, Takashi Kikuchi, Yûki Kubo, Satoshi Kurita, Haruhisa Matsumoto, Takefumi Mitani, Hiroko Miyahara, Yasunobu Miyoshi, Tsutomu Nagatsuma, Aoi Nakamizo, Satoko Nakamura, Hiroyuki Nakata, Naoto Nishizuka, Yuichi Otsuka, Shinji Saito, Susumu Saito, Takashi Sakurai, Tatsuhiko Sato, Toshifumi Shimizu, Hiroyuki Shinagawa, Kazuo Shiokawa, Daikou Shiota, Takeshi Takashima, Chihiro Tao, Shin Toriumi, Satoru Ueno, Kyoko Watanabe, Shinichi Watari, Seiji Yashiro, Kohei Yoshida, Akimasa Yoshikawa

Earth, Planets and Space   Vol. 73 ( 1 ) page： 159   2021.8

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<title>Abstract</title>Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

DOI： 10.1186/s40623-021-01486-1

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Other Link： https://link.springer.com/article/10.1186/s40623-021-01486-1/fulltext.html

What controls the luminosity of polar cap airglow patches?: Implication from airglow measurements in Eureka, Canada in comparison with SuperDARN convection pattern

K. Hosokawa, M. Nagata, K. Shiokawa, Y. Otsuka

Polar Science   Vol. 28   2021.6

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Polar cap patches are known as islands of enhanced electron density in the polar cap F region ionosphere. To observe the airglow signature of polar cap patches continuously at a fixed point near the center of the MLAT/MLT coordinate system, we started operating an all-sky imager in Eureka, Canada since 2015, where the magnetic latitude is ~87°. By statistically analyzing the 630-nm airglow images from Eureka, it was identified that the luminosity of patches, which is proportional to the electron density in the F region, increases during specific UT interval from 16 to 22 UT. This outstanding UT variation of patch luminosity can be explained simply by the systematic shift of the terminator in the MLAT/MLT coordinate system due to the offset between the geographic and geomagnetic pole. That is, the spatial relationship between the dense source plasma in the sunlit area and the high-latitude convection system controls the UT variation. We also found that when the IMF By is positive, the number of polar cap patches was twice of that in the negative IMF By. By deriving average convection patterns from the archived SuperDARN map potential data, we confirmed that the configuration of plasma convection is more appropriate for patches to be transported toward the magnetic pole during the positive IMF By condition.

DOI： 10.1016/j.polar.2020.100608

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The Occurrence Feature of Plasma Bubbles in the Equatorial to Midlatitude Ionosphere During Geomagnetic Storms Using Long-Term GNSS-TEC Data

T. Sori, A. Shinbori, Y. Otsuka, T. Tsugawa, M. Nishioka

Journal of Geophysical Research: Space Physics   Vol. 126 ( 5 )   2021.5

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We performed a superposed epoch analysis of solar wind, interplanetary magnetic field, geomagnetic index, and the rate of total electron content (TEC) index (ROTI) derived from global navigation satellite system-TEC data during 652 geomagnetic storm events (minimum SYM-H < −40 nT), to clarify the occurrence features and causes of storm-time plasma bubbles in the equatorial to mid-latitude ionosphere. In this analysis, we defined the time of the SYM-H minimum as the zero epoch. As a result, the ROTI enhancement started at the duskside magnetic equator and expanded to higher latitudes during the main phase. Approximately 1 h after the onset of the recovery phase, the ROTI values at the magnetic equator in the dusk-to-midnight sectors decreased while those in the dawn sector increased. This situation persisted for at least 12 h. The ratio of the ROTI during the main phase to that during the quiet period in the dusk sector is the largest in May–July. The ratio of the ROTI during the recovery phase decreased during dusk with increasing solar activity. Considering the requirement of the Rayleigh-Taylor instability, the difference in the magnetic local time of the ROTI signature, between the main and recovery phases, can be explained by a local time distribution of storm-time electric fields associated with a prompt penetration electric field and disturbance dynamo. This implies that the occurrence feature of the plasma bubble is different from that during quiet times when the input of solar wind energy to the magnetosphere and ionosphere increases significantly.

DOI： 10.1029/2020JA029010

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Simultaneous Observation of Two Isolated Proton Auroras at Subauroral Latitudes by a Highly Sensitive All-Sky Camera and Van Allen Probes

Kohki Nakamura, Kazuo Shiokawa, Yuichi Otsuka, Atsuki Shinbori, Yoshizumi Miyoshi, Martin Connors, Harlan Spence, Geoff Reeves, Herbert O. Funsten, Robert MacDowall, Charles Smith, John Wygant, John Bonnell

Journal of Geophysical Research: Space Physics   Vol. 126 ( 5 )   2021.5

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Isolated proton auroras (IPAs) appearing at subauroral latitudes are generated by energetic protons precipitating from the magnetosphere through interaction with electromagnetic ion cyclotron (EMIC) waves. An IPA thus indicates the spatial scale and temporal variation of wave-particle interactions in the magnetosphere. In this study, a unique event of simultaneous ground and magnetospheric satellite observations of two IPAs were conducted on March 16, 2015, using an all-sky imager at Athabasca, Canada and Van Allen Probes. The Van Allen Probes observed two isolated EMIC waves with frequencies of ∼1 and 0.4 Hz at L ≈ 5.0 when the satellite footprint crossed over the two IPAs. This suggests that the IPAs were caused by localized EMIC waves. Proton flux at 5–20 keV increased locally when the EMIC waves appeared. Electron flux at energies below ∼500 eV also increased. Temperature anisotropy of the energetic protons was estimated at 1.5–2.5 over a wide L-value range of 3.0–5.2. Electron density gradually decreased from L = 3.5 to 5.4, suggesting that the EMIC wave at L ≈ 5.0 was located in the gradual plasmapause. From these observations, we conclude that the localized IPAs and associated EMIC waves took place because of localized enhancement of energetic proton flux and plasma density structure near the plasmapause. The magnetic field observed by the satellite showed small variation during the wave observation, indicating that the IPAs were accompanied by the weak field-aligned current.

DOI： 10.1029/2020JA029078

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Influence of Zonal Wind Velocity Variation on Equatorial Plasma Bubble Occurrences Over Southeast Asia

I. Sarudin, N. S.A. Hamid, M. Abdullah, S. M. Buhari, K. Shiokawa, Y. Otsuka, K. Hozumi, P. Jamjareegulgarn

Journal of Geophysical Research: Space Physics   Vol. 126 ( 5 )   2021.5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

The present study aims to investigate the influence of the zonal wind velocity on equatorial plasma bubble (EPB) occurrences over Southeast Asia. The observation of the EPB occurrence is obtained from the GPS Rate of TEC change index. Meanwhile, the zonal winds were measured using a Fabry-Perot interferometer located at Kototabang and Chiang Mai stations, and the height of F layer was acquired using an ionosonde at Chumphon station near the magnetic equator. This is the first study to report the influence of zonal wind velocity variation on EPB occurrences with the presence and absence of EPB using GPS data in the Southeast Asian sector. The results illustrated that the average magnitude of zonal wind velocity during the presence of EPB (78 ± 23 m/s) was higher than that of its absence (68 ± 21 m/s). It was observed using long-term data analyses which led to in-depth analyses. The analysis of temporal variation of zonal wind variation demonstrated that the zonal winds during EPB were higher in the evening compared to midnight and postmidnight periods from medium to high solar activities. The dependence of zonal wind velocity on EPB over local time was obtained based on the analysis which utilized the data collected during equinox in high solar activity. Besides that, a positive correlation was obtained between the zonal wind velocity and EPB occurrences during pre-reversal enhancement (PRE) corroborated the effects of zonal wind influence on PRE, and thus EPB occurrences.

DOI： 10.1029/2020JA028994

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Model-based reproduction and validation of the total spectra of a solar flare and their impact on the global environment at the X9.3 event of September 6, 2017

Kyoko Watanabe, Hidekatsu Jin, Shohei Nishimoto, Shinsuke Imada, Toshiki Kawai, Tomoko Kawate, Yuichi Otsuka, Atsuki Shinbori, Takuya Tsugawa, Michi Nishioka

Earth, Planets and Space   Vol. 73 ( 1 )   2021.4

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<title>Abstract</title>We attempted to reproduce the total electron content (TEC) variation in the Earth's atmosphere from the temporal variation of the solar flare spectrum of the X9.3 flare on September 6, 2017. The flare spectrum from the Flare Irradiance Spectral Model (FISM), and the flare spectrum from the 1D hydrodynamic model, which considers the physics of plasma in the flare loop, are used in the GAIA model, which is a simulation model of the Earth's whole atmosphere and ionosphere, to calculate the TEC difference. We then compared these results with the observed TEC. When we used the FISM flare spectrum, the difference in TEC from the background was in a good agreement with the observation. However, when the flare spectrum of the 1D-hydrodynamic model was used, the result varied depending on the presence or absence of the background. This difference depending on the models is considered to represent which extreme ultraviolet (EUV) radiation is primarily responsible for increasing TEC. From the flare spectrum obtained from these models and the calculation result of TEC fluctuation using GAIA, it is considered that the enhancement in EUV emission by approximately 15<bold>–</bold>35 nm mainly contributes in increasing TEC rather than that of X-ray emission, which is thought to be mainly responsible for sudden ionospheric disturbance. In addition, from the altitude/wavelength distribution of the ionization rate of Earth's atmosphere by GAIA (Ground-to-topside Atmosphere and Ionosphere model for Aeronomy), it was found that EUV radiation of approximately 15<bold>–</bold>35 nm affects a wide altitude range of 120<bold>–</bold>300 km, and TEC enhancement is mainly caused by the ionization of nitrogen molecules.

DOI： 10.1186/s40623-021-01376-6

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Other Link： https://link.springer.com/article/10.1186/s40623-021-01376-6/fulltext.html

Multi‐Event Analysis of Plasma and Field Variations in Source of Stable Auroral Red (SAR) Arcs in Inner Magnetosphere During Non‐Storm‐Time Substorms

Yudai Inaba, Kazuo Shiokawa, Shin‐ichiro Oyama, Yuichi Otsuka, Martin Connors, Ian Schofield, Yoshizumi Miyoshi, Shun Imajo, Atsuki Shinbori, Artem Yu Gololobov, Yoichi Kazama, Shiang‐Yu Wang, Sunny W. Y. Tam, Tzu‐Fang Chang, Bo‐Jhou Wang, Kazushi Asamura, Shoichiro Yokota, Satoshi Kasahara, Kunihiro Keika, Tomoaki Hori, Ayako Matsuoka, Yoshiya Kasahara, Atsushi Kumamoto, Shoya Matsuda, Yasumasa Kasaba, Fuminori Tsuchiya, Masafumi Shoji, Masahiro Kitahara, Satoko Nakamura, Iku Shinohara, Harlan E. Spence, Geoff D. Reeves, Robert J. Macdowall, Charles W. Smith, John R. Wygant, John W. Bonnell

Journal of Geophysical Research: Space Physics   Vol. 126 ( 4 )   2021.4

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DOI： 10.1029/2020JA029081

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Formation of an additional density peak in the bottom side of the sodium layer associated with the passage of multiple mesospheric frontal systems

Viswanathan Lakshmi Narayanan, Satonori Nozawa, Shin-Ichiro Oyama, Ingrid Mann, Kazuo Shiokawa, Yuichi Otsuka, Norihito Saito, Satoshi Wada, Takuya D. Kawahara, Toru Takahashi

ATMOSPHERIC CHEMISTRY AND PHYSICS   Vol. 21 ( 4 ) page： 2343 - 2361   2021.2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS GESELLSCHAFT MBH  

We present a detailed investigation of the formation of an additional sodium density peak at altitudes of 79-85 km below the main peak of the sodium layer based on sodium lidar and airglow imager measurements made at Ramfjordmoen near Tromso, Norway, on the night of 19 December 2014. The airglow imager observations of OH emissions revealed four passing frontal systems that resembled mesospheric bores, which typically occur in ducting regions of the upper mesosphere. For about 1.5 h, the lower-altitude sodium peak had densities similar to that of the main peak of the layer around 90 km. The lower-altitude sodium peak weakened and disappeared soon after the fourth front had passed. The fourth front had weakened in intensity by the time it approached the region of lidar beams and disappeared soon afterwards. The column-integrated sodium densities increased gradually during the formation of the lower-altitude sodium peak. Temperatures measured with the lidar indicate that there was a strong thermal duct structure between 87 and 93 km. Furthermore, the temperature was enhanced below 85 km. Horizontal wind magnitudes estimated from the lidar showed strong wind shears above 93 km. We conclude that the combination of an enhanced stability region due to the temperature profile and intense wind shears have provided ideal conditions for evolution of multiple mesospheric bores revealed as frontal systems in the OH images. The downward motion associated with the fronts appeared to have brought air rich in H and O from higher altitudes into the region below 85 km, wherein the temperature was also higher. Both factors would have liberated sodium atoms from the reservoir species and suppressed the reconversion of atomic sodium into reservoir species so that the lower-altitude sodium peak could form and the column abundance could increase. The presented observations also reveal the importance of mesospheric frontal systems in bringing about significant variation of minor species over shorter temporal intervals.

DOI： 10.5194/acp-21-2343-2021

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Comparison of seasonal and longitudinal variation of daytime MSTID activity using GPS observation and GAIA simulations

Sivakandan, M; Otsuka, Y; Ghosh, P; Shinagawa, H; Shinbori, A; Miyoshi, Y

EARTH PLANETS AND SPACE   Vol. 73 ( 1 )   2021.2

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DOI： 10.1186/s40623-021-01369-5

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On the Generation of an Unseasonal EPB Over South East Asia

J. L. Currie, B. A. Carter, J. Retterer, T. Dao, R. Pradipta, R. Caton, K. Groves, Y. Otsuka, T. Yokoyama, K. Hozumi, T. Le Truong, M. Terkildsen

Journal of Geophysical Research: Space Physics   Vol. 126 ( 2 )   2021.2

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An unseasonal equatorial plasma bubble (EPB) event over South-East Asia was observed on July 22, 2014 that has not been studied before. An investigation into this event is presented with the 26th July, 2014 as a comparison, non-bubble day. The 22nd July EPB event occurred in the late post-sunset sector and was associated with a small upward plasma drift. This event was highlighted using a new filter on the SCINDA S4 data. Ionosonde data show that sporadic E was present during the growth period for the EPB event. Modeling results from Thermosphere-Ionosphere Electrodynamics (TEC) Global Circulation Model were used to conduct a numerical experiment investigating the direct effect of sporadic E on the linear R-T growth rate. It was shown that sporadic E located in the correct latitude and local time can increase the linear growth rate. The seeding conditions were investigated using TEC data from Patumwan, Thailand. Wave-like structures were observed for both days of interest, with larger amplitudes on 22nd July compared with the 26th July. Finally, simulations using the high-resolution model PBMOD showed that for forcing from above conditions similar to the days of interest, EPBs would form in the presence of large seed perturbations. Therefore, it is likely that this unseasonal event was caused by large seed perturbations in TEC.

DOI： 10.1029/2020JA028724

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Multi-Wavelength Imaging Observations of STEVE at Athabasca, Canada

Sneha Yadav, Kazuo Shiokawa, Yuichi Otsuka, Martin Connors, J. P. St Maurice

Journal of Geophysical Research: Space Physics   Vol. 126 ( 2 )   2021.2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We present the first multi-wavelength imaging observations of strong thermal emission velocity enhancement (STEVE) using an all-sky imager at Athabasca (magnetic latitude = 61.5°N), Canada. This study is based on three STEVE events which were accompanied by picket fence structures in the green-line. Although the STEVE arc was dominant in 630 and 557.7-nm, weak emissions were also found in other wavelengths including OI at 844.6, Hβ, Na, and the nominal background filter at 572.5-nm. As observed at 630 and 557.7-nm, the STEVE arc started as a faint arc close to the auroral oval and moved equatorward. The 557.7-nm arc exhibited picket fence structure at later times after it moved equatorward. The picket fence was sometimes found to persist even after the 630-nm arc had disappeared. During a particular event, the STEVE arcs in both the 630 and 557.7-nm were found to carry a ribbon-like motion moving along the arc. We have found that STEVE arcs are embedded in a region of weak diffuse auroral emissions. The STEVE arcs have sharp boundaries and these boundaries are different in red- and green-line. The sharp decrease in the intensity at the immediate poleward edge of the STEVE arc appears as a “dark-band” in the green-line images. Based on the horizontal component of the geomagnetic field at Fort Smith (magnetic latitude 67.28°N), we find that the STEVE arc detachment, its equatorward motion, and its brightness coincided with changes in the magnetic activity during the recovery phase of a substorm.

DOI： 10.1029/2020JA028622

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Solar activity dependence of medium-scale traveling ionospheric disturbances using GPS receivers in Japan

Otsuka, Y; Shinbori, A; Tsugawa, T; Nishioka, M

EARTH PLANETS AND SPACE   Vol. 73 ( 1 )   2021.1

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DOI： 10.1186/s40623-020-01353-5

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Challenges to Equatorial Plasma Bubble and Ionospheric Scintillation Short-Term Forecasting and Future Aspects in East and Southeast Asia

Li, GZ; Ning, BQ; Otsuka, Y; Abdu, MA; Abadi, P; Liu, ZZ; Spogli, L; Wan, WX

SURVEYS IN GEOPHYSICS   Vol. 42 ( 1 ) page： 201 - 238   2021.1

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DOI： 10.1007/s10712-020-09613-5

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Plasma and Field Observations in the Magnetospheric Source Region of a Stable Auroral Red (SAR) Arc by the Arase Satellite on 28 March 2017

Yudai Inaba, Kazuo Shiokawa, Shin‐ichiro Oyama, Yuichi Otsuka, Arto Oksanen, Atsuki Shinbori, Artem Yu Gololobov, Yoshizumi Miyoshi, Yoichi Kazama, Shiang‐Yu Wang, Sunny W. Y. Tam, Tzu‐Fang Chang, Bo‐Jhou Wang, Shoichiro Yokota, Satoshi Kasahara, Kunihiro Keika, Tomoaki Hori, Ayako Matsuoka, Yoshiya Kasahara, Atsushi Kumamoto, Yasumasa Kasaba, Fuminori Tsuchiya, Masafumi Shoji, Iku Shinohara, Claudia Stolle

Journal of Geophysical Research: Space Physics   Vol. 125 ( 10 )   2020.10

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DOI： 10.1029/2020JA028068

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Dilatory and Downward Development of 3-m Scale Irregularities in the Funnel-Like Region of a Rapidly Rising Equatorial Plasma Bubble

S. Tulasi Ram, K. K. Ajith, T. Yokoyama, M. Yamamoto, K. Hozumi, K. Shiokawa, Y. Otsuka, G. Li

Geophysical Research Letters   Vol. 47 ( 13 )   2020.7

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The equatorial plasma bubbles, once developed, grow nonlinearly into topside ionosphere, and simultaneous secondary instabilities lead to the development of shorter scale irregularities. The altitudinal growth and generation of smaller scale irregularities determine the spatio-temporal occurrence and the intensity of ionospheric scintillations at wide spectrum of radio waves and have significant implications on the GNSS/Satellite Based Augmentation Systems. In this letter, we present a unique equatorial plasma bubble observation from equatorial atmosphere radar that provides hitherto undisclosed evidence for the smaller (3-m) scale irregularities initially developing at higher altitudes and subsequently developing to lower altitudes in a narrow funnel-like structure. The responsible mechanisms for early development of shorter scale irregularities in the topside and their subsequent development at lower altitudes are discussed in light of difference between the time scales of altitudinal growth and cascading rate of secondary instabilities through high-resolution bubble model simulations.

DOI： 10.1029/2020GL087256

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Day-to-day variation of pre-reversal enhancement in the equatorial ionosphere based on GAIA model simulations

Ghosh, P; Otsuka, Y; Mani, S; Shinagawa, H

EARTH PLANETS AND SPACE   Vol. 72 ( 1 )   2020.7

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Language：Japanese  

DOI： 10.1186/s40623-020-01228-9

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Observations of equatorial plasma bubbles using a low-cost 630.0-nm all-sky imager in Ishigaki Island, Japan

Keisuke Hosokawa, Kohei Takami, Susumu Saito, Yasunobu Ogawa, Yuichi Otsuka, Kazuo Shiokawa, Chia Hung Chen, Chien Hung Lin

Earth, Planets and Space   Vol. 72 ( 1 )   2020.5

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© 2020, The Author(s). Here, we introduce a low-cost airglow imaging system developed for observing plasma bubble signatures in 630.0-nm airglow emission from the F region of the ionosphere. The system is composed of a small camera, optical filter, and fish-eye lens, and is operated using free software that automatically records video from the camera. A pilot system was deployed in Ishigaki Island in the southern part of Japan (Lat 24.4, Lon 124.4, Mlat 19.6) and was operated for ~ 1.5 years from 2014 to 2016 corresponding to the recent solar maximum period. The pilot observations demonstrated that it was difficult to identify the plasma bubble signature in the raw image captured every 4 s. However, the quality of the image could be improved by reducing the random noise of instrumental origin through an integration of 30 consecutive raw images obtained in 2 min and further by subtracting the 1-h averaged background image. We compared the deviation images to those from a co-existing airglow imager of OMTIs, which is equipped with a back-illuminated cooled CCD camera with a high quantum efficiency of ~ 90%. It was confirmed that the low-cost airglow imager is capable of imaging the spatial structure of plasma bubbles, including their bifurcating traces. The results of these pilot observations in Ishigaki Island will allow us to distribute the low-cost imager in a wide area and construct a network for monitoring plasma bubbles and their space weather impacts on satellite navigation systems.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-020-01187-1

Web of Science

Scopus

Probability of Ionospheric Plasma Bubble Occurrence as a Function of Pre-Reversal Enhancement Deduced from Ionosondes in Southeast Asia

Abadi, P; Otsuka, Y; Supriadi, S; Olla, A

7TH INTERNATIONAL SEMINAR ON AEROSPACE SCIENCE AND TECHNOLOGY (ISAST 2019)   Vol. 2226   2020

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Language：Japanese  

DOI： 10.1063/5.0002321

Web of Science

New observations of the total electron content and ionospheric scintillations over Ho Chi Minh city

Dao, T; Huy, ML; Carter, B; Le, Q; Trinh, TT; Phan, BN; Otsuka, Y

VIETNAM JOURNAL OF EARTH SCIENCES   Vol. 42 ( 4 ) page： 320 - 333   2020

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Language：Japanese  

DOI： 10.15625/0866-7187/42/4/15281

Web of Science

Capability of Geomagnetic Storm Parameters to Identify Severe Space Weather

Balan, N; Zhang, QH; Xing, ZY; Skoug, R; Shiokawa, K; Lühr, H; Ram, ST; Otsuka, Y; Zhao, LX

ASTROPHYSICAL JOURNAL   Vol. 887 ( 1 )   2019.12

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DOI： 10.3847/1538-4357/ab5113

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A Neural Network-Based Ionospheric Model Over Africa From Constellation Observing System for Meteorology, Ionosphere, and Climate and Ground Global Positioning System Observations

Daniel Okoh, Gopi Seemala, Babatunde Rabiu, John Bosco Habarulema, Shuanggen Jin, Kazuo Shiokawa, Yuichi Otsuka, Malini Aggarwal, Jean Uwamahoro, Patrick Mungufeni, Bolaji Segun, Aderonke Obafaye, Nada Ellahony, Chinelo Okonkwo, Mpho Tshisaphungo, Dadaso Shetti

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 124 ( 12 ) page： 10512 - 10532   2019.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The first regional total electron content (TEC) model over the entire African region (known as AfriTEC model) using empirical observations is developed and presented. Artificial neural networks were used to train TEC observations obtained from Global Positioning System receivers, both on ground and onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites for the African region from years 2000 to 2017. The neural network training was implemented using inputs that enabled the networks to learn diurnal variations, seasonal variations, spatial variations, and variations that are connected with the level of solar activity, for quiet geomagnetic conditions (-20 nT <= Dst <= 20 nT). The effectiveness of three solar activity indices (sunspot number, solar radio flux at 10.7-cm wavelength [F10.7], and solar ultraviolet [UV] flux at 1 AU) for the neural network trainings was tested. The F10.7 and UV were more effective, and the F10.7 was used as it gave the least errors on the validation data set used. Equatorial anomaly simulations show a reduced occurrence during the June solstice season. The distance of separation between the anomaly crests is typically in the range from about 11.5 +/- 1.0 degrees to 16.0 +/- 1.0 degrees. The separation is observed to widen as solar activity levels increase. During the December solstice, the anomaly region shifts southwards of the equinox locations; in year 2012, the trough shifted by about 1.5 degrees and the southern crest shifted by over 2.5 degrees.

DOI： 10.1029/2019JA027065

Web of Science

Investigation of Spatiotemporal Morphology of Plasma Bubbles Based on EAR Observations Reviewed

L.M. Joshi, L.‐C. Tsai, S.‐Y. Su, Y. Otsuka, T. Yokoyama, M. Yamamoto, S. Sarkhel, K. Hozumi, C.‐H. Lu

Journal of Geophysical Research: Space Physics   Vol. 124 ( 12 ) page： 10549 - 10563   2019.12

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union ({AGU})  

DOI： 10.1029/2019JA026839

Web of Science

Three-Dimensional Fourier Analysis of the Phase Velocity Distributions of Mesospheric and Ionospheric Waves Based on Airglow Images Collected Over 10 Years: Comparison of Magadan, Russia, and Athabasca, Canada Reviewed

Tsuchiya Satoshi, Shiokawa Kazuo, Fujinami Hatsuki, Otsuka Yuichi, Nakamura Takuji, Connors Martin, Schofield Ian, Shevtsov Boris, Poddelskiy Igor

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 124 ( 10 ) page： 8110 - 8124   2019.10

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We studied atmospheric gravity waves (AGWs) and nighttime medium-scale traveling ionospheric disturbances (MSTIDs) using a three-dimensional spectral analysis technique for airglow images at wavelengths of 557.7 nm (altitude: 90-100 km for AGWs) and 630.0 nm (200-300 km for MSTIDs), obtained from Athabasca (ATH), Canada (55 degrees N, 247 degrees E, 2005-2017), and Magadan (MGD), Russia (60 degrees N, 151 degrees E, 2008-2017), over 10-13 years. The AGW propagation direction in summer was from northwestward to northward in ATH and northeastward in MGD with phase speeds of 20-60 m/s. In winter at ATH, they are more omnidirectional with weak preference from northwestward to southward with a speed less than 40 m/s, while another weaker power exists from northeastward to southeastward from 70 to 120 m/s. In winter at MGD, there was no dominant direction in the phase-velocity spectra with spectral power an order smaller than ATH. We suggest that these AGW characteristics were caused by wind filtering and intensity and locations of tropospheric sources. The MSTIDs at ATH propagated southwestward in spring and winter and northeastward in summer and fall. The MSTIDs at MGD propagated northeastward, eastward, and westward in spring, fall, and winter, respectively, with weaker power than that at ATH. The phase speeds are mostly less than 100 m/s except for fall. The propagation direction tends to change from south-southwestward in the evening to north-northeastward after the midnight at both ATH and MGD. We discuss possible reasons for these MSTID characteristics at high latitudes based on Perkins and E-F coupling instabilities, high-latitude plasma convection, and thermospheric neutral winds.

DOI： 10.1029/2019JA026783

Web of Science

Observation and characterization of traveling ionospheric disturbances induced by solar eclipse of 20 March 2015 using incoherent scatter radars and GPS networks Reviewed

Panasenko Sergii V, Otsuka Yuichi, van de Kamp Max, Chernogor Leonid F, Shinbori Atsuki, Tsugawa Takuya, Nishioka Michi

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 191   2019.9

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Language：Japanese  

DOI： 10.1016/j.jastp.2019.05.015

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Thermospheric wind variations observed by a Fabry-Perot interferometer at Tromso, Norway, at substorm onsets Reviewed

Xu Heqiucen, Shiokawa Kazuo, Oyama Shin-ichiro, Otsuka Yuichi

EARTH PLANETS AND SPACE   Vol. 71 ( 1 )   2019.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGEROPEN  

Energy input from the magnetosphere during substorms can strongly affect the high-latitude thermosphere. The ionospheric current caused by thermospheric wind variations may also provide a feedback to the magnetosphere. In this study, we investigate the characteristics of high-latitude thermospheric wind variations at local substorm onsets at Tromso, Norway, as well as the possibility of such feedback mechanism. A Fabry-Perot interferometer (FPI) at Tromso provided wind measurements estimated from the Doppler shift of red-line emission (630.0 nm) of aurora and airglow. We analyzed wind data in 2009 with a time resolution of similar to 13 min. We first carefully identified the onset times of isolated local substorms at Tromso and extracted four wind measurements from red-line emission. All these events showed increases of eastward components at local substorm onsets. For northward components, these events showed decreases except for those at midnight. The observed wind variations at local substorm onsets were less than 49 m/s. These values are much smaller than the typical plasma convection speed in the auroral zone. We speculate that the ionospheric current caused by thermospheric wind variations at local substorm onsets does not provide strong feedback to the development of substorm expansion phase in the magnetotail. We discuss the possible causes of these wind variations in the context of plasma convection, diurnal tides, and arc-associated electric field.

DOI： 10.1186/s40623-019-1072-0

Web of Science

Direct Observations of Traveling Ionospheric Disturbances as Focusers of Solar Radiation: Spectral Caustics Reviewed

Koval Artem, Chen Yao, Tsugawa Akuya, Otsuka Yuichi, Shinbori Atsuki, Nishioka Michi, Brazhenko Anatoliy, Stanislaysky Aleksander, Konovalenko Aleksander, Zhang Qing-He, Monstein Christian, Gorgutsa Roman

ASTROPHYSICAL JOURNAL   Vol. 877 ( 2 )   2019.6

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DOI： 10.3847/1538-4357/ab1b52

Web of Science

Response of the Ionosphere-Plasmasphere Coupling to the September 2017 Storm:What Erodes the Plasmasphere so Severley? Reviewed

Obana Yuki, Maruyama Naomi, Shinbori Atsuki, Hashimoto Kumiko K, Fedrizzi Mariangel, Nose Masahito, Otsuka Yuichi, Nishitani Nozomu, Hori Tomoaki, Kumamoto Atsushi, Tsuchiya Fuminori, Matsuda Shoya, Matsuoka Ayako, Kasahara Yoshiya, Yoshikawa Akimasa, Miyoshi Yoshizumi, Shinohara Iku

SPACE WEATHER-THE INTERNATIONAL JOURNAL OF RESEARCH AND APPLICATIONS   Vol. 17 ( 6 ) page： 861 - 876   2019.6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report an extreme erosion of the plasmasphere arising from the September 2017 storm. The cold electron density is identified from the upper limit frequency of upper hybrid resonance waves observed by the Plasma Wave Experiment instrument onboard the Exploration of energization and Radiation in Geospace/Arase satellite. The electron density profiles reveal that the plasmasphere was severely eroded during the recovery phase of the storm and the plasmapause was located at L = 1.6-1.7 at 23 UT 8 September 2017. This is the first report of deep erosion of the plasmasphere (L-pp < 2) with the in situ observation of the electron density. The degree of the severity is much more than what is expected from the relatively moderate value of the SYM-H minimum (-146 nT). We attempt to find a possible explanation for the observed severe depletion by using both observational evidence and numerical simulations. Our results suggest that the middle latitude electric field had penetrated from the high-latitude storm time convection for several hours. Such an unusually long-lasting penetration event can cause this observed degree of severity.Plain Language Summary The plasmasphere is the region of cold, relatively dense ionized gas (mostly protons and helium ions) that resides on the magnetic field lines close to the Earth. It is understood that the plasmasphere is threaded by magnetic field flux tubes that are persistently "closed," so that plasma from the Earth's ionosphere has filled the flux tubes. The typical location of the outer boundary of the plasmasphere, known as the plasmapause, is usually 40,000-50,000 km from the Earth. Here we report that a magnetic storm during 7-10 September 2017 dramatically displaced the outer boundary of the plasmasphere inwards, to only similar to 4,000 km from Earth's surface. Our study suggests that the remarkable deformation is caused by the unusually long-lasting leakage of the convection electric field deep within the plasmasphere.

DOI： 10.1029/2019SW002168

Web of Science

Spatiotemporal development of pulsating auroral patch associated with discrete chorus elements: Arase and PWING observations Reviewed

Ozaki, M, K. Shiokawa, Y. Miyoshi, K. Hosokawa, S. Oyama, S. Yagitani, Y. Kasahara, Y. Kasaba, S. Matsuda, R. Kataoka, Y. Ebihara, Y. Ogawa, Y. Otsuka, S. Kurita, R. C. Moore, Y.-M. Tanaka, M. Nosé, T. Nagatsuma, M. Connors, N. Nishitani, M. Hikishima, A. Kumamoto, F. Tsuchiya, A. Kadokura, T. Nishiyama, T. Inoue, K. Imamura, A. Matsuoka, I. Shinohara

2019 URSI ASIA-PACIFIC RADIO SCIENCE CONFERENCE (AP-RASC)     page： 18776063   2019.6

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Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.23919/URSIAP-RASC.2019.8738444

Web of Science

Development of multivariate ionospheric TEC forecasting algorithm using linear time series model and ARMA over low-latitude GNSS station Reviewed

Ratnam D. Venkata, Otsuka Yuichi, Sivavaraprasad G, Dabbakuti J, R. K. Kumar

ADVANCES IN SPACE RESEARCH   Vol. 63 ( 9 ) page： 2848 - 2856   2019.5

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.asr.2018.03.024

Web of Science

Multicomponent Analysis of Ionospheric Scintillation Effects Using the Synchrosqueezing Technique for Monitoring and Mitigating their Impact on GNSS Signals Reviewed

Sivavaraprasad G, Ratnam D. Venkata, Otsuka Yuichi

JOURNAL OF NAVIGATION   Vol. 72 ( 3 ) page： 669 - 684   2019.5

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Language：Japanese  

DOI： 10.1017/S0373463318000929

Web of Science

Daytime Periodic Wave-like Structures in the Ionosphere Observed at Low Latitudes over the Asian-Australian Sector Using Total Electron Content from Beidou Geostationary Satellites Reviewed

Huang Fuqing, Otsuka Yuichi, Lei Jiuhou, Luan Xiaoli, Dou Xiankang, Li Guozhu

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 124 ( 3 ) page： 2312 - 2322   2019.3

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DOI： 10.1029/2018JA026443

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Observations of Low-Latitude Traveling Ionospheric Disturbances by a 630.0-nm Airglow Imager and the CHAMP Satellite Over Indonesia Reviewed

Moral Aysegul Ceren, Shiokawa Kazuo, Suzuki Shin, Liu Huixin, Otsuka Yuichi, Yatini Clara Yoko

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 124 ( 3 ) page： 2198 - 2212   2019.3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report the first comparison of ground and satellite measurements of low-latitude traveling ionospheric disturbances (TIDs). Three TID events were simultaneously observed by a 630.0-nm airglow imager and the CHAllenging Minisatellite Payload (CHAMP) satellite at Kototabang, Indonesia (geographic coordinates: 0.2 degrees S, 100.3 degrees E, geomagnetic latitude: 10.6 degrees S). In 630.0-nm airglow images of all three events, there are clear southward-moving structures. Events 1 and 2 are a single pulse with horizontal scales of similar to 500-1,000 km. Event 3 shows five wave fronts with a horizontal scale size of 500-1,000 km. All three TIDs are medium-scale TIDs. Horizontal wavelengths of both airglow intensity at an average emission altitude of 250 km and CHAMP neutral density variations measured at 400 km are estimated by fitting a sinusoidal function to the observed data. The estimated horizontal wavelengths for airglow and neutral density data are 1,031 and 880 km for event 1 and 560 and 420 km for event 3, respectively. These values between airglow and CHAMP are comparable, suggesting both instruments are observing the same wave. For event 1, the CHAMP electron density mapped along the geomagnetic field line onto the airglow altitude does not show wave structure similar to the airglow variation. For events 2 and 3, the plasma density did not show wavy structures similar to the waves seen in the airglow image and CHAMP neutral density. These results suggest that the TIDs observed in airglow images are not caused by ionospheric plasma instability but by gravity waves in the thermosphere.

DOI： 10.1029/2018JA025634

Web of Science

Visualization of rapid electron precipitation via chorus element wave–particle interactions Reviewed

Mitsunori Ozaki, Yoshizumi Miyoshi, Kazuo Shiokawa, Keisuke Hosokawa, Shin ichiro Oyama, Ryuho Kataoka, Yusuke Ebihara, Yasunobu Ogawa, Yoshiya Kasahara, Satoshi Yagitani, Yasumasa Kasaba, Atsushi Kumamoto, Fuminori Tsuchiya, Shoya Matsuda, Yuto Katoh, Mitsuru Hikishima, Satoshi Kurita, Yuichi Otsuka, Robert C. Moore, Yoshimasa Tanaka, Masahito Nosé, Tsutomu Nagatsuma, Nozomu Nishitani, Akira Kadokura, Martin Connors, Takumi Inoue, Ayako Matsuoka, Iku Shinohara

Nature Communications   Vol. 10 ( 1 )   2019.1

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© 2019, The Author(s). Chorus waves, among the most intense electromagnetic emissions in the Earth’s magnetosphere, magnetized planets, and laboratory plasmas, play an important role in the acceleration and loss of energetic electrons in the plasma universe through resonant interactions with electrons. However, the spatial evolution of the electron resonant interactions with electromagnetic waves remains poorly understood owing to imaging difficulties. Here we provide a compelling visualization of chorus element wave–particle interactions in the Earth’s magnetosphere. Through in-situ measurements of chorus waveforms with the Arase satellite and transient auroral flashes from electron precipitation events as detected by 100-Hz video sampling from the ground, Earth’s aurora becomes a display for the resonant interactions. Our observations capture an asymmetric spatial development, correlated strongly with the amplitude variation of discrete chorus elements. This finding is not theoretically predicted but helps in understanding the rapid scattering processes of energetic electrons near the Earth and other magnetized planets.

DOI： 10.1038/s41467-018-07996-z

Web of Science

Scopus

PubMed

Implementation of Hybrid Ionospheric TEC Forecasting Algorithm Using PCA-NN Method Reviewed

Mallika, IL, Ratnam, DV, Ostuka, Y, Sivavaraprasad, G, Raman, S

IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING   Vol. 12 ( 1 ) page： 371 - 381   2019.1

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DOI： 10.1109/JSTARS.2018.2877445

Rapid Loss of Relativistic Electrons by EMIC Waves in the Outer Radiation Belt Observed by Arase, Van Allen Probes, and the PWING Ground Stations Reviewed

Kurita, S, Miyoshi, Y, Shiokawa, K, Higashio, N, Mitani, T, Takashima, T, Matsuoka, A, Shinohara, I, Kletzing, CA, Blake, JB, Claudepierre, SG, Connors, M, Oyama, S, Nagatsuma, T, Sakaguchi, K, Baishev, D, Otsuka, Y

GEOPHYSICAL RESEARCH LETTERS   Vol. 45 ( 23 ) page： 12720 - 12729   2018.12

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2018GL080262

Web of Science

Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2018GL080262

Microscopic Observations of Pulsating Aurora Associated With Chorus Element Structures: Coordinated Arase Satellite-PWING Observations Reviewed

Ozaki, M, Shiokawa, K, Miyoshi, Y, Hosokawa, K, Oyama, S, Yagitani, S, Kasahara, Y, Kasaba, Y, Matsuda, S, Kataoka, R, Ebihara, Y, Ogawa, Y, Otsuka, Y, Kurita, S, Moore, RC, Tanaka, YM, Nose, M, Nagatsuma, T, Connors, M, Nishitani, N, Katoh, Y, Hikishima, M, Kumamoto, A, Tsuchiya, F, Kadokura, A, Nishiyama, T, Inoue, T, Imamura, K, Matsuoka, A, Shinohara, I

GEOPHYSICAL RESEARCH LETTERS   Vol. 45 ( 22 ) page： 12125 - 12134   2018.11

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Rapid (<1 s) intensity modulation of pulsating auroras is caused by successive chorus elements as a response to wave-particle interactions in the magnetosphere. Here we found that a pulsating auroral patch responds to the time spacing for successive chorus elements and possibly to chorus subpacket structures with a time scale of tens of milliseconds. These responses were identified from coordinated Arase satellite and ground (Gakona, Alaska) observations with a high-speed auroral imager (100 Hz). The temporal variations of auroral intensity in a few-hertz frequency range exhibited a spatial concentration at the lower-latitude edge of the auroral patch. The spatial evolution of the auroral patch showed repeated expansion/contraction with tens of kilometer scales in the ionosphere, which could be spatial behaviors in the wave-particle interactions. These observations indicate that chorus elements evolve coherently within the auroral patch, which is approximately 900 km in the radial and longitudinal directions at the magnetic equator.

DOI： 10.1029/2018GL079812

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Scopus

Statistical Analysis of SAR Arc Detachment From the Main Oval Based on 11-Year, AII-sky Imaging Observation at Athabasca, Canada Reviewed

Takagi Yuki, Shiokawa Kazuo, Otsuka Yuichi, Connors Martin, Schofield Ian

GEOPHYSICAL RESEARCH LETTERS   Vol. 45 ( 21 ) page： 11539 - 11546   2018.11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report the first statistical analysis of stable auroral red (SAR) arc detachments from the main auroral oval, using 630-nm all-sky cooled-Charge Coupled Device images obtained at Athabasca (magnetic latitude = 61.7 degrees N), Canada. SAR arc detachments from the main oval can be an important way of monitoring the characteristics of ring current particle injection in the inner magnetosphere. We analyzed all-sky images obtained for 11 years from 2006 to 2016 and found 163 SAR arc detachment events. The SAR arc detachments tend to occur in the premidnight sector, indicating the ring current ion drift to the dusk sector. The SAR arc detachments also tend to occur at the beginning of the substorm recovery phase, suggesting that the SAR arcs detach from the main oval as the main auroral oval returns to higher latitudes. The equatorward velocities of detached SAR arcs are from -100 m/s (poleward) to +200 m/s (equatorward), corresponding to magnetospheric electric fields from -1 to +2 mV/m.Plain Language, Summary Stable auroral red (SAR) arcs are the optical emissions caused by low-energy electron precipitation into the ionosphere from the inner magnetosphere close to the earth. In this paper, we report the first statistical analysis of SAR arc detachments from the main auroral oval, using all-sky cooled-Charge Coupled Device images measured at a wavelength of 630.0 nm obtained at Athabasca, Canada, which is located at latitudes just lower than the auroral zone latitudes. SAR arc detachments from the main oval can be an important way of monitoring the characteristics of high-energy particle injection in the inner magnetosphere. We analyzed 11-year all-sky images from 2006 to 2016 and found 163 SAR arc detachment events. The SAR arc detachments tend to occur premidnight, indicating the high-energy ion drift to the dusk sector. We also found that the SAR arc detachments tend to occur at the beginning of the substorm recovery phase. This likely indicates that the SAR arcs detach from the high-latitude aurora as the main aurora returns to higher latitudes at the beginning of the recovery phase. The equatorward velocities of detached SAR arcs are also estimated from the analysis.

DOI： 10.1029/2018GL079615

Web of Science

Review of the generation mechanisms of post-midnight irregularities in the equatorial and low-latitude ionosphere Reviewed

Otsuka Yuichi

PROGRESS IN EARTH AND PLANETARY SCIENCE   Vol. 5   2018.9

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DOI： 10.1186/s40645-018-0212-7

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Investigation of Nighttime MSTIDS Observed by Optical Thermosphere Imagers at Low Latitudes: Morphology, Propagation Direction, and Wind Filtering Reviewed

Figueiredo, CAOB, Takahashi, H, Wrasse, CM, Otsuka, Y, Shiokawa, K, Barros, D

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 123 ( 9 ) page： 7843 - 7857   2018.9

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DOI： 10.1029/2018JA025438

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Storm-Enhanced Development of Postsunset Equatorial Plasma Bubbles Around the Meridian 120 degrees E/60 degrees W on 7-8 September 2017 Reviewed

Li Guozhu, Ning Baiqi, Wang Chi, Abdu M. A, Otsuka Yuichi, Yamamoto M, Wu Jian, Chen Jinsong

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 123 ( 9 ) page： 7985 - 7998   2018.9

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DOI： 10.1029/2018JA025871

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Temporal and Spatial Variations of Storm Time Midlatitude Ionospheric Trough Based on Global GNSS-TEC and Arase Satellite Observations Reviewed

Shinbori Atsuki, Otsuka Yuichi, Tsugawa Takuya, Nishioka Michi, Kumamoto Atsushi, Tsuchiya Fuminori, Matsuda Shoya, Kasahara Yoshiya, Matsuoka Ayako, Ruohoniemi J. Michael, Shepherd Simon G, Nishitani Nozomu

GEOPHYSICAL RESEARCH LETTERS   Vol. 45 ( 15 ) page： 7362 - 7370   2018.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Temporal and spatial variations of the midlatitude ionospheric trough during a geomagnetic storm on 4 April 2017 have been investigated using Global Navigation Satellite System total electron content data together with Arase satellite observations. After the geomagnetic storm commencement, the trough minimum location moves equatorward from 60 to 48 degrees in geomagnetic latitude within 4 hr. The trough minimum location identified from the Global Navigation Satellite System total electron content data is located near the footprint of an abrupt drop of electron density detected by the Arase High-Frequency Analyzer instrument. The longitudinal variation of the trough minimum location shows a significant variation with a scale of 1,000-2,500 km during both storm and quiet times. This phenomenon has not yet been reported by previous studies. After the onset of the storm recovery phase, the trough minimum location rapidly moves poleward back to the quiet time location within 4 hr.Plain Language Summary Geomagnetic storms lead to a sever change in the plasma environment in the ionosphere and magnetosphere. Because their storm time disturbances in these regions cause an enhancement of positioning error and satellite anomaly due to the ionospheric electron density variation and magnetospheric high-energy particles, to clarify the characteristics of storm time variation of plasma environment and its physical mechanism is essential for prediction of the Geospace environmental change as space weather. In this study, we analyzed global positioning system total electron content data and Arase satellite observations in the inner magnetosphere to monitor a storm time change in the shape of the plasmasphere that controls the generation and propagation of plasma waves. Our analysis results show that the location of the midlatitude trough minimum identified from the total electron content data rapidly moves equatorward and poleward within 4 hr during the main and recovery phases. The location of the midlatitude trough minimum almost corresponds to that of the plasmapause detected by the Arase satellite. The longitudinal distribution of the midlatitude trough minimum shows a significant variation with its scale of 1,000-2,500 km. This feature is also seen during a geomagnetically quiet time. This phenomenon has not yet been reported by previous works.

DOI： 10.1029/2018GL078723

Web of Science

On the Role of Thermospheric Winds and Sporadic E Layers in the Formation and Evolution of Electrified MSTIDs in Geomagnetic Conjugate Regions Reviewed

Narayanan, VL, Shiokawa, K, Otsuka, Y, Neudegg, D

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 123 ( 8 ) page： 6957 - 6980   2018.8

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DOI： 10.1029/2018JA025261

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Statistical Analysis of the Phase Velocity Distribution of Mesospheric and Ionospheric Waves Observed in Airglow Images Over a 16-Year Period: Comparison Between Rikubetsu and Shigaraki, Japan Reviewed

Tsuchiya Satoshi, Shiokawa Kazuo, Fujinami Hatsuki, Otsuka Yuichi, Nakamura Takuji, Yamamoto Mamoru

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 123 ( 8 ) page： 6930 - 6947   2018.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Atmospheric gravity waves (AGWs) in the mesopause region and medium-scale traveling ionospheric disturbances (MSTIDs) in the thermosphere from 1999 through 2014 were studied by applying a three-dimensional spectral analysis technique to airglow images at wavelengths of 557.7 (emission altitudes: 90-100 km) and 630.0 nm (emission altitudes: 200-300 km) obtained at Rikubetsu (43.5 degrees N, 143.8 degrees E) and Shigaraki (34.8 degrees N, 136.1 degrees E), Japan. To our knowledge, such a long-term multipoint analysis of AGWs and MSTIDs using airglow images has not been reported previously. The propagation direction of mesospheric AGWs seen in 557.7-nm airglow images at both stations was northeastward in summer and southwestward in winter, probably due to wind filtering of these waves by the mesospheric jet. In winter, the propagation direction of AGWs shifted from southwestward to northwestward as time progressed from evening to morning at both stations, which can also be explained by the wind filtering effect. The propagation direction of AGWs changed from southwestward to northeastward at Rikubetsu during a zonal wind reversal at 60 degrees N at 10 hPa, caused by stratospheric sudden warming (SSW). No such a SSW-associated change was identified at Shigaraki, indicating that the effect of SSW wind reversal reached only the Rikubetsu latitudes. For MSTIDs, the major propagation direction was southwestward with a minor northeastward peak for all seasons at both stations. A negative correlation was found between the yearly variation in power spectral density and solar F10.7 flux. This negative correlation can be explained by considering the linear growth rate of the Perkins instability.

DOI： 10.1029/2018JA025585

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On the Solstice Maxima and Azimuth-Dependent Characteristics of the 150-km Echoes Observed Using the Equatorial Atmosphere Radar Reviewed

P. Pavan Chaitanya, A. K. Patra, Y. Otsuka, T. Yokoyama, M. Yamamoto

Journal of Geophysical Research: Space Physics   Vol. 123 ( 8 ) page： 6752 - 6759   2018.8

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DOI： 10.1029/2018JA025491

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Quantum criticality in the metal-superconductor transition of interacting Dirac fermions on a triangular lattice Reviewed

Otsuka Yuichi, Seki Kazuhiro, Sorella Sandro, Yunoki Seiji

PHYSICAL REVIEW B   Vol. 98 ( 3 )   2018.7

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DOI： 10.1103/PhysRevB.98.035126

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Equatorial plasma bubble seeding by MSTIDs in the ionosphere Reviewed

Hisao Takahashi, Cristiano Max Wrasse, Cosme Alexandre Oliveira Barros Figueiredo, Diego Barros, Mangalathayil Ali Abdu, Yuichi Otsuka, Kazuo Shiokawa

Progress in Earth and Planetary Science   Vol. 5 ( 1 )   2018.6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Berlin Heidelberg  

Occurrences of equatorial plasma bubbles (EPBs) and medium-scale traveling ionospheric disturbances (MSTIDs) were studied using GPS satellite data-based total electron content mapping, ionograms, and 630 nm all-sky airglow images observed over the South American continent during the period of 2014–2015. In many cases, we observed a close relationship between the inter-bubble distance and the horizontal wavelength of the MSTIDs. The MSTIDs followed by EPBs occurred primarily in the afternoon to evening period under strong tropospheric convective activities (cold fronts and/or intertropical convergence zones). The close relationship between EPBs and MSTIDs suggests that MSTIDs could be one of the seeding sources of EPBs. [Figure not available: see fulltext.].

DOI： 10.1186/s40645-018-0189-2

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Total Electron Content Observations by Dense Regional and Worldwide International Networks of GNSS

Tsugawa Takuya, Nishioka Michi, Ishii Mamoru, Hozumi Kornyanat, Saito Susumu, Shinbori Atsuki, Otsuka Yuichi, Saito Akinori, Buhari Suhaila M., Abdullah Mardina, Supnithi Pornchai

JOURNAL OF DISASTER RESEARCH   Vol. 13 ( 3 ) page： 535-545   2018.6

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Total Electron Content Observations by Dense Regional and Worldwide International Networks of GNSS Reviewed

Tsugawa Takuya, Nishioka Michi, Ishii Mamoru, Hozumi Kornyanat, Saito Susumu, Shinbori Atsuki, Otsuka Yuichi, Saito Akinori, Buhari Suhaila M, Abdullah Mardina, Supnithi Pornchai

JOURNAL OF DISASTER RESEARCH   Vol. 13 ( 3 ) page： 535 - 545   2018.6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：FUJI TECHNOLOGY PRESS LTD  

Two-dimensional ionospheric total electron content (TEC) maps have been derived from ground- based Global Navigation Satellite System (GNSS) receiver networks and applied to studies of various ionospheric disturbances since the mid-1990s. For the purpose of monitoring and researching ionospheric conditions and ionospheric space weather phenomena, we have developed TEC maps of areas over Japan using the dense GNSS network, GNSS Earth Observation NETwork (GEONET), which consists of about 1300 stations and is operated by the Geospatial Information Authority of Japan (GSI). Currently, we are providing high-resolution, two-dimensional maps of absolute TEC, detrended TEC, rate of TEC change index (ROTI), and loss-of-lock on GPS signal over Japan on a real- time basis. Such high-resolution TEC maps using dense GNSS receiver networks are one of the most effective ways to observe, on a scale of several 100 km to 1000 km, ionospheric variations caused by traveling ionospheric disturbances and/or equatorial plasma bubbles, which can degrade single-frequency and differential GNSS positioning/navigation. We have collected all the available GNSS receiver data in the world to expand the TEC observation area. Currently, however, dense GNSS receiver networks are available in only limited areas, such as Japan, North America, and Europe. To expand the two-dimensional TEC observation with high resolution, we have conducted the Dense Regional and Worldwide International GNSS TEC observation (DRAWING- TEC) project, which is engaged in three activities: (1) standardizing GNSSTEC data, (2) developing a new high-resolution TEC mapping technique, and (3) sharing the standardized TEC data or the information of GNSS receiver network. We have developed a new standardized TEC format, GNSS-TEC EXchange (GTEX), which is included in the Formatted Tables of ITU-R SG 3 Data-banks related to Recommendation ITU-R P. 311. Sharing the GTEX TEC data would be easier than sharing the GPS/GNSS data among those in the international ionospheric researcher community. The DRAWINGTEC project would promote studies of medium-scale ionospheric variations and their effect on GNSS.

DOI： 10.20965/jdr.2018.p0535

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Relationship between day-to-day variability of equatorial plasma bubble activity from GPS scintillation and atmospheric properties from Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) assimilation Reviewed

Mamoru Yamamoto, Yuichi Otsuka, Hidekatsu Jin, Yasunobu Miyoshi

PROGRESS IN EARTH AND PLANETARY SCIENCE   Vol. 5   2018.5

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The relationship between day-to-day variability of equatorial plasma bubbles (EPBs) and the neutral atmosphere is studied. This study is based on the previous study in which the GPS scintillation index and the tropospheric cloud-top temperature are used as proxies for EPB activity and atmospheric perturbations, respectively, and a correlation was found between their day-to-day variations. In this paper, we maintained the same GPS scintillation data but substituted the atmospheric data via an assimilation run of the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). Cross-correlation between the EPB activity and the atmospheric temperature is similar to the results in Ogawa et al. (Earth Planets Space 61: 397-410, 2009). The new findings from our study include (1) an enhanced correlation between the EPB activity and the neutral atmosphere is found in horizontally and vertically large areas, (2) the longitudinal disturbance of atmospheric temperature and wind velocity during the EPB-active days is enhanced, and (3) the enhancement of atmospheric disturbance during the EPB-active days shows a similarity to the characteristics of large-scale wave structures in the ionosphere. These results more clearly support couplings between EPBs and the neutral atmosphere.

DOI： 10.1186/s40645-018-0184-7

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Spatial and temporal characteristics of ionospheric total electron content over Indian equatorial and low-latitude GNSS stations Reviewed

G. Sivavaraprasad, Yuichi Otsuka, Nitin Kumar Tripathi, V Rajesh Chowdhary, D Venkata Ratnam, Mohammed Afzal Khan

2018 Conference on Signal Processing And Communication Engineering Systems, SPACES 2018   Vol. 2018-   page： 105 - 108   2018.3

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

The study and understanding of the intermittent characteristics of equatorial and low latitude ionosphere is crucial for modelling and forecasting the ionosphere and space weather conditions. The performance of space-based navigation systems such as Global Positioning System (GPS) is affected by the sporadic temporal and spatial variations of ionospheric Total Electron Content (TEC). The variability of ionospheric electron density over Indian low latitude sector is difficult to model due to Equatorial Ionization Anomaly (EIA). In this paper, Multi-fractal aspects of the GPS measured TEC is investigated during both high and low solar activity periods of 24th solar cycle. The vertical TEC (VTEC) data sets are obtained from two Indian low latitude stations namely, Bangalore (Geographical Latitude: 13.020 N, Geographical Longitude: 77.57o E), and Lucknow (Geographical Latitude: 26.830 N, Geographical Longitude: 80.92o E) for two year long period 2013 and 2015. The experimental results shows that the respective geographic sites have important scaling differences as well as similarities when their Multi-fractal signatures for VTEC are compared. These differences and similarities are interpreted in terms of the EIA conditions, where this phenomenon is an important source of intermittence due to the presence of the VTEC peaks at ±300 geomagnetic latitudes. During the high solar activity period, the intermittence characteristics of VTEC over EIA region (Lucknow) are relatively more complex than equatorial (Bengaluru) station, whereas during low solar activity period the scenario is reciprocal.

DOI： 10.1109/SPACES.2018.8316326

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Daily and seasonal variations in the linear growth rate of the Rayleigh-Taylor instability in the ionosphere obtained with GAIA Reviewed

Shinagawa, Hiroyuki, Jin, Hidekatsu, Miyoshi, Yasunobu, Fujiwara, Hitoshi, Yokoyama, Tatsuhiro, Otsuka, Yuichi

PROGRESS IN EARTH AND PLANETARY SCIENCE   Vol. 5   2018.3

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The linear growth rates of the Rayleigh-Taylor (R-T) instability in the ionosphere from 2011 to 2013 were obtained with a whole atmosphere-ionosphere coupled model GAIA (ground-to-topside model of atmosphere and ionosphere for aeronomy). The effects of thermospheric dynamics driven by atmospheric waves propagating from below on the R-T growth rate are included in the model by incorporating meteorological reanalysis data in the region below 30 km altitude. The daily maximum R-T growth rates for these periods are compared with the observed occurrence days of the equatorial plasma bubble (EPB) determined by the Equatorial Atmosphere Radar (EAR) and Global Positioning System (GPS) in West Sumatra, Indonesia. We found that a high R-T growth rate tends to correspond to the actual EPB occurrence, suggesting the possibility of predicting EPB occurrences with numerical models.

DOI： 10.1186/s40645-018-0175-8

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Medium-Scale Traveling Ionospheric Disturbances Observed by Detrended Total Electron Content Maps Over Brazil Reviewed

C. A.O.B. Figueiredo, H. Takahashi, C. M. Wrasse, Y. Otsuka, K. Shiokawa, D. Barros

Journal of Geophysical Research: Space Physics   Vol. 123 ( 3 ) page： 2215 - 2227   2018.3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

A ground-based network of Global Navigation Satellite Systems receivers has been used to monitor medium-scale traveling ionospheric disturbances (MSTIDs). MSTIDs were studied using total electron content perturbation maps and keograms over south-southeast of Brazil during the period from December 2012 to February 2016. In total, 826 MSTIDs were observed mainly in daytime, thus presenting median values of horizontal wavelength, period, and horizontal phase velocity of 452 ± 107 km, 24 ± 4 min. and 323 ± 81 m/s, respectively. The direction of propagation varies on the season: during the winter (June–August), the waves preferentially propagated to north-northeast, while in the other seasons the waves propagated to other directions. The anisotropy observed in the MSTID propagation direction could be associated with the region of the gravity wave generation that takes place in the troposphere. We also found that the MSTIDs were observed most frequently during the daytime, between 11 and 15 local time in winter and near to dusk solar terminator (17–19 local time) in the other seasons. Furthermore, the occurrence of MSTIDs was higher in winter. We suggest that atmospheric gravity waves in the thermosphere, mesosphere, and troposphere could play an important role in generating the MSTIDs and the propagation direction may depend on location of the wave sources.

DOI： 10.1002/2017JA025021

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Daytime F-region irregularity triggered by rocket-induced ionospheric hole over low latitude Reviewed

Guozhu Li, Baiqi Ning, M. A. Abdu, Chi Wang, Yuichi Otsuka, Weixing Wan, Jiuhou Lei, Michi Nishioka, Takuya Tsugawa, Lianhuan Hu, Guotao Yang, Chunxiao Yan

Progress in Earth and Planetary Science   Vol. 5 ( 1 )   2018.2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Berlin Heidelberg  

Unexpected daytime F-region irregularities following the appearance of an ionospheric hole have been observed over low latitude. The irregularities developed initially above the F-region peak height (~ 360 km) with a thickness of about 30 km and an east-west extension of more than 200 km around 1057 LT and then expanded upward to 500 km altitude behaving like the equatorial spread-F (ESF) irregularities of the nighttime ionosphere. These daytime F-region irregularities cannot be explained on basis of an earlier suggestion that the F-region irregularities observed during daytime are the continuation of the irregularities initially generated on the previous night. Based on the coincidence, both in space and time, with the appearance of an ionospheric hole, which was generated after the passage of a rocket, we conclude that the daytime F-region irregularities must have been artificially generated locally through a manifestation of plasma instability triggered by the rocket exhaust-induced ionospheric hole over low latitude. [Figure not available: see fulltext.].

DOI： 10.1186/s40645-018-0172-y

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Discovery of 1 Hz Range Modulation of Isolated Proton Aurora at Subauroral Latitudes Reviewed

M. Ozaki, K. Shiokawa, Y. Miyoshi, R. Kataoka, M. Connors, T. Inoue, S. Yagitani, Y. Ebihara, C. W. Jun, R. Nomura, K. Sakaguchi, Y. Otsuka, H. A. Uchida, I. Schofield, D. W. Danskin

Geophysical Research Letters   Vol. 45 ( 3 ) page： 1209 - 1217   2018.2

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Isolated proton aurora (IPA) is a manifestation of the wave-particle interaction visible at subauroral latitudes, with activity on many timescales. We herein present the first observational evidence of rapid luminous modulation of IPA correlated with simultaneously observed Pc1 waves observed on the ground, which are equivalent to the electromagnetic ion cyclotron (EMIC) waves in the magnetosphere. The fastest luminous modulation of IPA was observed in the 1 Hz frequency range, which was twice the frequency of the related Pc1 waves. The time lag between variations of Pc1 wave power and the IPA luminosity suggests that the source regions of IPA are distributed near the magnetic equator, suggesting an EMIC wave-energetic (a few tens of keV) proton or relativistic (MeV or sub-MeV) electron interaction. The generation mechanism of this 1 Hz luminous modulation remains an open issue, but this study supports the importance of nonlinear pitch angle scattering via wave-particle interactions.

DOI： 10.1002/2017GL076486

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Observations of Ultrawideband Signals in GPS TEC Variations over Europe during Solar Eclipse Reviewed

Panasenko Sergii V, Chernogor Leonid F, Lazorenko Oleg V, Otsuka Yuichi, van de Kamp Max

2018 9TH INTERNATIONAL CONFERENCE ON ULTRAWIDEBAND AND ULTRASHORT IMPULSE SIGNALS (UWBUSIS)     page： 115 - 118   2018

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Temporal and spatial variations of mid-latitude ionospheric trough during a geomagnetic storm based on global GNSS-TEC and Arase satellite observations Reviewed

2018 2ND URSI ATLANTIC RADIO SCIENCE MEETING (AT-RASC)     2018

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Web of Science

First Study on the Occurrence Frequency of Equatorial Plasma Bubbles over West Africa Using an All-Sky Airglow Imager and GNSS Receivers Reviewed

Daniel Okoh, Babatunde Rabiu, Kazuo Shiokawa, Yuichi Otsuka, Bolaji Segun, Elijah Falayi, Sylvester Onwuneme, Rafiat Kaka

Journal of Geophysical Research: Space Physics   Vol. 122 ( 12 ) page： 12430 - 12444   2017.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

This is the first paper that reports the occurrence frequency of equatorial plasma bubbles and their dependences of local time, season, and geomagnetic activity based on airglow imaging observations at West Africa. The all-sky imager, situated in Abuja (Geographic: 8.99°N, 7.38°E
Geomagnetic: 1.60°S), has a 180° fisheye view covering almost the entire airspace of Nigeria. Plasma bubbles are observed for 70 nights of the 147 clear-sky nights from 9 June 2015 to 31 January 2017. Differences between nighttime and daytime ROTIs were also computed as a proxy of plasma bubbles using Global Navigation Satellite Systems (GNSS) receivers within the coverage of the all-sky imager. Most plasma bubble occurrences are found during equinoxes and least occurrences during solstices. The occurrence rate of plasma bubbles was highest around local midnight and lower for hours farther away. Most of the postmidnight plasma bubbles were observed around the months of December to March, a period that coincides with the harmattan period in Nigeria. The on/off status of plasma bubble in airglow and GNSS observations were in agreement for 67.2% of the total 768 h, while we suggest several reasons responsible for the remaining 32.8% when the airglow and GNSS bubble status are inconsistent. A majority of the plasma bubbles were observed under relatively quiet geomagnetic conditions (Dst ≥ −40 and Kp ≤ 3), but there was no significant pattern observed in the occurrence rate of plasma bubbles as a function of geomagnetic activity. We suggest that geomagnetic activities could have either suppressed or promoted the occurrence of plasma bubbles.

DOI： 10.1002/2017JA024602

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Ground-based instruments of the PWING project to investigate dynamics of the inner magnetosphere at subauroral latitudes as a part of the ERG-ground coordinated observation network Reviewed

Kazuo Shiokawa, Yasuo Katoh, Yoshiyuki Hamaguchi, Yuka Yamamoto, Takumi Adachi, Mitsunori Ozaki, Shin-Ichiro Oyama, Masahito Nose, Tsutomu Nagatsuma, Yoshimasa Tanaka, Yuichi Otsuka, Yoshizumi Miyoshi, Ryuho Kataoka, Yuki Takagi, Yuhei Takeshita, Atsuki Shinbori, Satoshi Kurita, Tomoaki Hori, Nozomu Nishitani, Iku Shinohara, Fuminori Tsuchiya, Yuki Obana, Shin Suzuki, Naoko Takahashi, Kanako Seki, Akira Kadokura, Keisuke Hosokawa, Yasunobu Ogawa, Martin Connors, J. Michael Ruohoniemi, Mark Engebretson, Esa Turunen, Thomas Ulich, Jyrki Manninen, Tero Raita, Antti Kero, Arto Oksanen, Marko Back, Kirsti Kauristie, Jyrki Mattanen, Dmitry Baishev, Vladimir Kurkin, Alexey Oinats, Alexander Pashinin, Roman Vasilyev, Ravil Rakhmatulin, William Bristow, Marty Karjala

EARTH PLANETS AND SPACE   Vol. 69   2017.11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

The plasmas (electrons and ions) in the inner magnetosphere have wide energy ranges from electron volts to mega-electron volts (MeV). These plasmas rotate around the Earth longitudinally due to the gradient and curvature of the geomagnetic field and by the co-rotation motion with timescales from several tens of hours to less than 10 min. They interact with plasma waves at frequencies of mHz to kHz mainly in the equatorial plane of the magnetosphere, obtain energies up to MeV, and are lost into the ionosphere. In order to provide the global distribution and quantitative evaluation of the dynamical variation of these plasmas and waves in the inner magnetosphere, the PWING project (study of dynamical variation of particles and waves in the inner magnetosphere using ground-based network observations, http://www.isee.nagoya-u.ac.jp/dimr/PWING/) has been carried out since April 2016. This paper describes the stations and instrumentation of the PWING project. We operate all-sky airglow/aurora imagers, 64-Hz sampling induction magnetometers, 40-kHz sampling loop antennas, and 64-Hz sampling riometers at eight stations at subauroral latitudes (similar to 60 degrees geomagnetic latitude) in the northern hemisphere, as well as 100-Hz sampling EMCCD cameras at three stations. These stations are distributed longitudinally in Canada, Iceland, Finland, Russia, and Alaska to obtain the longitudinal distribution of plasmas and waves in the inner magnetosphere. This PWING longitudinal network has been developed as a part of the ERG (Arase)-ground coordinated observation network. The ERG (Arase) satellite was launched on December 20, 2016, and has been in full operation since March 2017. We will combine these ground network observations with the ERG (Arase) satellite and global modeling studies. These comprehensive datasets will contribute to the investigation of dynamical variation of particles and waves in the inner magnetosphere, which is one of the most important research topics in recent space physics, and the outcome of our research will improve safe and secure use of geospace around the Earth.

DOI： 10.1186/s40623-017-0745-9

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The Solar Flux Dependence of Ionospheric 150 km Radar Echoes and Implications Reviewed

A. K. Patra, P. Pavan Chaitanya, J.‐P. St.‐Maurice, Y. Otsuka, T. Yokoyama, M. Yamamoto

Geophysical Research Letters   Vol. 44 ( 22 ) page： 11257 - 11264   2017.11

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DOI： 10.1002/2017GL074678

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On the effect of thermospheric neutral winds on post-midnight field-aligned irregularities at low latitudes Reviewed

Tam Dao, Yuichi Otsuka, Kazuo Shiokawa, Michi Nishioka, Mamoru Yamamoto, Suhaila M. Buhari, Mardina Abdullah, Asnawi Husin

2017 32nd General Assembly and Scientific Symposium of the International Union of Radio Science, URSI GASS 2017   Vol. 2017-   page： 1 - 4   2017.11

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We investigated a post-midnight Field-Aligned Irregularities (FAIs) event observed with the Equatorial Atmosphere Radar (EAR) at Kototabang (0.2°S, 100.3°E, dip lat. 10.4°S) in Indonesia on the night of 9 July 2010, using a comprehensive dataset of both neutral and plasma parameters. We compared FAI echoes collocated with 630 nm airglow depletion detected by an all-sky imager. The thermospheric neutral winds and temperatures obtained by a Fabry-Perot interferometer at Kototabang and the altitudes of F-layer (h'F) observed with ionosondes at Kototabang, Chiang Mai, and Chumphon were also examined. We found that the 3-m scale post-midnight FAIs occurred within plasma bubbles. The convergence of the equatorward neutral winds happened in this particular event related to midnight temperature maximum (MTM) and that the equatorward winds in both northern and southern hemispheres could be responsible for the growth of plasma bubbles around midnight. The uplift of F-layer at low latitudes could increase the growth rate of the Rayleigh-Taylor instability. Eastward electric currents driven by the equatorward winds could also contribute to the generation of the irregularities at post-midnight.

DOI： 10.23919/URSIGASS.2017.8105106

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Geomagnetically conjugate observations of ionospheric and thermospheric variations accompanied by a midnight brightness wave at low latitudes (vol 69, 112, 2017) Reviewed

D. Fukushima, K. Shiokawa, Y. Otsuka, M. Kubota, T. Yokoyama, M. Nishioka, S. Komonjinda, C. Y. Yatini

EARTH PLANETS AND SPACE   Vol. 69   2017.11

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DOI： 10.1186/s40623-017-0729-9

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Characteristics of Seasonal Variation and Solar Activity Dependence of the Geomagnetic Solar Quiet Daily Variation Reviewed

Atsuki Shinbori, Yukinobu Koyama, Masahito Nosé, Tomoaki Hori, Yuichi Otsuka

Journal of Geophysical Research: Space Physics   Vol. 122 ( 10 ) page： 10796 - 10810   2017.10

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Characteristics of seasonal variation and solar activity dependence of the X and Y components of the geomagnetic solar quiet (Sq) daily variation at Memambetsu in midlatitudes and Guam near the equator have been investigated using long-term geomagnetic field data with 1 h time resolution from 1957 to 2016. The monthly mean Sq variation in the X and Y components (Sq-X and Sq-Y) shows a clear seasonal variation and solar activity dependence. The amplitude of seasonal variation increases significantly during high solar activities and is proportional to the solar F10.7 index. The pattern of the seasonal variation is quite different between Sq-X and Sq-Y. The result of the correlation analysis between the solar F10.7 index and the Sq-X and Sq-Y shows an almost linear relationship, but the slope of the linear fitted line varies as a function of local time and month. This implies that the sensitivity of Sq-X and Sq-Y to the solar activity is different for different local times and seasons. The pattern of the local time and seasonal variations of Sq-Y at Guam shows good agreement with that of a magnetic field produced by interhemispheric field-aligned currents (FACs), which flow from the summer to winter hemispheres in the dawn and dusk sectors and from the winter to summer hemispheres in the prenoon to afternoon sectors. The direction of the interhemispheric FAC in the dusk sector is opposite to the concept of Fukushima's model.

DOI： 10.1002/2017JA024342

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Geomagnetically conjugate observations of ionospheric and thermospheric variations accompanied by a midnight brightness wave at low latitudes Reviewed

D. Fukushima, K. Shiokawa, Y. Otsuka, M. Kubota, T. Yokoyama, M. Nishioka, S. Komonjinda, C. Y. Yatini

EARTH PLANETS AND SPACE   Vol. 69   2017.8

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We conducted geomagnetically conjugate observations of 630-nm airglow for a midnight brightness wave (MBW) at Kototabang, Indonesia [geomagnetic latitude (MLAT): 10.0 degrees S], and Chiang Mai, Thailand (MLAT: 8.9 degrees N), which are geomagnetically conjugate points at low latitudes. An airglow enhancement that was considered to be an MBW was observed in OI (630-nm) airglow images at Kototabang around local midnight from 2240 to 2430 LT on February 7, 2011. This MBW propagated south-southwestward, which is geomagnetically poleward, at a velocity of 290 m/s. However, a similar wave was not observed in the 630-nm airglow images at Chiang Mai. This is the first evidence of an MBW that does not have geomagnetic conjugacy, which also implies generation of MBW only in one side of the hemisphere from the equator. We simultaneously observed thermospheric neutral winds observed by a co-located Fabry-Perot interferometer at Kototabang. The observed meridional winds turned from northward (geomagnetically equatorward) to southward (geomagnetically poleward) just before the wave was observed. This indicates that the observed MBW was generated by the poleward winds which push ionospheric plasma down along geomagnetic field lines, thereby increasing the 630-nm airglow intensity. The bottomside ionospheric heights observed by ionosondes rapidly decreased at Kototabang and slightly increased at Chiang Mai. We suggest that the polarization electric field inside the observed MBW is projected to the northern hemisphere, causing the small height increase observed at Chiang Mai. This implies that electromagnetic coupling between hemispheres can occur even though the original disturbance is caused purely by the neutral wind.

DOI： 10.1186/s40623-017-0698-z

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Daytime zonal drifts in the ionospheric 150 km and E regions estimated using EAR observations Reviewed

P. Pavan Chaitanya, A. K. Patra, Y. Otsuka, T. Yokoyama, M. Yamamoto, R. A. Stoneback, R. A. Heelis

Journal of Geophysical Research: Space Physics   Vol. 122 ( 8 ) page： 9045 - 9055   2017.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley-Blackwell  

DOI： 10.1002/2017JA024589

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Equinoctial asymmetry in the zonal distribution of scintillation as observed by GPS receivers in Indonesia Reviewed

P. Abadi, Y. Otsuka, K. Shiokawa, A. Husin, Huixin Liu, S. Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 122 ( 8 ) page： 8947 - 8958   2017.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We investigate the azimuthal distribution of amplitude scintillation observed by Global Positioning System (GPS) ground receivers at Pontianak (0.0 degrees S, 109.3 degrees E; magnetic latitude: 9.8 degrees S) and Bandung (6.9 degrees S, 107.6 degrees E; magnetic latitude: 16.7 degrees S) in Indonesia in March and September from 2011 to 2015. The scintillation is found to occur more to the west than to the east in March at both stations, whereas no such zonal difference is found in September. We also analyze the zonal scintillation drift as estimated using three closely spaced single-frequency GPS receivers at Kototabang (0.2 degrees S, 100.3 degrees E; magnetic latitude: 9.9 degrees S) in Indonesia during 2003-2015 and the zonal thermospheric neutral wind as measured by the CHAMP satellite at longitudes of 90 degrees-120 degrees E during 2001-2008. We find that the velocities of both the zonal scintillation drift and the neutral wind decrease with increasing latitudes. Interestingly, the latitudinal gradients of both the zonal scintillation drift and the neutral wind are steeper in March than in September. These steeper March gradients may be responsible for the increased westward altitudinal and latitudinal tilting of plasma bubbles in March. This equinoctial asymmetry could be responsible for the observed westward bias in scintillation in March, because the scintillation is more likely to occur when radio waves pass through longer lengths of plasma irregularities in the plasma bubbles.

DOI： 10.1002/2017JA024146

Web of Science

Sixteen year variation of horizontal phase velocity and propagation direction of mesospheric and thermospheric waves in airglow images at Shigaraki, Japan Reviewed

D. Takeo, K. Shiokawa, H. Fujinami, Y. Otsuka, T. S. Matsuda, M. K. Ejiri, T. Nakamura, M. Yamamoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 122 ( 8 ) page： 8770 - 8780   2017.8

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We analyzed the horizontal phase velocity of gravity waves and medium-scale traveling ionospheric disturbances (MSTIDs) by using the three-dimensional fast Fourier transform method developed by Matsuda et al. (2014) for 557.7 nm (altitude: 90-100 km) and 630.0 nm (altitude: 200- 300 km) airglow images obtained at Shigaraki MU Observatory (34.8 degrees N, 136.1 degrees E, dip angle: 49 degrees) over similar to 16 years from 16 March 1999 to 20 February 2015. The analysis of 557.7 nm airglow images shows clear seasonal variation of the propagation direction of gravity waves in the mesopause region. In spring, summer, fall, and winter, the peak directions are northeastward, northeastward, northwestward, and southwestward, respectively. The difference in east-west propagation direction between summer and winter is probably caused by the wind filtering effect due to the zonal mesospheric jet. Comparison with tropospheric reanalysis data shows that the difference in north-south propagation direction between summer and winter is caused by differences in the latitudinal location of wave sources due to convective activity in the troposphere relative to Shigaraki. The analysis of 630.0 nm airglow images shows that the propagation direction of MSTIDs is mainly southwestward with a minor northeastward component throughout the 16 years. A clear negative correlation is seen between the yearly power spectral density of MSTIDs and F-10.7 solar flux. This negative correlation with solar activity may be explained by the linear growth rate of the Perkins instability and secondary wave generation of gravity waves in the thermosphere.

DOI： 10.1002/2017JA023919

Web of Science

Sixteen-year variation of horizontal phase velocity and propagation direction of mesospheric and thermospheric waves in airglow images at Shigaraki Reviewed

Takeo, D, K. Shiokawa, H. Fujinami, Y. Otsuka, T. S. Matsuda, M. K. Ejiri, T. Nakamura, M. Yamamoto

Journal of Geophysical Research -Atmospheres   Vol. 122   page： 8770-8780   2017.8

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DOI： 10.0002/2017JA023919

Coordinated observations of postmidnight irregularities and thermospheric neutral winds and temperatures at low latitudes Reviewed

Tam Dao, Yuichi Otsuka, Kazuo Shiokawa, Michi Nishioka, Mamoru Yamamoto, Suhaila M. Buhari, Mardina Abdullah, Asnawi Husin

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 122 ( 7 ) page： 7504 - 7518   2017.7

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We investigated a postmidnight field-aligned irregularity (FAI) event observed with the Equatorial Atmosphere Radar at Kototabang (0.2 degrees S, 100.3 degrees E, dip latitude 10.4 degrees S) in Indonesia on the night of 9 July 2010 using a comprehensive data set of both neutral and plasma parameters. We examined the rate of total electron content change index (ROTI) obtained from GPS receivers in Southeast Asia, airglow images detected by an all-sky imager, and thermospheric neutral winds and temperatures obtained by a Fabry-Perot interferometer at Kototabang. Altitudes of the F layer (h'F) observed by ionosondes at Kototabang, Chiang Mai, and Chumphon were also surveyed. We found that the postmidnight FAIs occurred within plasma bubbles and coincided with kilometer-scale plasma density irregularities. We also observed an enhancement of the magnetically equatorward thermospheric neutral wind at the same time as the increase of h'F at low-latitude stations, but h'F at a station near the magnetic equator remained invariant. Simultaneously, a magnetically equatorward gradient of thermospheric temperature was identified at Kototabang. The convergence of equatorward neutral winds from the Northern and Southern Hemispheres could be associated with a midnight temperature maximum occurring around the magnetic equator. Equatorward neutral winds can uplift the F layer at low latitudes and increase the growth rate of Rayleigh-Taylor instabilities, causing more rapid extension of plasma bubbles. The equatorward winds in both hemispheres also intensify the eastward Pedersen current, so a large polarization electric field generated in the plasma bubble might play an important role in the generation of postmidnight FAIs.

DOI： 10.1002/2017JA024048

Web of Science

Detection of Plasma Bubble in Ionosphere using GPS Receivers in Southeast Asia Reviewed

Suhaila M. Buhari, Mardina Abdullah, Yuichi Otsuka, Tatsuhiro Yokoyama, Michi Nishioka, Alina Marie Hasbi, Takuya Tsugawa

SAINS MALAYSIANA   Vol. 46 ( 6 ) page： 879 - 885   2017.6

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Language：Malay   Publishing type：Research paper (scientific journal)   Publisher：UNIV KEBANGSAAN MALAYSIA  

The equatorial ionosphere most often shows a nighttime plasma irregularity that is commonly referred as equatorial plasma bubble (PBB). The occurrence of PBB could cause rapid fluctuations in the amplitude and phase of the propagation radio signals and crucial to communication and navigation systems. The PBB normally occur successively where one structure rising after another during the sunset time. However, the onset time and location of the PBB are ubiquitous because the seed of the initial perturbation is not completely understood. Although various observation systems have been developed to capture the EPB, each of the measurement is limited with space and time resolution. This study aims to observe 2D structure of the PBB using high-density GPS receivers in Southeast Asia. GPS data was collected from 127 GPS receivers in Southeast Asia with the spacing distances of 30-120 km from each other. Total electron content (TEC) was derived from the difference between two signals from each GPS satellite. The signature of the PBB was detected using rate of TEC change index (ROTI) for all the available satellites to receiver paths. The 2D structure of the PBB was obtained by averaging GPS ROTI into 0.45 degrees latitude x 0.45 degrees longitude grid and projected at 300 km altitude. A case study on the night of 18 Mac 2011 showed the births of six PBB structures during the passage of the solar terminator along the 95 degrees E to 120 degrees E longitude. The separation distance between the PBB structures varied from 300 to 600 km. The separation distance between the EPB structures play an important role in determining the source of the seeding mechanism that believed in a form of wavelike structure.

DOI： 10.17576/jsm-2017-4606-06

Web of Science

Measurement of thermospheric temperatures using OMTI Fabry-Perot interferometers with 70-mm etalon Reviewed

Y. Nakamura, K. Shiokawa, Y. Otsuka, S. Oyama, S. Nozawa, T. Komolmis, S. Komonjida, Dave Neudegg, Colin Yuile, J. Meriwether, H. Shinagawa, H. Jin

EARTH PLANETS AND SPACE   Vol. 69   2017.4

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Fabry-Perot interferometer (FPI) is an instrument that can measure the temperature and wind velocity of the thermosphere through observations of airglow emission at a wavelength of 630.0 nm. The Solar-Terrestrial Environment Laboratory/Institute for Space-Earth Environmental Research, Nagoya University, has recently developed four new ground-based FPIs. One of those FPIs, possessing a large-aperture etalon (diameter: 116 mm), was installed in Tromso (FP01), Norway, in 2009. The other three small FPIs, using 70-mm-diameter etalons, were installed in Thailand (FP02), Indonesia (FP03) and Australia (FP04) in 2010-2011. They use highly sensitive cooled-CCD cameras with 1024 x 1024 pixels to obtain interference fringes. However, appropriate temperature has not been obtained from the interference fringes using these new small-aperture FPIs. In the present study we improved the analysis procedure of temperature determination using these FPIs. Each of FPIs measures north, south, east and west directions repeatedly by rotating two mirrors mounted on top of the FPI. We estimated center pixel of laser fringe and airglow fringes for each direction and found significant differences in the center pixel locations (a few pixels) among the measurement directions. These differences are considered to be caused by movement of the scanning mirror on the top of the optics, resulting in mechanical distortion of the optics body. By calculating the fringe center separately for each direction, we could correct these center pixel variations and determine the temperature with random errors of 10-40 K. This new method was employed to the all measurements from four FPIs after 2009 and provided temperatures with reasonably small errors. However, we found that temperatures below 400 K were obtained associated with weak airglow intensities and concluded using a model calculation that they are due to contamination of OH line emissions in the upper mesosphere. By defining an appropriate threshold of the fringe peak count, we successfully eliminated these unrealistic temperature values, and the corrected temperature values became comparable to those provided by the MSIS-90E and GAIA models.

DOI： 10.1186/s40623-017-0643-1

Web of Science

Large-scale traveling ionospheric disturbances observed by GPS dTEC maps over North and South America on Saint Patrick's Day storm in 2015 Reviewed

C. A. O. B. Figueiredo, C. M. Wrasse, H. Takahashi, Y. Otsuka, K. Shiokawa, D. Barros

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 122 ( 4 ) page： 4755 - 4763   2017.4

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Large-scale traveling ionospheric disturbances (LSTIDs) were detected in both Northern and Southern Hemispheres over American sector during the geomagnetic storm on 17-18 March 2015, also known as the Saint Patrick's Day storm. Detrended total electronic content (dTEC) maps were made using dense GNSS network receiver data. The retrieved LSTIDs showed wavelengths of 1000 to 2000 km, phase velocity of similar to 300-1000 m/s, and period of similar to 30-50 min. Among them, three couples of LSTIDs were observed propagating from the polar regions to low latitudes. Two wave events observed in daytime showed the propagation direction of southwest in the Northern Hemisphere and northeast in the Southern Hemisphere, which means an asymmetric propagation against the geographic equator. The other wave event observed during the evening hour showed symmetric propagation direction, i.e., southwest in the Northern Hemisphere and northwest in the Southern Hemisphere, whereas their wavelength and phase velocity are significantly different between NH and SH. These observations indicate that the two groups of LSTID have different propagation conditions from polar to low-latitude regions. The observed asymmetric/symmetric propagation forms suggest asymmetric/symmetric auroral current activity between the northern and southern polar regions.

DOI： 10.1002/2016JA023417

Web of Science

Climatology of successive equatorial plasma bubbles observed by GPS ROTI over Malaysia Reviewed

S. M. Buhari, M. Abdullah, T. Yokoyama, Y. Otsuka, M. Nishioka, A. M. Hasbi, S. A. Bahari, T. Tsugawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 122 ( 2 ) page： 2174 - 2184   2017.2

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The occurrence rate of the equatorial plasma bubble (EPB) with season, solar activity, and geomagnetic conditions are investigated using long-term data sets of Malaysia Real-Time Kinematics Network (MyRTKnet) from 2008 to 2013. The rate of TEC (total electron content) change index (ROTI) in 5 min was derived from MyRTKnet data to detect the EPB with scale sizes around tens of kilometers. Then, the daily east-west cross sections of 2-D ROTI maps were used to examine the EPB features over 100 degrees E-119 degrees E longitudes. The EPBs tend to occur successively in one night along the observational coverage of MyRTKnet during equinoxes in high solar activity years. The perturbations in a form of wavelike structures along the observed longitudes might be responsible for the development of successive EPBs due to high growth rate of the Rayleigh-Taylor instability (RTI) process. On the contrary, the occurrence of successive EPBs is infrequent and the occurrence day of EPB remains active during equinoctial months in low solar activity years. The small growth rate of the RTI process during low solar activity years might require a strong seed perturbation to generate the EPB structure. The occurrence probability of the EPB was found to be similar during quiet and disturbed geomagnetic conditions. The results imply that the strong perturbations play an important role in the development of the EPB in low solar activity years. Nonetheless, the high growth rate of the RTI could cause the successive occurrence of the EPB in high solar activity years.

DOI： 10.1002/2016JA023202

Web of Science

STATISTICAL ANALYSIS OF THE OCCURRENCES OF MSTIDs OBSERVED BY ALL-SKY IMAGER IN LOW MAGNETIC LATITUDE Reviewed

2017 IEEE INTERNATIONAL GEOSCIENCE AND REMOTE SENSING SYMPOSIUM (IGARSS)     page： 4425 - 4428   2017

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The first long-term all-sky imager observation of lunar sodium tail Reviewed

Masaki N. Nishino, Kazuo Shiokawa, Yuichi Otsuka

ICARUS   Vol. 280   page： 199 - 204   2016.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

The Moon possesses a long tail of neutral sodium atoms that are emitted from the lunar surface and transported anti-sunward by the solar radiation pressure. Since the earth crosses the lunar sodium tail for a few days around the new moon, the resonant light emission from sodium atoms can be detected from the ground. Here we show the first long-term (16 years) observation of the lunar sodium tail, using an all-sky imager at Shigaraki Observatory (35 degrees N, 136 degrees E), Japan. We have surveyed our database of all-sky sodium images at a wavelength of 5893 nm to find more than 20 events in which a bright spot emerges around the anti-lunar point during the new moon periods. We could not find any clear correlation between the sodium brightness and solar wind parameters (density, speed, dynamic pressure, and F10.7 index). In particular, no enhancement of the sodium spot brightness is detected even under very high density solar wind conditions (70 cm(-3); an order-of-magnitude higher than usual), which means that solar wind sputtering is not a principal mechanism of the formation of the lunar sodium tail. (C) 2016 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.icarus.2016.08.004

Web of Science

Three years of concentric gravity wave variability in the mesopause as observed by IMAP/VISI Reviewed

S. Perwitasari, T. Sakanoi, T. Nakamura, M. K. Ejiri, M. Tsutsumi, Y. Tomikawa, Y. Otsuka, A. Yamazaki, A. Saito

GEOPHYSICAL RESEARCH LETTERS   Vol. 43 ( 22 ) page： 11528 - 11535   2016.11

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We report a statistical study on concentric gravity waves (CGWs) in the mesopause (similar to 95 km) using 3 years nightglow data obtained by Ionosphere, Mesosphere, upper Atmosphere and Plasmasphere/Visible and near-Infrared Spectral Imager. The 235 CGWs events were found with horizontal wavelength ranging from 40 to 250 km and maximum radius of 200 to 3000 km. The latitudinal distribution of the CGWs centers had peaks in mid latitude (40 degrees N and 40 degrees S) and minimum at low latitudes (10 degrees S). More events were found in the summer hemisphere midlatitudes, with a rapid transition between northern and Southern Hemisphere around the equinoxes. The occurrence probability was significantly higher during nonsolstice months (February-May and August-November) than solstice months (June-July and December-January), suggesting that there was a little breaking or critical level absorption so the waves could reach the mesopause more often during these periods. The global distribution showed several preferable regions but very few events over tropical convective regions.

DOI： 10.1002/2016GL071511

Web of Science

Ionospheric TEC Weather Map Over South America Reviewed

H. Takahashi, C. M. Wrasse, C. M. Denardini, M. B. Padua, E. R. de Paula, S. M. A. Costa, Y. Otsuka, K. Shiokawa, J. F. Galera Monico, A. Ivo, N. Sant'Anna

SPACE WEATHER-THE INTERNATIONAL JOURNAL OF RESEARCH AND APPLICATIONS   Vol. 14 ( 11 ) page： 937 - 949   2016.11

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Ionospheric weather maps using the total electron content (TEC) monitored by ground-based Global Navigation Satellite Systems (GNSS) receivers over South American continent, TECMAP, have been operationally produced by Instituto Nacional de Pesquisas Espaciais's Space Weather Study and Monitoring Program (Estudo e Monitoramento Brasileiro de Clima Especial) since 2013. In order to cover the whole continent, four GNSS receiver networks, (Rede Brasileiro de Monitoramento Continuo) RBMC/Brazilian Institute for Geography and Statistics, Low-latitude Ionospheric Sensor Network, International GNSS Service, and Red Argentina de Monitoreo Satelital Continuo, in total similar to 140 sites, have been used. TECMAPs with a time resolution of 10min are produced in 12h time delay. Spatial resolution of the map is rather low, varying between 50 and 500km depending on the density of the observation points. Large day-to-day variabilities of the equatorial ionization anomaly have been observed. Spatial gradient of TEC from the anomaly trough (total electron content unit, 1TECU=10(16)elm(-2) (TECU) &lt;10) to the crest region (TECU&gt;80) causes a large ionospheric range delay in the GNSS positioning system. Ionospheric plasma bubbles, their seeding and development, could be monitored. This plasma density (spatial and temporal) variability causes not only the GNSS-based positioning error but also radio wave scintillations. Monitoring of these phenomena by TEC mapping becomes an important issue for space weather concern for high-technology positioning system and telecommunication.

DOI： 10.1002/2016SW001474

Web of Science

On the fresh development of equatorial plasma bubbles around the midnight hours of June solstice Reviewed

K. K. Ajith, S. Tulasi Ram, M. Yamamoto, Y. Otsuka, K. Niranjan

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 121 ( 9 ) page： 9051 - 9062   2016.9

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Using the 47MHz Equatorial Atmosphere Radar (EAR) at Kototabang, Indonesia, the nocturnal evolution of equatorial plasma bubbles (EPBs) was examined during the moderate solar activity years 2011-2012. While the evolution of EPBs was mostly (86%) confined to post sunset hours (1900-2100LT) during equinoxes, in contrast, the majority of EPBs (similar to 71%) in June solstice found evolve around the midnight hours (2200-0300LT). The mechanisms behind the fresh evolution of summer time midnight EPBs were investigated, for the first time, through SAMI2 model simulations with a realistic input of background ExB drift variation derived from CINDI IVM on board C/NOFS satellite. The term-by-term analysis of linear growth rate of RT instability indicates that the formation of high flux tube electron content height gradient (K-F) (steep vertical gradient) region at higher altitudes is the key factor for the enhanced growth rate of RT instability. The responsible factors are discussed in light of relatively weak westward zonal electric field in the presence of equatorward neutral wind and bottomside recombination around the midnight hours of June solstice. The effects of neutral winds and weak westward electric fields on the uplift of equatorial F layer were examined separately using controlled SAMI2 simulations. The results indicate that relatively larger linear growth rate is more likely to occur around midnight during June solstice because of relatively weak westward electric field than other local times in the presence of equatorward meridional wind.

DOI： 10.1002/2016JA023024

Web of Science

Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes Reviewed

M. Ozaki, K. Shiokawa, Y. Miyoshi, R. Kataoka, S. Yagitani, T. Inoue, Y. Ebihara, C. -W Jun, R. Nomura, K. Sakaguchi, Y. Otsuka, M. Shoji, I. Schofield, M. Connors, V. K. Jordanova

GEOPHYSICAL RESEARCH LETTERS   Vol. 43 ( 15 ) page： 7859 - 7866   2016.8

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To understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave-particle interactions at subauroral latitudes, high-time-resolution (1/8s) images of proton-induced N-2(+) emissions were recorded using a new electron multiplying charge-coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising-tone elements with a spacing for each element of 100s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensity showed a similar repetition period of 100s and an unpredicted fast modulation of a few tens of seconds. These results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator.

DOI： 10.1002/2016GL070008

Web of Science

Enhanced ionospheric plasma bubble generation in more active ITCZ Reviewed

Guozhu Li, Yuichi Otsuka, Baiqi Ning, M. A. Abdu, M. Yamamoto, Weixing Wan, Libo Liu, Prayitno Abadi

GEOPHYSICAL RESEARCH LETTERS   Vol. 43 ( 6 ) page： 2389 - 2395   2016.3

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A close link between the atmospheric Intertropical Convergence Zone (ITCZ) and ionospheric plasma bubble has been proposed since the last century. But this relationship has often appeared to be less than convincing due to the simultaneous roles played by several other factors in shaping the global distribution of ionospheric bubbles. From simultaneous collaborative radar multibeam steering measurements at Kototabang (0.2 degrees S, 100.3 degrees E) and Sanya (18.4 degrees N, 109.6 degrees E), conducted during September-October of 2012 and 2013, we find that the total numbers of nights with bubble (i.e., occurrence rates) at the two closely located longitudes (Kototabang and Sanya) are comparable. But interestingly, the total number of nights with locally generated bubble (i.e., generation rate) over Kototabang is clearly more than that over Sanya. Further analysis reveals that a more active ITCZ is situated around the longitude of Kototabang. We surmise that the enhanced ionospheric bubble generation at Kototabang longitude could be caused by a higher gravity wave activity associated with the more active ITCZ.

DOI： 10.1002/2016GL068145

Web of Science

Altitude development of postmidnight F region field-aligned irregularities observed using Equatorial Atmosphere Radar in Indonesia Reviewed

Tam Dao, Yuichi Otsuka, Kazuo Shiokawa, S. Tulasi Ram, Mamoru Yamamoto

GEOPHYSICAL RESEARCH LETTERS   Vol. 43 ( 3 ) page： 1015 - 1022   2016.2

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For the first time, vertical rise velocities of postmidnight field-aligned irregularities (FAIs) at low geomagnetic latitudes have been examined near the June solstice by using two-dimensional maps of F region FAI echoes observed with the Equatorial Atmosphere Radar in Indonesia for 3years starting in May 2010. We found 15 freshly growing FAIs at postmidnight between May and August during the 3years. The rise velocities of FAIs are smaller at postmidnight than at postsunset, and most postmidnight FAIs do not exceed an altitude of 450km. Based on the rise velocities, a lower limit for the creation time of the postmidnight FAIs is estimated to be between 21:30LT and 02:00LT for 14 of the 15 events, indicating that this class of FAIs is distinct from the postsunset FAIs.

DOI： 10.1002/2015GL067432

Web of Science

Pulsating proton aurora caused by rising tone Pc1 waves Reviewed

R. Nomura, K. Shiokawa, Y. Omura, Y. Ebihara, Y. Miyoshi, K. Sakaguchi, Y. Otsuka, M. Connors

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 121 ( 2 ) page： 1608 - 1618   2016.2

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We found rising tone emissions with a dispersion of approximate to 1Hz per several tens of seconds in the dynamic spectrum of a Pc1 geomagnetic pulsation (Pc1) observed on the ground. These Pc1 rising tones were successively observed over approximate to 30min from 0250UT on 14 October 2006 by an induction magnetometer at Athabasca, Canada (54.7 degrees N, 246.7 degrees E, magnetic latitude 61.7 degrees N). Simultaneously, a Time History of Events and Macroscale Interactions during Substorms panchromatic (THEMIS) all-sky camera detected pulsations of an isolated proton aurora with a period of several tens of seconds, approximate to 10% variations in intensity, and fine structures of 3 degrees in magnetic longitudes. The pulsations of the proton aurora close to the zenith of ATH have one-to-one correspondences with the Pc1 rising tones. This suggests that these rising tones scatter magnetospheric protons intermittently at the equatorial region. The radial motion of the magnetospheric source, of which the isolated proton aurora is a projection, can explain the central frequency increase of Pc1, but not the shorter period (tens of seconds) frequency increase of approximate to 1Hz in Pc1 rising tones. We suggest that EMIC-triggered emissions generate the frequency increase of Pc1 rising tones on the ground and that they also cause the Pc1 pearl structure, which has a similar characteristic time.

DOI： 10.1002/2015JA021681

Web of Science

Duskside enhancement of equatorial zonal electric field response to convection electric fields during the St. Patrick's Day storm on 17 March 2015 Reviewed

S. Tulasi Ram, T. Yokoyama, Y. Otsuka, K. Shiokawa, S. Sripathi, B. Veenadhari, R. Heelis, K. K. Ajith, V. S. Gowtam, S. Gurubaran, P. Supnithi, M. Le Huy

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 121 ( 1 ) page： 538 - 548   2016.1

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The equatorial zonal electric field responses to prompt penetration of eastward convection electric fields (PPEF) were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 March 2015. At dusk sector (Indian longitudes), a rapid uplift of equatorial F layer to &gt;550km and development of intense equatorial plasma bubbles (EPBs) were observed. These EPBs were found to extend up to 27.13 degrees N and 25.98 degrees S magnetic dip latitudes indicating their altitude development to similar to 1670km at apex. In contrast, at few degrees east in the premidnight sector (Thailand-Indonesian longitudes), no significant height rise and/or EPB activity has been observed. The eastward electric field perturbations due to PPEF are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo (IDD) fields, whereas they were mostly counter balanced by the IDD fields in the premidnight sector. In situ observations from SWARM-A and SWARM-C and Communication/Navigation Outage Forecasting System satellites detected a large plasma density depletion near Indian equatorial region due to large electrodynamic uplift of F layer to higher than satellite altitudes. Further, this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator. This study brings out the significantly enhanced equatorial zonal electric field in response to PPEF that is uniquely confined to dusk sector. The responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field B-z.

DOI： 10.1002/2015JA021932

Web of Science

ISS-IMAPによる大気光波状構造の観測

齊藤昭則, 穂積裕太, 坂野井健, S. Perwitasari, 吉川一朗, 山崎敦, 大塚雄一, 山本衛

    2016

TEC variation during high and low solar activities over South American sector Reviewed

O. F. Jonah, E. R. de Paula, M. T. A. H. Muella, S. L. G. Dutra, E. A. Kherani, P. M. S. Negreti, Y. Otsuka

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 135   page： 22 - 35   2015.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Using dual frequency GPS receivers in the South American sector, the measurement of absolute ionospheric Total Electron Content (TEC) has been estimated applying the Nagoya ionospheric model for both the years of 2009 and 2001, which represent low and high solar activities, respectively. The diurnal, day-to-day, monthly, seasonal, latitudinal and longitudinal variations of TEC were studied for equatorial and low latitude regions of South America. The strength and characteristics of the Equatorial Ionization Anomaly (EIA) were equally analyzed. The analyses reveal the diurnal, seasonal and semidiurnal TEC variation, as well as the nighttime variability during the low and high solar activities. Wavelet power spectra analysis was employed to check the periodicities of the TEC data, F10.7 and zonal and meridional wind velocities measured by Meteor radar at similar to 100 km altitude. Periods such as 27, 16, 8-10, 1-5 days were found to be dominant in the zonal and meridional wind velocity corresponding with those of TEC periodicities. Hence, besides the solar radiation, we suggest that there are contributions of tides and planetary waves in spatial and temporal TEC enhancement and variations during the geomagnetic quiet periods of both solar activities. (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2015.10.005

Web of Science

A direct link between chorus emissions and pulsating aurora on timescales from milliseconds to minutes: A case study at subauroral latitudes Reviewed

Mitsunori Ozaki, Satoshi Yagitani, Kaoru Sawai, Kazuo Shiokawa, Yoshizumi Miyoshi, Ryuho Kataoka, Akimasa Ieda, Yusuke Ebihara, Martin Connors, Ian Schofield, Yuto Katoh, Yuichi Otsuka, Naoki Sunagawa, Vania K. Jordanova

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 120 ( 11 ) page： 9617 - 9631   2015.11

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A correlation was observed between chorus emissions and pulsating aurora (PA) from observations at Athabasca (L approximate to 4.3) in Canada at 9: 00-9: 20 UT on 7 February 2013, using an electron multiplying charge-coupled device camera and a VLF loop antenna with sampling rates of 110 Hz and 100 kHz, respectively. Pulsating aurora having a quasiperiodic variation in luminosity and a few hertz modulation was observed together with chorus emissions consisting of a group of successive rising-tone elements. The repetition period and modulation frequency of the PA are in good agreement with those of the modulated chorus. After 9: 11 UT, the temporal features of the aurora became aperiodic PA of indistinct modulation. Simultaneously, the rising-tone chorus turned into chorus emissions consisting of numerous rising-tone elements. The equatorial geomagnetic field inhomogeneity calculated using the Tsyganenko 2002 model shows a decreasing trend during the period. This result is consistent with nonlinear wave growth theory having a small geomagnetic field inhomogeneity, which contributes to a decrease in the threshold amplitude to trigger discrete chorus elements. These observations show a close connection between chorus emissions and PA on timescales from milliseconds for generation of discrete chorus elements on the microphysics of wave-particle interaction to minutes for the variations of the geomagnetic field inhomogeneity related with the substorm activity.

DOI： 10.1002/2015JA021381

Web of Science

Coordinated airglow observations between IMAP/VISI and a ground-based all-sky imager on concentric gravity wave in the mesopause Reviewed

S. Perwitasari, T. Sakanoi, A. Yamazaki, Y. Otsuka, Y. Hozumi, Y. Akiya, A. Saito, K. Shiokawa, S. Kawamura

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 120 ( 11 ) page： 9706 - 9721   2015.11

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We present a study of concentric gravity waves (CGWs) event from the coordinated observation between Ionosphere, Mesosphere, upper Atmosphere, and Plasmasphere mapping (IMAP)/Visible and near-Infrared Spectral Imager (VISI), all-sky camera at Rikubetsu, Multi-functional Transport Satellite (MTSAT), Tropical Rainfall Measuring Mission, and MF radar at Wakkanai combined with Modern-Era Retrospective Analysis for Research and Application data. IMAP/VISI is the first space-based imager that capable of imaging the airglow in the mesosphere and lower thermosphere region in the nadir-looking direction. Therefore, it has a unique ability to observe a great extend of CGWs propagation. Arc-like shaped, part of CGWs pattern was observed around themesopause (similar to 95 km) in the O-2 762 nm airglow emission obtained by IMAP/VISI at 1204 UT on 18 October 2012. Similar patterns were also observed by the all-sky imager at Rikubetsu (43.5 degrees N, 143.8 degrees E) in OI 557.7 nm and OH band airglow emissions from similar to 1100 to 1200 UT. Horizontal wavelengths of the observed small-scale gravity waves are similar to 50 km (OH band and OI 557.7nm) and similar to 67 km (O-2 762 nm). The source is suggested to be a deep convective activity over Honshu Island which likely was an enhanced convective activity related to a typhoon in the south of Japan. The data showed that the CGWs could propagate up to similar to 1400-1500km horizontally from the source to the mesopause but not farther away. Using atmospheric temperature profiles obtained by Thermospheric Ionosphere Mesosphere Energetics Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry, we conclude that this long-distance propagation of the waves could be caused by thermal duct in the middle atmosphere. The arc-like shaped instead of full circle pattern points out that the wind filtering effect is significant for the particular direction of wave propagation.

DOI： 10.1002/2015JA021424

Web of Science

Multi-instrument, high-resolution imaging of polar cap patch transportation Reviewed

E. G. Thomas, K. Hosokawa, J. Sakai, J. B. H. Baker, J. M. Ruohoniemi, S. Taguchi, K. Shiokawa, Y. Otsuka, A. J. Coster, J. -P. St.-Maurice, K. A. McWilliams

RADIO SCIENCE   Vol. 50 ( 9 ) page： 904 - 915   2015.9

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Transionospheric radio signals in the high-latitude polar cap are susceptible to degradation when encountering sharp electron density gradients associated with discrete plasma structures, or patches. Multi-instrument measurements of polar cap patches are examined during a geomagnetic storm interval on 22 January 2012. For the first time, we monitor the transportation of patches with high spatial and temporal resolution across the polar cap for 1-2 h using a combination of GPS total electron content (TEC), all-sky airglow imagers (ASIs), and Super Dual Auroral Radar Network (SuperDARN) HF radar backscatter. Simultaneous measurements from these data sets allow for continuous tracking of patch location, horizontal extent, and velocity despite adverse observational conditions for the primary technique (e.g., sunlit regions in the ASI data). Spatial collocation between patch-like features in relatively coarse but global GPS TEC measurements and those mapped by high-resolution ASI data was very good, indicating that GPS TEC can be applied to track patches continuously as they are transported across the polar cap. In contrast to previous observations of cigar-shaped patches formed under weakly disturbed conditions, the relatively narrow dawn-dusk extent of patches in the present interval (500-800 km) suggests association with a longitudinally confined plasma source region, such as storm-enhanced density (SED) plume. SuperDARN observations show that the backscatter power enhancements corresponded to the optical patches, and for the first time we demonstrate that the motion of the optical patches was consistent with background plasma convection velocities.

DOI： 10.1002/2015RS005672

Web of Science

Plasma bubble monitoring by TEC map and 630 nm airglow image Reviewed

H. Takahashi, C. M. Wrasse, Y. Otsuka, A. Ivo, V. Gomes, I. Paulino, A. F. Medeiros, C. M. Denardini, N. Sant'Anna, K. Shiokawa

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 130   page： 151 - 158   2015.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Equatorial ionosphere plasma bubbles over the South American continent were successfully observed by mapping the total electron content (TECMAP) using data provided by ground-based GNSS receiver networks. The TECMAP could cover almost all of the continent within 4000 km distance in longitude and latitude, monitoring TEC variability continuously with a time resolution of 10 min. Simultaneous observations of OI 630 nm all-sky image at Cachoeira Paulista (22.7 degrees S, 45.0 degrees W) and Cariri (7.4 degrees S, 36.5 degrees W) were used to compare the bubble structures. The spatial resolution of the TECMAP varied from 50 km to 1000 km, depending on the density of the observation sites. On the other hand, optical imaging has a spatial resolution better than 15 km, depicting the fine structure of the bubbles but covering a limited area (similar to 1600 km diameter). TECMAP has an advantage in its spatial coverage and the continuous monitoring (day and night) form. The initial phase of plasma depletion in the post-sunset equatorial ionization anomaly (PS-EIA) trough region, followed by development of plasma bubbles in the crest region, could be monitored in a progressive way over the magnetic equator. In December 2013 to January 2014, periodically spaced bubble structures were frequently observed. The longitudinal spacing between the bubbles was around 600-800 km depending on the day. The periodic form of plasma bubbles may suggest a seeding process related to the solar terminator passage in the ionosphere. (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2015.06.003

Web of Science

Statistical study of auroral fragmentation into patches Reviewed

Ayumi Hashimoto, Kazuo Shiokawa, Yuichi Otsuka, Shin-ichiro Oyama, Satonori Nozawa, Tomoaki Hori, Mark Lester, Magnar Gullikstad Johnsen

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 120 ( 8 ) page： 6207 - 6217   2015.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The study of auroral dynamics is important when considering disturbances of the magnetosphere. Shiokawa et al. (2010, 2014) reported observations of finger-like auroral structures that cause auroral fragmentation. Those structures are probably produced by macroscopic instabilities in the magnetosphere, mainly of the Rayleigh-Taylor type. However, the statistical characteristics of these structures have not yet been investigated. Here based on observations by an all-sky imager at TromsO (magnetic latitude=67.1 degrees N), Norway, over three winter seasons, we statistically analyzed the occurrence conditions of 14 large-scale finger-like structures that developed from large-scale auroral regions including arcs and 6 small-scale finger-like structures that developed in auroral patches. The large-scale structures were seen from midnight to dawn local time and usually appeared at the beginning of the substorm recovery phase, near the low-latitude boundary of the auroral region. The small-scale structures were primarily seen at dawn and mainly occurred in the late recovery phase of substorms. The sizes of these large- and small-scale structures mapped in the magnetospheric equatorial plane are usually larger than the gyroradius of 10keV protons, indicating that the finger-like structures could be caused by magnetohydrodynamic instabilities. However, the scale of small structures is only twice the gyroradius of 10keV protons, suggesting that finite Larmor radius effects may contribute to the formation of small-scale structures. The eastward propagation velocities of the structures are -40 to +200m/s and are comparable with those of plasma drift velocities measured by the colocating Super Dual Auroral Radar Network radar.

DOI： 10.1002/2015JA021000

Web of Science

Fresh and evolutionary-type field-aligned irregularities generated near sunrise terminator due to overshielding electric fields Reviewed

S. Tulasi Ram, K. K. Ajith, M. Yamamoto, Y. Otsuka, T. Yokoyama, K. Niranjan, S. Gurubaran

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 120 ( 7 ) page： 5922 - 5930   2015.7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The unusual evolution of fresh and intense field-aligned irregularities (FAI) near sunrise terminator which further sustained for more than 90min of postsunrise period was observed by Equatorial Atmosphere Radar at Kototabang during a minor geomagnetic storm period. These FAI echoes were initially observed around 250-350km altitudes, growing upward under eastward polarization electric fields indicating the plasma bubbles that are fully depleted along the flux tube. The background low-latitude F layer dynamics that lead to the development of these dawn time FAI have been investigated from two ionosondes at near magnetic conjugate low-latitude locations. A minor geomagnetic storm was in progress which did not appear to cause any large electric field perturbations at preceding postsunset to midnight period over Indonesian sector. However, the prompt penetration of overshielding electric fields associated with sudden northward turning of interplanetary magnetic field B-z caused spectacular ascent of F layer and development of fresh, intense, and upward evolutionary plasma bubbles near sunrise terminator.

DOI： 10.1002/2015JA021427

Web of Science

Effects of pre-reversal enhancement of E x B drift on the latitudinal extension of plasma bubble in Southeast Asia Reviewed

Prayitno Abadi, Yuichi Otsuka, Takuya Tsugawa

EARTH PLANETS AND SPACE   Vol. 67   2015.5

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We investigated the effects of the F region bottomside altitude (h'F), maximum upward E x B drift velocity, duration of pre-reversal enhancement and the integral of upward E x B drift on the latitudinal extension of equatorial plasma bubbles in the Southeast Asian sector using the observations recorded by three GPS receivers and two ionosondes. The GPS receivers are installed at Kototabang (0.2 degrees S, 100.3 degrees E; 9.9 degrees S magnetic latitude), Pontianak (0.02 degrees S, 109.3 degrees E; 9.8 degrees S magnetic latitude) and Bandung (6.9 degrees S, 107.6 degrees E; 16.7 degrees S magnetic latitude) in Indonesia. The ionosondes are installed at magnetically equatorial stations, Chumphon (10.7 degrees N, 99.4 degrees E; 0.86 degrees N magnetic latitude) in Thailand and Bac Lieu (9.3 degrees N, 105.7 degrees E; 0.62 degrees N magnetic latitude) in Vietnam. We analysed those observations acquired in the equinoctial months (March, April, September and October) in 2010-2012, when the solar activity index F-10.7 was in the range from 75 to 150. Assuming that plasma bubbles are the major source of scintillations, the latitudinal extension of the bubbles was determined according to the S4 index. We have found that the peak of h'F, maximum upward E x B drift and the integral of upward E x B drift during the pre-reversal enhancement period are positively correlated with the maximum latitude extension of plasma bubbles, but that duration of pre-reversal enhancement does not show correlation. The plasma bubbles reached magnetic latitudes of 10 degrees-20 degrees in the following conditions: (1) the peak value of h'F is greater than 250-450 km, (2) the maximum upward E x B drift is greater than 10-70 m/s and (3) the integral of upward E x B drift is greater than 50-250 m/s. These results suggest that the latitudinal extension of plasma bubbles is controlled mainly by the magnitude of pre-reversal enhancement and the peak value of h'F at the initial phase of development of plasma bubbles (or equatorial spread F) rather than by the duration of pre-reversal enhancement.

DOI： 10.1186/s40623-015-0246-7

Web of Science

Airglow-imaging observation of plasma bubble disappearance at geomagnetically conjugate points Reviewed

Kazuo Shiokawa, Yuichi Otsuka, Kenneth J. W. Lynn, Philip Wilkinson, Takuya Tsugawa

EARTH PLANETS AND SPACE   Vol. 67   page： 1 - 12   2015.3

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We report the first observation of the disappearance of a plasma bubble over geomagnetically conjugate points. It was observed by airglow imagers at Darwin, Australia (magnetic latitude: -22 degrees N) and Sata, Japan (21 degrees N) on 8 August 2002. The plasma bubble was observed in 630-nm airglow images from 1530 (0030 LT) to 1800 UT (0300 LT) and disappeared equatorward at 1800 to 1900 UT (0300 to 0400 LT) in the field of view. The ionograms at Darwin and Yamagawa (20 km north of Sata) show strong spread-F signatures at approximately 16 to 21 UT. At Darwin, the F-layer virtual height suddenly increased from approximately 200 to approximately 260 km at the time of bubble disappearance. However, a similar F-layer height increase was not observed over the conjugate point at Yamagawa, indicating that this F-layer rise was caused not by an eastward electric field but by enhancement of the equatorward thermospheric wind over Darwin. We think that this enhancement of the equatorward neutral wind was caused by an equatorward-propagating large-scale traveling ionospheric disturbance, which was identified in the north-south keogram of 630-nm airglow images. We speculate that polarization electric field associated with this equatorward neutral wind drive plasma drift across the magnetic field line to cause the observed bubble disappearance.

DOI： 10.1186/s40623-015-0202-6

Web of Science

Geomagnetically conjugate observation of plasma bubbles and thermospheric neutral winds at low latitudes Reviewed

D. Fukushima, K. Shiokawa, Y. Otsuka, M. Nishioka, M. Kubota, T. Tsugawa, T. Nagatsuma, S. Komonjinda, C. Y. Yatini

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 120 ( 3 ) page： 2222 - 2231   2015.3

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This is the first paper that reports simultaneous observations of zonal drift of plasma bubbles and the thermospheric neutral winds at geomagnetically conjugate points in both hemispheres. The plasma bubbles were observed in the 630nm nighttime airglow images taken by using highly sensitive all-sky airglow imagers at Kototabang, Indonesia (geomagnetic latitude (MLAT): 10.0 degrees S), and Chiang Mai, Thailand (MLAT: 8.9 degrees N), which are nearly geomagnetically conjugate stations, for 7h from 13 to 20UT (from 20 to 03LT) on 5 April 2011. The bubbles continuously propagated eastward with velocities of 100-125m/s. The 630nm images at Chiang Mai and those mapped to the conjugate point of Kototabang fit very well, which indicates that the observed plasma bubbles were geomagnetically connected. The eastward thermospheric neutral winds measured by two Fabry-Perot interferometers were 70-130m/s at Kototabang and 50-90m/s at Chiang Mai. We compared the observed plasma bubble drift velocity with the velocity calculated from the observed neutral winds and the model conductivity, to investigate the F region dynamo contribution to the bubble drift velocity. The estimated drift velocities were 60-90% of the observed velocities of the plasma bubbles, suggesting that most of the plasma bubble velocity can be explained by the F region dynamo effect.

DOI： 10.1002/2014JA020398

Web of Science

Explicit characteristics of evolutionary-type plasma bubbles observed from Equatorial Atmosphere Radar during the low to moderate solar activity years 2010-2012 Reviewed

K. K. Ajith, S. Tulasi Ram, M. Yamamoto, T. Yokoyama, V. Sai Gowtam, Y. Otsuka, T. Tsugawa, K. Niranjan

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 120 ( 2 ) page： 1371 - 1382   2015.2

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Using the fan sector backscatter maps of 47MHz Equatorial Atmosphere Radar (EAR) at Kototabang (0.2 degrees S geographic latitude, 100.3 degrees E geographic longitude, and 10.4 degrees S geomagnetic latitude), Indonesia, the spatial and temporal evolution of equatorial plasma bubbles (EPBs) were examined to classify the evolutionary-type EPBs from those which formed elsewhere and drifted into the field of view of radar. A total of 535 EPBs were observed during the low to moderate solar activity years 2010-2012, out of which about 210 (similar to 39%) are of evolving type and the remaining 325 (similar to 61%) are drifting-in EPBs. In general, both the evolving-type and drifting-in EPBs exhibit predominance during the postsunset hours of equinoxes and December solstices. Interestingly, a large number of EPBs were found to develop even a few minutes prior to the apex sunset during equinoxes. Further, the occurrence of evolving-type EPBs exhibits a clear secondary peak around midnight (2300-0100 LT), primarily, due to higher rate of occurrence during the postmidnight hours of June solstices. A significant number (similar to 33%) of postmidnight EPBs generated during June solstices did not exhibited any clear zonal drift, while about 14% of EPBs drifted westward. Also, the westward drifting EPBs are confined only to June solstices. The responsible mechanisms for the genesis of fresh EPBs during postmidnight hours were discussed in light of equatorward meridional winds in the presence of weak westward electric fields.

DOI： 10.1002/2014JA020878

Web of Science

CDF data archive and integrated data analysis platform for ERG-related ground data developed by ERG Science Center (ERG-SC) Reviewed

Hori, T, Y. Miyashita, Y. Miyoshi, K. Seki, T. Segawa, Y.-M. Tanaka, K. Keika, M. Shoji, I. Shinohara, K. Shiokawa, Y. Otsuka, S. Abe, A. Yoshikawa, K. Yumoto, Y. Obana, N. Nishitani, A. S. Yukimatu, T. Nagatsuma, M. Kunitake, K. Hosokawa, Y. Ogawa, K. T. Murata, M. Nosé, H. Kawano, T. Sakanoi

Journal of Space Science Informatics Japan   Vol. JAXA-RR-14-009   page： 75 - 90   2015

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GAST-D flight experiment results with disturbed and quiet ionospheric conditions Reviewed

Susumu Saito, Takayuki Yoshihara, Atsushi Kezuka, Shinji Saitoh, Sonosuke Fukushima, Yuichi Otsuka

PROCEEDINGS OF THE 28TH INTERNATIONAL TECHNICAL MEETING OF THE SATELLITE DIVISION OF THE INSTITUTE OF NAVIGATION (ION GNSS+ 2015)     page： 1494 - 1499   2015

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：INST NAVIGATION  

GAST-D ground prototype and and GAST-D airborne experimental subsystem have been developed. The GAST-D ground prototype was installed at New Ishigaki Airport located in the low magnetic latitude region. With the ground prototype and the airborne experimental system, GAST-D flight experiments in the low magnetic latitude region under ionospheric quiet and disturbed conditions for the first time in the world.
Two flight campaigns were conducted in March and September 2014. In the campaign carried out in September 2014, the vertical error performances were comparable between ionospheric quiet and disturbed condition. However, availability degradation during the severely disturbed condition is an issue. The DSIGMA monitor which is one of the airborne integrity monitors was confirmed by all-sky airglow measurements to react ionospheric disturbance.
The next steps of the study include parameter tuning of the ground prototype and airborne experimental system to enhance service availability during ionospheric disturbed conditions.

Web of Science

Direct observations of blob deformation during a substorm Reviewed

T. Ishida, Y. Ogawa, A. Kadokura, K. Hosokawa, Y. Otsuka

ANNALES GEOPHYSICAE   Vol. 33 ( 5 ) page： 525 - 530   2015

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS GESELLSCHAFT MBH  

Ionospheric blobs are localized plasma density enhancements, which are mainly produced by the transportation process of plasma. To understand the deformation process of a blob, observations of plasma parameters with good spatial-temporal resolution are desirable. Thus, we conducted the European Incoherent Scatter radar observations with high-speed meridional scans (60-80 s) during October and December 2013, and observed the temporal evolution of a blob during a substorm on 4 December 2013. This paper is the first report of direct observations of blob deformation during a substorm. The blob deformation arose from an enhanced plasma flow shear during the substorm expansion phase, and then the blob split into two smaller-scale blobs, whose scale sizes were more than similar to 100 km in latitude. Our analysis indicates that the Kelvin-Helmholtz instability and dissociative recombination could have deformed the blob structure.

DOI： 10.5194/angeocom-33-525-2015

Web of Science

Climatology of Equatorial Plasma Bubble Observed by MyRTKnet over the Years 2008-2013 Reviewed

S. M. Buhari, M. Abdullah, T. Yokoyama, A. M. Hasbi, Y. Otsuka, M. Nishioka, S. A. Bahari, T. Tsugawa

2015 INTERNATIONAL CONFERENCE ON SPACE SCIENCE AND COMMUNICATION (ICONSPACE)     page： 101 - 105   2015

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Malaysia Real-Time Kinematics GNSS Network (MyRTKnet) which consists of 78 GPS receivers was used to investigate the occurrence of equatorial plasma bubble (EPB) along 96 degrees E - 120 degrees E longitude. In this study, we present the monthly occurrence rate of EPB along the geographical longitudes of 96 degrees E - 120 degrees E for a half of solar cycle period (2008 - 2013). A 2D map of rate of TEC change index (ROTI) projected at 300 km altitude was derived from the signal paths between GPS satellites and the receivers. A ROTI keogram for one day period was obtained from the east-west cross section of the 2D ROTI maps at 4 degrees N for every 5 min. The occurrence day of EPB was determined from the keogram by the existence of ROTI larger than 0.1 TECU/min within the 96 degrees E - 120 degrees E longitude. The results show that the occurrence of EPB along the 96 degrees E - 120 degrees E has maximum during equinoctial months and is consistent with previous studies. The occurrence rate of EPB during equinoctial months shows similar characteristics in low and high solar activity due to the broad observational coverage of the MyRTKnet. In contrast, the occurrence rate of EPB during solstice months shows significant relation with solar activity. Solstice months recorded high occurrence rate of EPB in high solar activity that might be attributed to post-midnight irregularities.

DOI： 10.1109/IconSpace.2015.7283752

Web of Science

Long-term variation in the upper atmosphere as seen in the geomagnetic solar quiet daily variation Reviewed

Atsuki Shinbori, Yukinobu Koyama, Masahito Nose, Tomoaki Hori, Yuichi Otsuka, Akiyo Yatagai

EARTH PLANETS AND SPACE   Vol. 66   2014.12

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Characteristics of long-term variation in the amplitude of solar quiet (Sq) geomagnetic field daily variation have been investigated using 1-h geomagnetic field data obtained from 69 geomagnetic observation stations within the period of 1947 to 2013. The Sq amplitude observed at these geomagnetic stations showed a clear dependence on the 10- to 12-year solar activity cycle and tended to be enhanced during each solar maximum phase. The Sq amplitude was the smallest around the minimum of solar cycle 23/24 in 2008 to 2009. The relationship between the solar F10.7 index and Sq amplitude was approximately linear but about 53% of geomagnetic stations showed a weak nonlinear relation to the solar F10.7 index. In order to remove the effect of solar activity seen in the long-term variation of the Sq amplitude, we calculated a linear or second-order fitting curve between the solar F10.7 index and Sq amplitude during 1947 to 2013 and examined the residual Sq amplitude, which is defined as the deviation from the fitting curve. As a result, the majority of trends in the residual Sq amplitude that passed through a trend test showed negative values over a wide region. This tendency was relatively strong in Europe, India, the eastern part of Canada, and New Zealand. The relationship between the magnetic field intensity at 100-km altitude and residual Sq amplitude showed an anti-correlation for about 71% of the geomagnetic stations. Furthermore, the residual Sq amplitude at the equatorial station (Addis Ababa) was anti-correlated with the absolute value of the magnetic field inclination. This implies movement of the equatorial electrojet due to the secular variation of the ambient magnetic field.

DOI： 10.1186/s40623-014-0155-1

Web of Science

CME front and severe space weather Reviewed

N. Balan, R. Skoug, S. Tulasi Ram, P. K. Rajesh, K. Shiokawa, Y. Otsuka, I. S. Batista, Y. Ebihara, T. Nakamura

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 119 ( 12 )   2014.12

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Thanks to the work of a number of scientists who made it known that severe space weather can cause extensive social and economic disruptions in the modern high-technology society. It is therefore important to understand what determines the severity of space weather and whether it can be predicted. We present results obtained from the analysis of coronal mass ejections (CMEs), solar energetic particle (SEP) events, interplanetary magnetic field (IMF), CME-magnetosphere coupling, and geomagnetic storms associated with the major space weather events since 1998 by combining data from the ACE and GOES satellites with geomagnetic parameters and the Carrington event of 1859, the Quebec event of 1989, and an event in 1958. The results seem to indicate that (1) it is the impulsive energy mainly due to the impulsive velocity and orientation of IMF B-z at the leading edge of the CMEs (or CME front) that determine the severity of space weather. (2) CMEs having high impulsive velocity (sudden nonfluctuating increase by over 275 km s(-1) over the background) caused severe space weather (SvSW) in the heliosphere (failure of the solar wind ion mode of Solar Wind Electron Proton Alpha Monitor in ACE) probably by suddenly accelerating the high-energy particles in the SEPs ahead directly or through the shocks. (3) The impact of such CMEs which also show the IMF B-z southward from the leading edge caused SvSW at the Earth including extreme geomagnetic storms of mean Dst(MP) &lt; -250 nT during main phases, and the known electric power outages happened during some of these SvSW events. (4) The higher the impulsive velocity, the more severe the space weather, like faster weather fronts and tsunami fronts causing more severe damage through impulsive action. (5) The CMEs having IMF B-z northward at the leading edge do not seem to cause SvSW on Earth, although, later when the IMF B-z turns southward, they can lead to super geomagnetic storms of intensity (Dst(min)) less than even -400 nT.

DOI： 10.1002/2014JA020151

Web of Science

Continuous generation and two-dimensional structure of equatorial plasma bubbles observed by high-density GPS receivers in Southeast Asia Reviewed

S. M. Buhari, M. Abdullah, A. M. Hasbi, Y. Otsuka, T. Yokoyama, M. Nishioka, T. Tsugawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 119 ( 12 )   2014.12

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High-density GPS receivers located in Southeast Asia (SEA) were utilized to study the two-dimensional structure of ionospheric plasma irregularities in the equatorial region. The longitudinal and latitudinal variations of tens of kilometer-scale irregularities associated with equatorial plasma bubbles (EPBs) were investigated using two-dimensional maps of the rate of total electron content change index (ROTI) from 127 GPS receivers with an average spacing of about 50-100 km. The longitudinal variations of the two-dimensional maps of GPS ROTI measurement on 5 April 2011 revealed that 16 striations of EPBs were generated continuously around the passage of the solar terminator. The separation distance between the subsequent onset locations varied from 100 to 550 km with 10 min intervals. The lifetimes of the EPBs observed by GPS ROTI measurement were between 50 min and over 7 h. The EPBs propagated 440-3000 km toward the east with velocities of 83-162 m s(-1). The longitudinal variations of EPBs by GPS ROTI keogram coincided with the depletions of 630 nm emission observed using the airglow imager. Six EPBs were observed by GPS ROTI along the meridian of Equatorial Atmosphere Radar (EAR), while only three EPBs were detected by the EAR. The high-density GPS receivers in SEA have an advantage of providing time continuous descriptions of latitudinal/longitudinal variations of EPBs with both high spatial resolution and broad geographical coverage. The spatial periodicity of the EPBs could be associated with a wavelength of the quasiperiodic structures on the bottomside of the F region which initiate the Rayleigh-Taylor instability.

DOI： 10.1002/2014JA020433

Web of Science

Airglow observations of nighttime medium-scale traveling ionospheric disturbances from Yonaguni: Statistical characteristics and low-latitude limit Reviewed

V. Lakshmi Narayanan, K. Shiokawa, Y. Otsuka, S. Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 119 ( 11 ) page： 9268 - 9282   2014.11

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The characteristics of nighttime medium-scale traveling ionospheric disturbance (MSTID) features observed over Yonaguni (24.5 degrees N, 123.0 degrees E; 19.3 degrees N dip latitude), Japan are studied using all-sky imaging of OI 630.0nm airglow emission. The uniqueness of these observations is that the area observed by the imager covers the transition region between low to middle latitudes in the ionosphere. Typical low-latitude limit of midlatitude-type nighttime MSTIDs possessing phase front alignments along the northwest to the southeast occurs in this region. These MSTID features are rarely sighted at dip latitudes below 15 degrees. We selected 2 year period for analysis in which 1 year corresponded to the solar minimum conditions and another year to the solar maximum conditions. The MSTIDs were observed to extend to farther lower latitudes during the solar minimum conditions than during the solar maximum periods. Their observed range of wavelengths, phase velocities, phase front alignment, and propagation directions are similar to those observed at typical midlatitude sites. However, on many occasions the phase fronts of the observed MSTIDs did not extend over the whole field of view of the imager indicating that some process inhibits their extension to further lower latitudes. Detailed investigation suggests that the poleward propagating enhancement of airglow intensity, probably associated with the midnight pressure bulge, causes the MSTID features to disappear when they reach lower latitudes later in the night. When the MSTIDs reach lower latitudes well before midnight, they are found to be inhibited by the equatorial ionization anomaly crest region.

DOI： 10.1002/2014JA020368

Web of Science

First spaceborne observation of the entire concentric airglow structure caused by tropospheric disturbance Reviewed

Y. Akiya, A. Saito, T. Sakanoi, Y. Hozumi, A. Yamazaki, Y. Otsuka, M. Nishioka, T. Tsugawa

GEOPHYSICAL RESEARCH LETTERS   Vol. 41 ( 19 ) page： 6943 - 6948   2014.10

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Spaceborne imagers are able to observe the airglow structures with wide field of views regardless of the tropospheric condition that limits the observational time of the ground-based imagers. Concentric wave structures of the O-2 airglow in 762 nm wavelength were observed over North America on 1 June 2013 from the International Space Station. This was the first observation in which the entire image of the structure was captured from space, and its spatial scale size was determined to be 1200 km radius without assumptions. The apparent horizontal wavelength was 80 km, and the amplitude in the intensity was approximately 20% of the background intensity. The propagation velocity of the structure was derived as 125 +/- 62 m/s and atmospheric gravity waves were estimated to be generated for 3. 5 +/- 1. 7 h. Concentric structures observed in this event were interpreted to be generated by super cells that caused a tornado in its early phase.

DOI： 10.1002/2014GL061403

Web of Science

Auroral fragmentation into patches Reviewed

Kazuo Shiokawa, Ayumi Hashimoto, Tomoaki Hori, Kaori Sakaguchi, Yasunobu Ogawa, Eric Donovan, Emma Spanswick, Martin Connors, Yuichi Otsuka, Shin-Ichiro Oyama, Satonori Nozawa, Kathryn McWilliams

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 119 ( 10 ) page： 8249 - 8261   2014.10

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Auroral patches in diffuse auroras are very common features in the postmidnight local time. However, the processes that produce auroral patches are not yet well understood. In this paper we present two examples of auroral fragmentation which is the process by which uniform aurora is broken into several fragments to form auroral patches. These examples were observed at Athabasca, Canada (geomagnetic latitude: 61.7 degrees N), and TromsO, Norway (67.1 degrees N). Captured in sequences of images, the auroral fragmentation occurs as finger-like structures developing latitudinally with horizontal-scale sizes of 40-100 km at ionospheric altitudes. The structures tend to develop in a north-south direction with speeds of 150-420 m/s without any shearing motion, suggesting that pressure-driven instability in the balance between the earthward magnetic-tension force and the tailward pressure gradient force in the magnetosphere is the main driving force of the auroral fragmentation. Therefore, these observations indicate that auroral fragmentation associated with pressure-driven instability is a process that creates auroral patches. The observed slow eastward drift of aurora during the auroral fragmentation suggests that fragmentation occurs in low-energy ambient plasma.

DOI： 10.1002/2014JA020050

Web of Science

Diagnostics of equatorial and low latitude ionosphere by TEC mapping over Brazil Reviewed

H. Takahashi, S. Costa, Y. Otsuka, K. Shiokawa, J. F. G. Monico, E. Paula, P. Nogueira, C. M. Denardini, F. Becker-Guedes, C. M. Wrasse, A. S. Ivo, V. C. F. Gomes, W. Gargarela, N. Sant'Anna, R. Gatto

ADVANCES IN SPACE RESEARCH   Vol. 54 ( 3 ) page： 385 - 394   2014.8

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The total electron content (TEC) in the equatorial and low-latitude ionosphere over Brazil was monitored in two dimensions by using 2011 data from the ground-based global navigation satellite system (GNSS) receiver network operated by the Brazilian Institute for Geography and Statistics. It was possible to monitor the spatial and temporal variations in TEC over Brazil continuously during both day and night with a temporal interval of 10 mm and a spatial resolution of about 400 km. The daytime equatorial ionization anomaly (ETA) and post-sunset plasma enhancement (PS-ETA) were monitored over an area corresponding to a longitudinal extension of 4000 km in South America. Considerable day-to-day variation was observed in ETA and PS-ETA. A large latitudinal and longitudinal gradient of TEC indicated a significant ionospheric range error in application of the GNSS positioning system. Large-scale plasma bubbles after sunset were also mapped over a wide range. Depletions with longitudinally separated by more than 800 km were observed. They were extended by more than 2000 km along the magnetic field lines and drifted eastward. It is expected that 2-dimensional TEC mapping can serve as a useful tool for diagnosing ionospheric weather, such as temporal and spatial variation in the equatorial plasma trough and crest, and particularly for monitoring the dynamics of plasma bubbles. (C) 2014 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2014.01.032

Web of Science

Observations of GPS scintillation during an isolated auroral substorm Reviewed

Hosokawa, K, Y. Otsuka, Y. Ogawa, T. Tsugawa

Progress in Earth and Planetary Sciencs   Vol. 96693290710.1186/2197-4284-1-1   2014.8

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Vertical ExB drifts from radar and C/NOFS observations in the Indian and Indonesian sectors: Consistency of observations and model Reviewed

A. K. Patra, P. Pavan Chaitanya, Y. Otsuka, T. Yokoyama, M. Yamamoto, R. A. Stoneback, R. A. Heelis

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 119 ( 5 ) page： 3777 - 3788   2014.5

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In this paper, we analyze vertical ExB drifts obtained from the Doppler shifts of the daytime 150km radar echoes from two radar stations located off the magnetic equator, namely, Gadanki in India and Kototabang in Indonesia, and compare those with corresponding Coupled Ion Neutral Dynamics Investigation (CINDI) observations onboard the C/NOFS satellite and the Scherliess-Fejer model in an effort to understand to what extent the low-latitude vertical ExB drifts of the 150km region represent the F region vertical ExB drifts. The radar observations were made during 9-16 LT in January, June, July, and December 2009. A detailed comparison reveals that vertical ExB drifts observed by the radars at both locations agree well with those of CINDI and differ remarkably from those of the model. Importantly, the model and observed drifts show large disagreement when the observed drifts are either large or downward. Further, while the CINDI as well as the radar observations from the two longitudes are found to agree with each other on the average, they differ remarkably on several occasions when compared on a one-to-one basis. The observed difference in detail is due to measurements made in different volumes linked with latitudinal and/or longitudinal differences and underlines the role of neutral dynamics linked with tides and gravity waves in the two longitude sectors on the respective vertical ExB drifts. The results presented here are the first of their kind and are expected to have wider applications in furthering our understanding on fine-scale longitudinal variabilities in the ionosphere in general and ionospheric electrodynamics in the Indian and Indonesian sectors in particular.

DOI： 10.1002/2013JA019732

Web of Science

Global imaging of polar cap patches with dual airglow imagers Reviewed

K. Hosokawa, S. Taguchi, K. Shiokawa, Y. Otsuka, Y. Ogawa, M. Nicolls

GEOPHYSICAL RESEARCH LETTERS   Vol. 41 ( 1 ) page： 1 - 6   2014.1

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During a 2 h interval from 2240 to 2440 UT on 12 November 2012, regions of increased 630.0 nm airglow emissions were simultaneously detected by dual all-sky imagers in the polar cap, one at Longyearbyen, Norway (78.1 degrees N, 15.5 degrees E) and the other at Resolute Bay, Canada (74.7 degrees N, 265.1 degrees E). The Resolute Bay incoherent scatter radar observed clear enhancements of the F region electron density up to 10(12) m(-3) within these airglow structures which indicates that these are optical manifestations of polar cap patches propagating across the polar cap. During this interval of simultaneous airglow imaging, the nightside/dawnside (dayside/duskside) half of the patches was captured by the imager at Longyearbyen (Resolute Bay). This unique situation enabled us to estimate the dawn-dusk extent of the patches to be around 1500 km, which was at least 60-70% of the width of the antisunward plasma stream seen in the Super Dual Auroral Radar Network convection maps. In contrast to the large extent in the dawn-dusk direction, the noon-midnight thickness of each patch was less than 500 km. These observations demonstrate that there exists a class of patches showing cigar-shaped structures. Such patches could be produced in a wide range of local time on the dayside nearly simultaneously and spread across many hours of local time soon after their generation.

DOI： 10.1002/2013GL058748

Web of Science

Drift Velocities of 150-km Field-Aligned Irregularities Observed by the Equatorial Atmosphere Radar Reviewed

Otsuka Y., Mizutani, N., Shiokawa, K., Patra, A., Yokoyama, T., and Yamamoto

Antarctic Record     page： 印刷中   2014

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The observation of equatorial plasma bubble using all sky imager and GPS TEC measurement Reviewed

Suhaila M. Buhari, Mardina Abdullah, Alina Marie Hasbi, Yuichi Otsuka, Siti Aminah Bahari, Mohd Hezri Mokhtar, Michi Nishioka, Takuya Tsugawa

2014 XXXITH URSI GENERAL ASSEMBLY AND SCIENTIFIC SYMPOSIUM (URSI GASS)     2014

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In this study, the two-dimensional horizontal structure of EPB was observed using GPS total electron content (TEC) measurement in South East Asia region. Rate of TEC index (ROTI) is calculated from GPS TEC measurement and plotted onto two-dimensional map in geographic coordinate. Depletion of The OI 630.0 nm emission is completely coincided with ROTI enhancement region from GPS TEC measurement. Therefore, the observation using GPS TEC measurement is able to provide spatial and temporal properties of EPB in SEA region.

Web of Science

国際宇宙ステーションからの超高層大気撮像観測ミッションISS-IMAPの現状

齊藤 昭則, 秋谷祐亮, 穂積裕太, 山崎 敦, 阿部 琢美, 鈴木 睦, 村上豪, 坂野井 健, 吉川 一朗, 大塚 雄一, 藤原 均, 田口 真, 山本 衛, 中村 卓司, 江尻 省, 菊池 雅行, 河野 英昭, Huixin Liu, 石井 守, 久保田 実, 津川 卓也, 星野尾 一明, 坂野井 和代, IMAPワーキンググループ

第14回宇宙科学シンポジウム 講演集     2014

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国際宇宙ステーションからの大気光観測による赤道域電離圏擾乱の研究

山田貴宣, 大塚雄一, 坂野井健, 山崎敦, 齊藤昭則, 秋谷祐亮

第28回大気圏シンポジウム講演集     2014

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Vertical ExB drifts from radar and C/NOFS observations in the Indian and Indonesian sectors: Consistency of observations and model Reviewed

A. K. Patra, P. Pavan Chaitanya, Y. Otsuka, T. Yokoyama, M. Yamamoto, R. A. Stoneback, R. A. Heelis

Journal of Geophysical Research: Space Physics   Vol. 119 ( 5 ) page： 3777 - 3788   2014

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

In this paper, we analyze vertical ExB drifts obtained from the Doppler shifts of the daytime 150 km radar echoes from two radar stations located off the magnetic equator, namely, Gadanki in India and Kototabang in Indonesia, and compare those with corresponding Coupled Ion Neutral Dynamics Investigation (CINDI) observations onboard the C/NOFS satellite and the Scherliess-Fejer model in an effort to understand to what extent the low-latitude vertical ExB drifts of the 150 km region represent the F region vertical ExB drifts. The radar observations were made during 9-16 LT in January, June, July, and December 2009. A detailed comparison reveals that vertical ExB drifts observed by the radars at both locations agree well with those of CINDI and differ remarkably from those of the model. Importantly, the model and observed drifts show large disagreement when the observed drifts are either large or downward. Further, while the CINDI as well as the radar observations from the two longitudes are found to agree with each other on the average, they differ remarkably on several occasions when compared on a one-to-one basis. The observed difference in detail is due to measurements made in different volumes linked with latitudinal and/or longitudinal differences and underlines the role of neutral dynamics linked with tides and gravity waves in the two longitude sectors on the respective vertical ExB drifts. The results presented here are the first of their kind and are expected to have wider applications in furthering our understanding on fine-scale longitudinal variabilities in the ionosphere in general and ionospheric electrodynamics in the Indian and Indonesian sectors in particular. Key Points First comparison of 150 km echo and C/NOFS ExB drifts over India and Indonesia These observations agree at both locations and show some longitudinal difference Radar and C/NOFS observations differ remarkably from model ©2014. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2013JA019732

Scopus

Two-dimensional structure of equatorial plasma bubble observed using GPS networks in South East Asia region Reviewed

Suhaila M. Buhari, Mardina Abdullah, Alina Marie Hasbi, Yuichi Otsuka

2014 International Conference on Electronics, Information and Communications (ICEIC)     2014

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Malaysia is located at equatorial region, which is from 3 degrees to 8 degrees South of the magnetic equator where equatorial plasma bubbles (EPBs) frequently occur. EPBs could disrupt radio communication and navigational systems in this region. We present the two-dimensional structure of EPBs observed using GPS networks over South East Asia (SEA) near the equatorial region. 59 days with EPB structure were observed using GPS networks in 2011. The results show that; 1) the initial EPBs onset time occurs mostly after post-sunset between 1900 to 2100 local time (LT); 2) the duration of the occurrences of EPB is mostly from 3 to 6 hours; 3) the GPS networks in SEA region are able to observe east-west size of EPBs from 50 km to 650 km with accuracy +/-50 km.

Web of Science

Drift Velocities of 150-km Field-Aligned Irregularities Observed by the Equatorial Atmosphere Radar Reviewed

Otsuka Y., Mizutani, N., Shiokawa, K., Patra, A., Yokoyama, T., and Yamamoto, M.

  Vol. 57 ( 3 ) page： 369-378   2013.11

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Typhoon-induced concentric airglow structures in the mesopause region Reviewed

S. Suzuki, S. L. Vadas, K. Shiokawa, Y. Otsuka, S. Kawamura, Y. Murayama

Geophysical Research Letters   Vol. 40 ( 22 ) page： 5983 - 5987   2013.11

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We present the first reported gravity wave patterns in the mesopause region caused by a typhoon in the troposphere. On 10 December 2002, concentric rings of gravity waves in OH airglow were observed simultaneously by all-sky imagers in the Optical Mesosphere and Thermosphere Imager system in Japan, located at Rikubetsu (43.5°N, 143.8°E), Shigaraki (34.9°N, 136.1°E), and Sata (31.0°N, 130.7°E). The airglow structures, which were well defined and formed a coherent wave pattern expanding concentrically, were identified over 8 h (2135-2947 LT). We estimate the horizontal wavelength, horizontal phase speed, and wave period as 34.5 km, 50.2 m s-1, and 11.5 min, respectively. Infrared cloud images from the Geostationary Meteorological Satellite show that the center of the rings estimated from the airglow data corresponds to a spiral band of Typhoon Pongsona (T0226). This unique event provides new insight into coupling between the lower and upper atmosphere. ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2013GL058087

Scopus

Typhoon-induced concentric airglow structures in the mesopause region Reviewed

S. Suzuki, S. L. Vadas, K. Shiokawa, Y. Otsuka, S. Kawamura, Y. Murayama

GEOPHYSICAL RESEARCH LETTERS   Vol. 40 ( 22 ) page： 5983 - 5987   2013.11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We present the first reported gravity wave patterns in the mesopause region caused by a typhoon in the troposphere. On 10 December 2002, concentric rings of gravity waves in OH airglow were observed simultaneously by all-sky imagers in the Optical Mesosphere and Thermosphere Imager system in Japan, located at Rikubetsu (43.5 degrees N, 143.8 degrees E), Shigaraki (34.9 degrees N, 136.1 degrees E), and Sata (31.0 degrees N, 130.7 degrees E). The airglow structures, which were well defined and formed a coherent wave pattern expanding concentrically, were identified over 8 h (2135-2947 LT). We estimate the horizontal wavelength, horizontal phase speed, and wave period as 34.5 km, 50.2 m s(-1), and 11.5 min, respectively. Infrared cloud images from the Geostationary Meteorological Satellite show that the center of the rings estimated from the airglow data corresponds to a spiral band of Typhoon Pongsona (T0226). This unique event provides new insight into coupling between the lower and upper atmosphere.

DOI： 10.1002/2013GL058087

Web of Science

Two-dimensional simulation of ionospheric variations in the vicinity of the epicenter of the Tohoku-oki earthquake on 11 March 2011 Reviewed

H. Shinagawa, T. Tsugawa, M. Matsumura, T. Iyemori, A. Saito, T. Maruyama, H. Jin, M. Nishioka, Y. Otsuka

GEOPHYSICAL RESEARCH LETTERS   Vol. 40 ( 19 ) page： 5009 - 5013   2013.10

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Unusual ionospheric variations were observed in the M9.0 Tohoku-oki earthquake on 11 March 2011. Among various kinds of features in the ionosphere, significant depletion of total electron content (TEC) near the epicenter was observed after the earthquake. Although previous studies have suggested that the coseismic ionospheric variations are associated with atmospheric perturbation caused by vertical displacement of the sea surface, the mechanism of the TEC depletion has not been fully understood. In this paper, a two-dimensional nonlinear nonhydrostatic compressible atmosphere-ionosphere model is employed to investigate the ionospheric variations in the vicinity of the epicenter. The simulation results reveal that an impulsive pressure pulse produced by a sudden uplift of the sea surface leads to local atmospheric expansion in the thermosphere and that the expansion of the thermosphere combined with the effect of inclined magnetic field lines in the ionosphere causes the sudden TEC depletion above the epicenter region.

DOI： 10.1002/2013GL057627

Web of Science

Longitudinal characteristics of spread F backscatter plumes observed with the EAR and Sanya VHF radar in Southeast Asia Reviewed

Guozhu Li, Baiqi Ning, M. A. Abdu, Yuchi Otsuka, T. Yokoyama, M. Yamamoto, Libo Liu

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 118 ( 10 ) page： 6544 - 6557   2013.10

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The development of equatorial plasma irregularity plumes can be well recorded by steerable backscatter radars operated at and off the magnetic equator due to the fact that the vertically extended plume structures are tracers of magnetically north-south aligned larger scale structures. From observations during March 2012, using two low latitude steerable backscatter radars in Southeast Asia, the Equatorial Atmosphere Radar (EAR) (0.2 degrees S, 100.3 degrees E; dip lat 10.4 degrees S) and the Sanya VHF radar (18.4 degrees N, 109.6 degrees E; dip lat 12.8 degrees N), the characteristics of backscatter plumes over the two sites separated in longitude by similar to 1000 km were simultaneously investigated. The beam steering measurements reveal frequent occurrences of multiple plumes over both radar sites, of which two cases are analyzed here. The observations on 30 March 2012 show plume structures initiated within the radar scanned area, followed by others drifting from the west of the radar beam over both stations. A tracing analysis on the onset locations of plasma plumes reveals spatially well-separated backscatter plumes, with a maximum east-west wavelength of about 1000 km, periodically generated in longitudes between 85 degrees E and 110 degrees E. The postsunset backscatter plumes seen by the Sanya VHF radar are found to be due to the passage of sunset plumes initiated around the longitude of EAR. Most interestingly, the EAR measurements on the night of 21 March 2012 show multiple plume structures that developed successively in the radar scanned area with east-west separation of similar to 50 km, with however no sunset plasma plume over Sanya. Colocated ionogram measurements show that spread F irregularities occurred mainly in the bottomside F region at Sanya, whereas satellite traces in ionograms that are indications of large-scale wave structures were observed on that night at both stations. Possible causes for the longitudinal difference in the characteristics of radar backscatter plumes are discussed.

DOI： 10.1002/jgra.50581

Web of Science

Observation of nighttime medium-scale travelling ionospheric disturbances by two 630-nm airglow imagers near the auroral zone Reviewed

K. Shiokawa, M. Mori, Y. Otsuka, S. Oyama, S. Nozawa, S. Suzuki, M. Connors

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 103   page： 184 - 194   2013.10

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This is the first statistical study of high-latitude nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed by all-sky airglow imagers in the European and Canadian longitudinal sectors. We investigate the MSTIDs observed in the 630-nm airglow imagers at Tromso (magnetic latitude: 67.1 degrees N), Norway, for two winters, and at Athabasca (61.7 degrees N), Canada, for 2 years. At both stations, the MSTIDs were observed mostly before midnight with an occurrence rate of more than 50% of clear observation hours at Tromso and similar to 30% at Athabasca. The average wavelengths, phase velocities, and periods of the observed MSTIDs were 150-200 km, 50-80 m/s, and 30-60 min, respectively. We found that MSTIDs at Tromso tend to show eastward motion in addition to the typical westward and southwestward motion at middle latitudes. At Athabasca, westward and southwestward motions prevail except for the summer when characteristic northward-moving MSTIDs with larger wavelengths and faster phase velocities were observed. At both stations, some MSTIDs showed characteristic changes of their phase velocity and directions in association with auroral activity, suggesting that they are plasma structures affected by auroral electric field. Vertical wavelengths of gravity waves were estimated by using simultaneous thermospheric wind data obtained at Tromso, showing that most of these MSTIDs can exist as gravity waves in the thermosphere. On the basis of these results, we conclude that the high-latitude nighttime MSTIDs are caused mainly by the Perkins and E-F coupling instabilities similar to those at middle latitudes and that an additional source by atmospheric gravity waves from lower altitudes also comes into play. (C) 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2013.03.024

Web of Science

Ionogram-based range-time displays for observing relationships between ionosonde satellite traces, spread F and drifting optical plasma depletions Reviewed

K. J. W. Lynn, Y. Otsuka, K. Shiokawa

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 98   page： 105 - 112   2013.6

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A form of range-time plots derived from ionograms taken from a standard digital ionosondes, situated at the low latitude sites of Vanimo, Port Moresby and Darwin, exhibit bursts of spread F at the center of descending and ascending off-angle reflectors. This particular type of event has since been identified with the passage of optically imaged ionospheric plasma depletions (bubbles) over a Darwin ionosonde. This paper describes the process for producing this form of range-time display and its relationship to ionospheric height, satellite traces and range spread F as seen on individual ionograms. First hop satellite traces are proposed to be via direct reflection from the steep electron density gradients at the base of bubbles while second hop satellite traces then involve a single additional ground reflection. Measurements of night equatorial drift velocity were made from the range-time displays and found to be in the range 20-220 m/s peaking at approximately 90-100 m/s in good agreement with values derived from drift measurements made by a variety of other types of equipment. (C) 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2013.03.020

Web of Science

Physical mechanisms of the ionospheric storms at equatorial and higher latitudes during the recovery phase of geomagnetic storms Reviewed

N. Balan, Y. Otsuka, M. Nishioka, J. Y. Liu, G. J. Bailey

Journal of Geophysical Research: Space Physics   Vol. 118 ( 5 ) page： 2660 - 2669   2013.5

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The paper studies the physical mechanisms of the ionospheric storms at equatorial and higher latitudes, which are generally opposite both during the main phase (MP) and recovery phase (RP) of geomagnetic storms. The mechanisms are based on the natural tendency of physical systems to occupy minimum energy state which is most stable. The paper first illustrates the recent developments in the understanding of the mechanisms during daytime MPs when generally negative ionospheric storms (in Nmax and TEC) develop at equatorial latitudes and positive storms occur at higher latitudes, including why the storms are severe only in some cases. The paper then investigates the relative importance of the physical mechanisms of the positive ionospheric storms observed at equatorial latitudes (within ±15°) during daytime RPs when negative storms occur at higher latitudes using CHAMP Ne and GPS-TEC data and Sheffield University Plasmasphere Ionosphere Model. The results indicate that the mechanical effect of the storm-time equatorward neutral winds that causes plasma convergence at equatorial F region could be a major source for the positive storms, with the downwelling effect of the winds and zero or westward electric field, if present, acting as minor sources. Key PointsPositive ionospheric storms during RPs due to plasma convergencePhysical mechanisms based on stable minimum energy stateSevere space weather seems to be due to the rate of energy release during CMEs ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/jgra.50275

Web of Science

Scopus

Evidence of gravity wave ducting in the mesopause region from airglow network observations Reviewed

S. Suzuki, K. Shiokawa, Y. Otsuka, S. Kawamura, Y. Murayama

Geophysical Research Letters   Vol. 40 ( 3 ) page： 601 - 605   2013.2

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We present observational evidence of gravity wave ducting and show, for the first time, that the ducting occurs over large horizontal distances in the mesopause region. An optical network of four all-sky imagers in Japan identified two-dimensional patterns of small-scale gravity wave bands in OH airglow images on 13 June 2004. The wave signatures clearly showed northward propagation of more than 1800 km over a wide range of latitudes. The horizontal wavelength, horizontal phase speed, and wave period were estimated from the airglow data as 33.4 km, 42.8 m s&lt
sup&gt
-1&lt
/sup&gt
, and 13.4 min, respectively. The wave structure lasted for the whole 5-h airglow observation period. Simultaneous MF radar wind data and TIMED/SABER measurements suggested that the wave was trapped and ducted at the airglow height. The ducting likely contributed to the remarkable coherence of the wave as it propagated northward. Key Points An airglow imaging network in Japan provides gravity wave ducting signatures Gravity wave bands appear over a horizontal extent of more than 1800 km Simultaneous winds and temperature are consistent with gravity wave ducting © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012GL054605

Web of Science

Scopus

GPS observations of medium-scale traveling ionospheric disturbances over Europe Reviewed

Otsuka, Y., Suzuki, K., Nakagawa, S., Nishioka, M., Shiokawa, K., and Tsugawa, T.

Ann. Geophys.   Vol. 31   page： 163-172   2013.2

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Two-dimensional structures of medium-scale traveling ionospheric disturbances (MSTIDs) over Europe have been revealed, for the first time, by using maps of the total electron content (TEC) obtained from more than 800 GPS receivers of the European GPS receiver networks. From statistical analysis of the TEC maps obtained 2008, we have found that the observed MSTIDs can be categorized into two groups: daytime MSTID and nighttime MSTID. The daytime MSTID frequently occurs in winter. Its maximum occurrence rate in monthly and hourly bin exceeds 70% at lower latitudes over Europe, whereas it is approximately 45% at higher latitudes. Since most of the daytime MSTIDs propagate southward, we speculate that they could be caused by atmospheric gravity waves in the thermosphere. The nighttime MSTIDs also frequently occur in winter but most of them propagate southwestward, in a direction consistent with the theory that polarization electric fields play an important role in generating the nighttime MSTIDs. The nighttime MSTID occurrence rate shows distinct latitudinal difference: The maximum of the occurrence rate in monthly and hourly bin is approximately 50% at lower latitudes in Europe, whereas the nighttime MSTID was rarely observed at higher latitudes. We have performed model calculations of the plasma density perturbations caused by a gravity wave and an oscillating electric field to reproduce the daytime and nighttime MSTIDs, respectively. We find that TEC perturbations caused by gravity waves do not show dip angle dependencies, while those caused by the oscillating electric field have a larger amplitude at lower latitudes. These dip angle dependencies of the TEC perturbation amplitude could contribute to the latitudinal variation of the MSTID occurrence rate. Comparing with previous studies, we discuss the longitudinal difference of the nighttime MSTID occurrence rate, along with the E- and F-region coupling processes. The seasonal variation, of the nighttime MSTID occurrence rate in Europe, is not consistent with the theory that the longitudinal and seasonal variations of the nighttime MSTID occurrence could be attributed to those of the Es layer occurrence.


DOI： doi:10.5194/angeo-31-163-2013

Drift velocities of 150-km field-aligned irregularities observed by the equatorial atmosphere radar Reviewed

Otsuka Y, Mizutani N, Shiokawa K, Patra A, Yokoyama T, Yamamoto M

Antarctic Record   Vol. 57 ( 3 ) page： 369 - 378   2013

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DOI： 10.15094/00009711

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J-GLOBAL

Other Link： http://orcid.org/0000-0001-8392-6455

GPS observations of medium-scale traveling ionospheric disturbances over Europe Reviewed

Y. Otsuka, K. Suzuki, S. Nakagawa, M. Nishioka, K. Shiokawa, T. Tsugawa

ANNALES GEOPHYSICAE   Vol. 31 ( 2 ) page： 163 - 172   2013

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS GESELLSCHAFT MBH  

Two-dimensional structures of medium-scale traveling ionospheric disturbances (MSTIDs) over Europe have been revealed, for the first time, by using maps of the total electron content (TEC) obtained from more than 800 GPS receivers of the European GPS receiver networks. From statistical analysis of the TEC maps obtained 2008, we have found that the observed MSTIDs can be categorized into two groups: daytime MSTID and nighttime MSTID. The daytime MSTID frequently occurs in winter. Its maximum occurrence rate in monthly and hourly bin exceeds 70 % at lower latitudes over Europe, whereas it is approximately 45 % at higher latitudes. Since most of the daytime MSTIDs propagate southward, we speculate that they could be caused by atmospheric gravity waves in the thermosphere. The nighttime MSTIDs also frequently occur in winter but most of them propagate southwestward, in a direction consistent with the theory that polarization electric fields play an important role in generating the nighttime MSTIDs. The nighttime MSTID occurrence rate shows distinct latitudinal difference: The maximum of the occurrence rate in monthly and hourly bin is approximately 50 % at lower latitudes in Europe, whereas the nighttime MSTID was rarely observed at higher latitudes. We have performed model calculations of the plasma density perturbations caused by a gravity wave and an oscillating electric field to reproduce the daytime and nighttime MSTIDs, respectively. We find that TEC perturbations caused by gravity waves do not show dip angle dependencies, while those caused by the oscillating electric field have a larger amplitude at lower latitudes. These dip angle dependencies of the TEC perturbation amplitude could contribute to the latitudinal variation of the MSTID occurrence rate. Comparing with previous studies, we discuss the longitudinal difference of the nighttime MSTID occurrence rate, along with the E- and F-region coupling processes. The seasonal variation, of the nighttime MSTID occurrence rate in Europe, is not consistent with the theory that the longitudinal and seasonal variations of the nighttime MSTID occurrence could be attributed to those of the Es layer occurrence.

DOI： 10.5194/angeo-31-163-2013

Web of Science

Longitudinal characteristics of spread F backscatter plumes observed with the EAR and Sanya VHF radar in Southeast Asia Reviewed

Guozhu Li, Baiqi Ning, M. A. Abdu, Yuchi Otsuka, T. Yokoyama, M. Yamamoto, Libo Liu

Journal of Geophysical Research: Space Physics   Vol. 118 ( 10 ) page： 6544 - 6557   2013

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

The development of equatorial plasma irregularity plumes can be well recorded by steerable backscatter radars operated at and off the magnetic equator due to the fact that the vertically extended plume structures are tracers of magnetically north-south aligned larger scale structures. From observations during March 2012, using two low latitude steerable backscatter radars in Southeast Asia, the Equatorial Atmosphere Radar (EAR) (0.2°S, 100.3°E
dip lat 10.4°S) and the Sanya VHF radar (18.4°N, 109.6°E
dip lat 12.8°N), the characteristics of backscatter plumes over the two sites separated in longitude by ~1000 km were simultaneously investigated. The beam steering measurements reveal frequent occurrences of multiple plumes over both radar sites, of which two cases are analyzed here. The observations on 30 March 2012 show plume structures initiated within the radar scanned area, followed by others drifting from the west of the radar beam over both stations. A tracing analysis on the onset locations of plasma plumes reveals spatially well-separated backscatter plumes, with a maximum east-west wavelength of about 1000 km, periodically generated in longitudes between 85°E and 110°E. The postsunset backscatter plumes seen by the Sanya VHF radar are found to be due to the passage of sunset plumes initiated around the longitude of EAR. Most interestingly, the EAR measurements on the night of 21 March 2012 show multiple plume structures that developed successively in the radar scanned area with east-west separation of ~50 km, with however no sunset plasma plume over Sanya. Colocated ionogram measurements show that spread F irregularities occurred mainly in the bottomside F region at Sanya, whereas satellite traces in ionograms that are indications of large-scale wave structures were observed on that night at both stations. Possible causes for the longitudinal difference in the characteristics of radar backscatter plumes are discussed. Key Points Simultaneous measurements of ESF plumes by two closely located radars Sunset plume onset and post-sunset periodic plumes are detected by both radars Smaller scale longitudinal differences in the plume occurrence are presented ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/jgra.50581

Scopus

IMAP-WG国際宇宙ステーションからの地球超高層大気撮像観測ISS-IMAPミッションの初期成果

齊藤昭則, 山崎 敦, 阿部琢美, 鈴木 睦, 坂野井 健, 吉川一朗, 大塚雄一, 藤原 均, 田口 真, 山本 衛, 中村卓司, 江尻 省, 菊池雅行, 河野英昭, Huixin Liu, 石井 守, 久保田 実, 津川卓也, 星野尾一明, 坂野井和代

第13回宇宙科学シンポジウム     2013

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Language：Japanese   Publishing type：Research paper (other academic)  

Small-scale ionospheric delay variation associated with plasma bubbles studied with GNSS and optical measurements and its impact on GBAS Reviewed

S. Saito, T. Yoshihara, Y. Otsuka

PROCEEDINGS OF THE 26TH INTERNATIONAL TECHNICAL MEETING OF THE SATELLITE DIVISION OF THE INSTITUTE OF NAVIGATION (ION GNSS 2013)     page： 1869 - 1874   2013

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：INST NAVIGATION  

For differential GNSS systems such as the ground-based augmentation system (GBAS), spatial variation (or spatial gradient) in ionospheric delay is a critical issue.
In the low latitude region, the dominant ionospheric disturbance is the plasma bubble. Plasma bubbles are local ionospheric depletions that often occur after sunset. They accompany ionospheric irregularities with various scale sizes from 100 km down to a meter. However, the characteristics of the low latitude ionospheric variations in teens of a specific threat to GBAS have not been well studied yet.
This paper studies the spatial relationship between the ionospheric delay gradients and the large-scale plasma bubble structure. The ionospheric delay gradients are measured with spaced receivers at Ishigaki, Japan. Large-scale plasma bubble structures are measured with an all-sky airglow images observed at Yonaguni, Japan.
The ionospheric delay gradients are shown to have large values at the edges of plasma bubbles, and theeir directions are consistent with the direction of the electron density gradient inferred from the airglow images as well as the expectation that the gradient.
In some cases, however, the ionospheric delay gradients have large values had large values at locations other than the plasma bubble edges. The absolute ionospheric delay with dual-frequency measurements and the drift velocity will be analyzed to investigate the scale size and the amplitude of the irregularities that cause the ionospheric delay gradients. Co-site observations of the ionospheric delay gradient and the airglow at Ishigaki to have nearly common line of sights for both the measurements.

Web of Science

A comparative study of equatorial daytime vertical E x B drift in the Indian and Indonesian sectors based on 150 km echoes Reviewed

A. K. Patra, P. Pavan Chaitanya, N. Mizutani, Y. Otsuka, T. Yokoyama, M. Yamamoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

In this paper, we study for the first time the daytime vertical E x B drift velocities from Gadanki and Kototabang using the Doppler shifts of the 150-km echoes observed during 2008-2010, a period of low solar activity. Drift velocities are mostly positive and confined to 35 m s(-1) at both the locations, except for Gadanki where on a few occasions negative drift velocities have been observed in the afternoon hours. Drift velocities generally show a decreasing trend with local time and the largest drift is generally observed in the forenoon hours consistent with extensively reported observations and models of E x B drift. Drift velocities from Gadanki and Kototabang compared exceeding well on some days and differed remarkably on many days despite the fact that they are longitudinally separated by only 20 degrees. The day-to-day variation in the drift velocity could be as high as 15 m s(-1) at Gadanki and 7 m s(-1) at Kototabang. Seasonal mean drifts over Gadanki are found to be generally larger than those of Kototabang. The observations have been compared in detail with those reported earlier based on ground- and satellite- based observations and also with the Scherliess-Fejer model. The observed differences in the drifts at the two locations, including the downward drifts, have been discussed in the light of current understanding of the longitudinal variability of E x B drift.

DOI： 10.1029/2012JA018053

Web of Science

Motion of high-latitude nighttime medium-scale traveling ionospheric disturbances associated with auroral brightening Reviewed

Shiokawa, K, Mori, M, Otsuka, Y, Oyama, S, Nozawa, S

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.10

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DOI： 10.1029/2012JA017928

Motion of high-latitude nighttime medium-scale traveling ionospheric disturbances associated with auroral brightening Reviewed

Shiokawa K, Mori M, Otsuka Y, Oyama S, Nozawa S

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.10

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DOI： 10.1029/2012JA017928

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Observation of equatorial nighttime medium-scale traveling ionospheric disturbances in 630-nm airglow images over 7 years Reviewed

D. Fukushima, K. Shiokawa, Y. Otsuka, T. Ogawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.10

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report on nighttime medium- scale traveling ionospheric disturbances (MSTIDs) observed at Kototabang, Indonesia (geographic longitude: 100.3 degrees E; geographic latitude: 0.2 degrees S; and geomagnetic latitude: 10.6 degrees S) during a 7-year period from October 2002 to October 2009. MSTIDs were observed in 630-nm nighttime airglow images by using a highly sensitive all-sky airglow imager at Kototabang. The averages and standard deviations of horizontal phase velocity, period, and horizontal wavelength of MSTIDs observed during the 7 years were 320 +/- 170 m/s, 42 +/- 11 min, and 790 +/- 440 km, respectively. The occurrence rate of the observed MSTIDs decreased with decreasing solar activity. The average horizontal wavelength of MSTIDs increased with decreasing solar activity. Southward MSTIDs were dominant throughout the 7 years of observations. These facts are consistent with the hypothesis that the observed MSTIDs are caused by gravity waves in the thermosphere. Moreover, we compared the propagation directions of the observed MSTIDs with the locations of tropospheric convection activity for the events where gravity waves producing the observed MSTIDs could have existed in the lower atmosphere. Strong tropospheric convection was found within +/- 30 degrees from the source directions of MSTIDs in 81% of the MSTID events. In such events, gravity waves were possibly generated from deep convection in the troposphere and directly propagated into the thermosphere.

DOI： 10.1029/2012JA017758

Web of Science

On post-midnight field-aligned irregularities observed with a 30.8-MHz radar at a low latitude: Comparison with F-layer altitude near the geomagnetic equator Reviewed

M. Nishioka, Y. Otsuka, K. Shiokawa, T. Tsugawa, Effendy, P. Supnithi, T. Nagatsuma, K. T. Murata

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.8

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We investigated the relationship between post-midnight F-region field aligned irregularities (FAIs) and F-layer altitude by analyzing data of a 30.8-MHz radar installed 5at Kototabang, Indonesia (0.2 degrees S, 100.3 degrees E; geomagnetic latitude 10.4 degrees S) and an ionosonde installed at Chumphon, Thailand (10.7 degrees N, 99.4 degrees E; geomagnetic latitude 3.3 degrees N). Chumphon is located near the geomagnetic equator on approximately the same meridian as Kototabang. Case studies show that the altitude of the F-layer rose at Chumphon a half hour before the post-midnight FAIs appeared at Kototabang. The Doppler velocity of the E-region FAIs observed simultaneously by the 30.8-MHz radar was downward, indicating that the F-layer uplift was not caused by the electric field. We also investigated seasonal variations of the post-midnight FAI occurrence and the F-layer altitude. Both the post-midnight FAIs and the uplift of the F-layer were frequently seen around midnight between May and August. The seasonal variation of the midnight F-layer uplift around the geomagnetic equator coincided with that of the post-midnight FAI occurrence at Kototabang. These results suggest that the uplift of the F-layer would play an important role in the generation of post-midnight FAIs. We evaluated the linear growth rate of the Rayleigh-Taylor instability based on the altitude of the F-layer observed at Chumphon. The result shows that the uplift of the F-layer can enhance the growth rate because gravity-driven eastward electric current increases. Therefore, we interpret that the observed FAIs were accompanied by plasma bubble, the growth rate of which was reinforced by the uplifted F-layer.

DOI： 10.1029/2012JA017692

Web of Science

Response of low-latitude ionosphere to medium-term changes of solar and geomagnetic activity Reviewed

Ivan Kutiev, Yuichi Otsuka, Dora Pancheva, Rod Heelis

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.8

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The paper presents the medium-term quasi periodic (similar to 9-27 day) response of middle and low-latitude ionosphere to solar [F10.7) and geomagnetic (Kp-index) forcing. The ionospheric response is examined by wavelet analysis of the relative deviations of TEC over Japan for the period of time 2000-2008. It is found that the similar to 27-day rTEC oscillations correlate well with the same oscillations of the solar index F10.7 particularly in the solar maximum and its early declining phase (2001-2005). During the declining phase of solar activity (for example, year of 2005) the Kp-index variability exhibits additionally strong oscillations with periods 13.5- and 9-days. Similar oscillations are found in rTEC as well but they do not follow the geomagnetic forcing as faithfully as those associated with F10.7. During solar minimum the quasi periodic rTEC variability is shaped mainly by the recurrent geomagnetic activity. An attempt is made to investigate the latitudinal dependence of the similar to 9-27-day rTEC response over Japan as well as the phase relationship between the forcing and response.

DOI： 10.1029/2012JA017641

Web of Science

Disappearance of equatorial plasma bubble after interaction with mid-latitude medium-scale traveling ionospheric disturbance Reviewed

Otsuka, Y., K. Shiokawa, and T. Ogawa

Geophys. Res. Lett.   Vol. 39 ( L14105 )   2012.7

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DOI： doi:10.1029/2012GL052286

Disappearance of equatorial plasma bubble after interaction with mid-latitude medium-scale traveling ionospheric disturbance Reviewed

Y. Otsuka, K. Shiokawa, T. Ogawa

GEOPHYSICAL RESEARCH LETTERS   Vol. 39   2012.7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report simultaneous observations of an equatorial plasma bubble and a Medium-Scale Traveling Ionospheric Disturbance (MSTID) in 630-nm airglow images taken with an all-sky airglow imager at Shigaraki (34.9 degrees N, 136.1 degrees E; dip angle of the geomagnetic field similar to 49 degrees), Japan. Clear depletion of the 630-nm airglow intensity was observed as the equatorial plasma bubble propagated eastward, whereas the MSTID, which had a wavefront aligned from northwest to southeast, propagated southwestward. This result indicates that MSTIDs do not propagate at the same velocity as the ambient plasma, which is clearly shown by the eastward motion of the plasma bubbles. We found that the airglow depletion caused by the plasma bubble disappeared when the plasma bubble encountered the MSTID. The plasma depletion could be filled with ambient rich plasma that moved into the plasma-depleted region by E x B drift associated with the MSTID, indicating that MSTIDs are accompanied by electric field perturbations.

DOI： 10.1029/2012GL052286

Web of Science

Overview of Nighttime Ionospheric Instabilities at Low- and Mid-Latitudes: Coupling Aspects Resulting in Structuring at the Mesoscale Reviewed

Jonathan J. Makela, Yuichi Otsuka

SPACE SCIENCE REVIEWS   Vol. 168 ( 1-4 ) page： 419 - 440   2012.6

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Language：English   Publisher：SPRINGER  

We present a review of the current state of understanding regarding two classes of irregularities causing mesoscale structuring (hundreds of kilometers) in the nighttime ionosphere at low- and mid-latitudes. Additionally, current state of understanding of equatorial plasma bubbles at low latitudes, and medium-scale traveling ionospheric disturbances at mid latitudes and their relationship to possible seeding from lower altitudes are described. In each case, well-developed linear theories exist to explain the general properties of the irregularities. However, these linear theories have growth rates too low to explain the actual observations, giving rise to the need to invoke seeding mechanisms. We describe the observational databases that have been compiled over the decades and discuss possible coupling and seeding mechanisms that would overcome the low growth rate and explain the observed structuring at the mesoscale. Future research directions are also briefly discussed.

DOI： 10.1007/s11214-011-9816-6

Web of Science

GPS total electron content variations associated with poleward moving Sun-aligned arcs Reviewed

P. T. Jayachandran, K. Hosokawa, K. Shiokawa, Y. Otsuka, C. Watson, S. C. Mushini, J. W. MacDougall, P. Prikryl, R. Chadwick, T. D. Kelly

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.5

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GPS total electron content (TEC) has shown quasiperiodic oscillations of varying amplitude associated with poleward moving Sun-aligned arcs. The amplitude of TEC variations showed a maximum of similar to 3 TECU and seemed to decrease as the arcs moved poleward from the source/generation region. Simultaneous DMSP data showed that fluctuations in TEC and optical intensification were caused by precipitation of high-energy (&gt;500 eV) particles. Concurrent ionosonde observations also exhibited quasiperiodic variations (within limit of the resolution of the data) in peak ionospheric electron density of the ionosphere. Bottom height of the ionospheric layers produced by precipitating particles varied between 130 km (upper E region) and 300 km (F region), indicating variable particle precipitation energy. Frequency analysis of high-resolution TEC data showed a broad range of discrete frequency components from 1.60 mHz to 22.80 mHz present in the TEC oscillations, which may provide insight into the energization/modulation of precipitating particles by these oscillations. A broad distribution of equivalent vertical thickness of arcs was calculated using GPS TEC and ionosonde measurements of peak electron density. This distribution showed a minimum thickness of 21 km, a maximum of 84 km, and an average of 49 km. The equivalent vertical thickness also showed a linear relationship with bottomside height of the ionospheric layer (auroral arc). The relationship showed an increase in the vertical thickness with an increase in bottomside height of the layer. This relationship is a consequence of variations in the energy of the precipitating particles causing different ionospheric production profiles.

DOI： 10.1029/2011JA017423

Web of Science

VHF Radar Observations of Post-Midnight F-Region Field-Aligned Irregularities over Indonesia during Solar Minimum Reviewed

Otsuka, Y., K. Shiokawa, M. Nishioka and Effendy

Indian Journal of Radio and Space Physics (IJRSP)   ( 41 ) page： 199-207   2012.4

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VHF Radar Observations of Post-Midnight F-Region Field-Aligned Irregularities over Indonesia during Solar Minimum Reviewed

Otsuka, Y, K. Shiokawa, M. Nishioka, Effendy

Indian Journal of Radio and Space Physics (IJRSP)   ( 41 ) page： 199-207   2012.4

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Seasonal and Local Time Variations of E-Region Field-Aligned Irregularities Observed with 30.8-MHz Radar at Kototabang, Indonesia Reviewed

Otsuka, Y.

International Journal of Geophysics   Vol. 2012 ( 695793 )   2012.3

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DOI： doi:10.1155/2012/695793

Polarization of Pc1/EMIC waves and related proton auroras observed at subauroral latitudes Reviewed

R. Nomura, K. Shiokawa, K. Sakaguchi, Y. Otsuka, M. Connors

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We have investigated the polarization of Pc1 geomagnetic pulsations and related proton auroras at subauroral latitudes, using an induction magnetometer and an all-sky camera at Athabasca, Canada (54.7 degrees N, 246.7 degrees E, magnetic latitude (mlat) 61.7 degrees N). Isolated proton auroras often appear in association with Pc1 pulsations, because of proton scattering by electromagnetic ion cyclotron (EMIC) waves in the magnetosphere. We used the proton aurora as a proxy for the location and size of the Pc1 ionospheric source. For 27 Pc1 events with simultaneously observed proton auroras over 4 years from September 7, 2005 to September 6, 2009, we calculated the distances between the Pc1 ionospheric source and the observation site, normalized by the scale size of the source. We tried three different definitions of the scale size of Pc1 ionospheric source to calculate the normalized distances. We found that the rotation angle theta between the Pc1 polarization ellipse and the direction to the proton aurora changes from 90 degrees to 0 degrees as the normalized distance increases. For the definition of the scale sizes that gives most clear theta-transition from 90 degrees to 0 degrees, the transition occurs at the normalized distance similar to 2.0-4.0, while it was similar to 1.0-2.0 by the model calculation of Fujita and Tamao (1988). The averaged major axes tend to point toward the proton aurora at larger distances. The difference of the transition location may imply that the Pc1 ionospheric sources are larger than the isolated proton auroras, or that the inhomogenuity of the ionospheric conductivity by the proton precipitation affects the transition distances.

DOI： 10.1029/2011JA017241

Web of Science

Ionospheric and thermospheric storms at equatorial latitudes observed by CHAMP, ROCSAT, and DMSP Reviewed

N. Balan, J. Y. Liu, Y. Otsuka, S. Tulasi Ram, H. Luehr

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 117   2012.1

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Analysis of the dayside electron density (Ne) and neutral mass density (N) at 400 km height measured by CHAMP during 12 intense geomagnetic storms in 2000-2004, and ion densities at 600 km and 840 km heights measured by ROCSAT and DMSP during a few of the intense storms, reveal some new aspects. Thermospheric storms (change of N) reach the equator within 1.5 to 3 hours from the main phase (MP) onset of intense storms having short and steady MPs. The responses of the equatorial ionosphere (at CHAMP) to both MPs and RPs (recovery phases) of the storms are generally opposite to those at higher latitudes. In addition to the known opposite responses during MPs, the analysis reveals that positive ionospheric storms develop at equatorial latitudes (within about +/- 15 degrees magnetic latitudes) during daytime RPs, while conventional negative storms occur at higher latitudes. Ionospheric storms also extend to the topside ionosphere beyond 850 km height and are generally positive (at DMSP), especially during MPs. The positive storms around the equatorial ionospheric peak during RPs are interpreted in terms of the potential sources such as (1) zero or westward electric fields due to disturbance dynamo and/or prompt penetration, (2) plasma convergence due to the mechanical effects of storm-time equatorward neutral winds and waves, (3) increase of atomic oxygen density and decrease of molecular nitrogen density due to the downwelling effect of the winds, and (4) photoionization. The positive storms in the topside ionosphere during MPs involve the rapid upward drift of plasma due to eastward PPEFs, reduction in the downward diffusion of plasma along the field lines, and plasma convergence due to equatorward winds and waves.

DOI： 10.1029/2011JA016903

Web of Science

A comparative study of equatorial daytime vertical e × B drift in the Indian and Indonesian sectors based on 150 km echoes Reviewed

A. K. Patra, P. Pavan Chaitanya, N. Mizutani, Y. Otsuka, T. Yokoyama, M. Yamamoto

Journal of Geophysical Research: Space Physics   Vol. 117 ( 11 )   2012

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In this paper, we study for the first time the daytime vertical E × B drift velocities from Gadanki and Kototabang using the Doppler shifts of the 150-km echoes observed during 2008-2010, a period of low solar activity. Drift velocities are mostly positive and confined to 35 m s-1 at both the locations, except for Gadanki where on a few occasions negative drift velocities have been observed in the afternoon hours. Drift velocities generally show a decreasing trend with local time and the largest drift is generally observed in the forenoon hours consistent with extensively reported observations and models of E × B drift. Drift velocities from Gadanki and Kototabang compared exceeding well on some days and differed remarkably on many days despite the fact that they are longitudinally separated by only 20. The day-to-day variation in the drift velocity could be as high as 15 m s-1 at Gadanki and 7 m s-1 at Kototabang. Seasonal mean drifts over Gadanki are found to be generally larger than those of Kototabang. The observations have been compared in detail with those reported earlier based on ground-and satellite-based observations and also with the Scherliess-Fejer model. The observed differences in the drifts at the two locations, including the downward drifts, have been discussed in the light of current understanding of the longitudinal variability of E × B drift.

DOI： 10.1029/2012JA018053

Scopus

Development of low-cost sky-scanning Fabry-Perot interferometers for airglow and auroral studies Reviewed

K. Shiokawa, Y. Otsuka, S. Oyama, S. Nozawa, M. Satoh, Y. Katoh, Y. Hamaguchi, Y. Yamamoto, J. Meriwether

EARTH PLANETS AND SPACE   Vol. 64 ( 11 ) page： 1033 - 1046   2012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

We have developed new Fabry-Perot interferometers (FPIs) that are designed to measure thermospheric winds and temperatures as well as mesospheric winds through the airglow/aurora emissions at wavelengths of 630.0 nm and 557.7 nm, respectively. One FPI (FP01), possessing a large aperture etalon (diameter: 116 mm), was installed at the EISCAT Tromso site in 2009. The other FPIs, using 70-mm diameter etalons, were installed in Thailand, Indonesia, and Australia in 2010-2011 (FP02-FP04) by the Solar-Terrestrial Environment Laboratory, and in Peru (Nazca and Jicamarca) and Alaska (Poker Flat) by Clemson University. The FPIs with 70-mm etalons are low-cost compact instruments, suitable for multipoint network observations. All of these FPIs use low-noise cooled-CCD detectors with 1024 x 1024 pixels combined with a 4-stage thermoelectric cooling system that can cool the CCD temperature down to -80 degrees C. The large incident angle (maximum: 1.3 degrees-1.4 degrees) to the etalon achieved by the use of multiple orders increases the throughput of the FPIs. The airglow and aurora observations at Tromso by FP01 show wind velocities with typical random errors ranging from 2 to 13 m s(-1) and from 4 to 27 m s(-1) for mesosphere (557.7 nm) and thermosphere (630.0 nm) measurements, respectively. The 630.0-nm airglow observations at Shigaraki, Japan, by FP02-FP04 and by the American FPI instruments give thermospheric wind velocities with typical random errors that vary from 2 m s(-1) to more than 50 m s(-1) depending on airglow intensity.

DOI： 10.5047/eps.2012.05.004

Web of Science

Early observation and future plan of the Ionosphere, Mesosphere, upper Atmosphere, and Plasmasphere mapping observation mission

SAITO Akinori, YAMAZAKI Atsushi, SAKANOI Takeshi, YOSHIKAWA Ichiro, ABE Takumi, OTSUKA Yuichi, TAGUCHI Makoto, SUZUKI Makoto, KIKUCHI Masayuki, NAKAMURA Takuji, YAMAMOTO Mamoru, KAWANO Hideaki, LIU Huixin, ISHII Mamoru, TSUGAWA Takuya, SAKANOI Kazuyo, FUJIWARA Hitoshi, KUBOTA Minoru, EJIRI Mitsumu K, SAITO Akinori

地球電磁気・地球惑星圏学会総会及び講演会予稿集(CD-ROM)   Vol. 132nd   page： ROMBUNNO.B005-18   2012

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J-GLOBAL

GPS total electron content variations associated with poleward moving Sun-aligned arcs Reviewed

P. T. Jayachandran, K. Hosokawa, K. Shiokawa, Y. Otsuka, C. Watson, S. C. Mushini, J. W. MacDougall, P. Prikryl, R. Chadwick, T. D. Kelly

Journal of Geophysical Research: Space Physics   Vol. 117 ( 5 )   2012

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GPS total electron content (TEC) has shown quasiperiodic oscillations of varying amplitude associated with poleward moving Sun-aligned arcs. The amplitude of TEC variations showed a maximum of ∼3 TECU and seemed to decrease as the arcs moved poleward from the source/generation region. Simultaneous DMSP data showed that fluctuations in TEC and optical intensification were caused by precipitation of high-energy (&gt
500 eV) particles. Concurrent ionosonde observations also exhibited quasiperiodic variations (within limit of the resolution of the data) in peak ionospheric electron density of the ionosphere. Bottom height of the ionospheric layers produced by precipitating particles varied between 130km (upper E region) and 300km (F region), indicating variable particle precipitation energy. Frequency analysis of high-resolution TEC data showed a broad range of discrete frequency components from 1.60 mHz to 22.80 mHz present in the TEC oscillations, which may provide insight into the energization/modulation of precipitating particles by these oscillations. A broad distribution of equivalent vertical thickness of arcs was calculated using GPS TEC and ionosonde measurements of peak electron density. This distribution showed a minimum thickness of 21km, a maximum of 84km, and an average of 49km. The equivalent vertical thickness also showed a linear relationship with bottomside height of the ionospheric layer (auroral arc). The relationship showed an increase in the vertical thickness with an increase in bottomside height of the layer. This relationship is a consequence of variations in the energy of the precipitating particles causing different ionospheric production profiles. Copyright 2012 by the American Geophysical Union.

DOI： 10.1029/2011JA017423

Scopus

Motion of high-latitude nighttime medium-scale traveling ionospheric disturbances associated with auroral brightening Reviewed

K. Shiokawa, M. Mori, Y. Otsuka, S. Oyama, S. Nozawa

Journal of Geophysical Research: Space Physics   Vol. 117 ( 10 )   2012

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We report for the first time the rapid oscillating motion of nighttime medium-scale traveling ionospheric disturbances (MSTIDs) based on airglow imaging observations at Troms (magnetic latitude: 67.1N), Norway on 8 December, 2009. The MSTIDs appeared in 630-nm airglow images at 1530 UT as wave-like structures south of Troms with a horizontal wavelength of ∼200km and a phase surface of north to south. They moved eastward with velocities of 30-60m/s. The velocity was faster in the poleward-side of the MSTIDs, forming a northeast-southwest phase surface at later times. This phase surface direction is opposite to that of midlatitude MSTIDs. The MSTIDs show sudden oscillations and phase jump in the east-west direction with a timescale of ∼10min at 1730 UT. The oscillations were associated with an auroral brightening observed at the poleward edge of the images and small magnetic field perturbations observed by ground magnetometers. The Doppler measurement of the 630-nm airglow by a Fabry-Perot interferometer at Troms showed a stable southeastward thermospheric wind with a velocity of ∼150m/s. These observations indicate that the MSTID oscillations were linked to auroral electric field in the ionosphere, implying that the observed MSTIDs are ionospheric plasma structures. We suggest that the observed MSTIDs were created by atmospheric gravity waves at the beginning, left as fossil plasma structures even after the gravity wave packet dissipated in the thermosphere, moved eastward according to the background electric field driven by the F-region dynamo, and oscillated associated with the auroral electric field. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017928

Scopus

Low-latitude mesosphere, thermosphere, and ionosphere Reviewed

Y. Sahai, R. S. Dabas, Y. Otsuka, M. Klimenko

International Journal of Geophysics   Vol. 2012   2012

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DOI： 10.1155/2012/671240

Scopus

Ionospheric and thermospheric storms at equatorial latitudes observed by CHAMP, ROCSAT, and DMSP Reviewed

N. Balan, J. Y. Liu, Y. Otsuka, S. Tulasi Ram, H. Lühr

Journal of Geophysical Research: Space Physics   Vol. 117 ( 1 )   2012

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Analysis of the dayside electron density (Ne) and neutral mass density (N) at 400km height measured by CHAMP during 12 intense geomagnetic storms in 2000-2004, and ion densities at 600km and 840km heights measured by ROCSAT and DMSP during a few of the intense storms, reveal some new aspects. Thermospheric storms (change of N) reach the equator within 1.5 to 3 hours from the main phase (MP) onset of intense storms having short and steady MPs. The responses of the equatorial ionosphere (at CHAMP) to both MPs and RPs (recovery phases) of the storms are generally opposite to those at higher latitudes. In addition to the known opposite responses during MPs, the analysis reveals that positive ionospheric storms develop at equatorial latitudes (within about ±15° magnetic latitudes) during daytime RPs, while conventional negative storms occur at higher latitudes. Ionospheric storms also extend to the topside ionosphere beyond 850km height and are generally positive (at DMSP), especially during MPs. The positive storms around the equatorial ionospheric peak during RPs are interpreted in terms of the potential sources such as (1) zero or westward electric fields due to disturbance dynamo and/or prompt penetration, (2) plasma convergence due to the mechanical effects of storm-time equatorward neutral winds and waves, (3) increase of atomic oxygen density and decrease of molecular nitrogen density due to the downwelling effect of the winds, and (4) photoionization. The positive storms in the topside ionosphere during MPs involve the rapid upward drift of plasma due to eastward PPEFs, reduction in the downward diffusion of plasma along the field lines, and plasma convergence due to equatorward winds and waves.

DOI： 10.1029/2011JA016903

Scopus

Observation of equatorial nighttime medium-scale traveling ionospheric disturbances in 630-nm airglow images over 7 years Reviewed

D. Fukushima, K. Shiokawa, Y. Otsuka, T. Ogawa

Journal of Geophysical Research: Space Physics   Vol. 117 ( 10 )   2012

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[1] We report on nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed at Kototabang, Indonesia (geographic longitude: 100.3°E
geographic latitude: 0.2°S
and geomagnetic latitude: 10.6°S) during a 7-year period from October 2002 to October 2009. MSTIDs were observed in 630-nm nighttime airglow images by using a highly sensitive all-sky airglow imager at Kototabang. The averages and standard deviations of horizontal phase velocity, period, and horizontal wavelength of MSTIDs observed during the 7 years were 320 ±170 m/s, 42 ±11 min, and 790 ± 440 km, respectively. The occurrence rate of the observed MSTIDs decreased with decreasing solar activity. The average horizontal wavelength of MSTIDs increased with decreasing solar activity. Southward MSTIDs were dominant throughout the 7 years of observations. These facts are consistent with the hypothesis that the observed MSTIDs are caused by gravity waves in the thermosphere. Moreover, we compared the propagation directions of the observed MSTIDs with the locations of tropospheric convection activity for the events where gravity waves producing the observed MSTIDs could have existed in the lower atmosphere. Strong tropospheric convection was found within ±30 degrees from the source directions of MSTIDs in 81% of the MSTID events. In such events, gravity waves were possibly generated from deep convection in the troposphere and directly propagated into the thermosphere. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017758

Scopus

Seasonal and local time variations of e -Region field-aligned irregularities observed with 30.8-mhz radar at kototabang, indonesia Reviewed

Y. Otsuka

International Journal of Geophysics   Vol. 2012   2012

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A VHF backscatter radar with operating frequency 30.8 MHz has been operated at Kototabang (0.20°S, 100.32°E
dip latitude 10.36°S), Indonesia, since February 2006. We analyzed E-region field-aligned irregularities (FAIs) observed by this radar through a year of 2007 and found that the E-region FAI observed at Kototabang can be classified into two groups. One is "descending FAI". Altitude of the FAI echo region descends with time from 102 km to 88 km altitude during 0700-1000 and 1900-0000 LT in June solstice season. The other is "low-altitude FAI", which is observed in an altitude range from 88 to 94 km mainly during nighttime. The observed Doppler velocity show distinct local time and altitude dependence. The seasonally averaged zonal velocity above (below) approximately 94 km altitude is westward (eastward) during daytime and eastward (westward) during nighttime. Meridional/vertical velocity perpendicular to the geomagnetic fields is upward during daytime and downward during nighttime. The direction of the FAI velocity above approximately 94 km altitude is consistent with that of the background E × B plasma drifts reported previously. © 2012 Y. Otsuka.

DOI： 10.1155/2012/695793

Scopus

Response of low-latitude ionosphere to medium-term changes of solar and geomagnetic activity Reviewed

Ivan Kutiev, Yuichi Otsuka, Dora Pancheva, Rod Heelis

Journal of Geophysical Research: Space Physics   Vol. 117 ( 8 )   2012

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The paper presents the medium-term quasi periodic (∼9-27 day) response of middle and low-latitude ionosphere to solar [F10.7) and geomagnetic (Kp-index) forcing. The ionospheric response is examined by wavelet analysis of the relative deviations of TEC over Japan for the period of time 2000-2008. It is found that the ∼27-day rTEC oscillations correlate well with the same oscillations of the solar index F10.7 particularly in the solar maximum and its early declining phase (2001-2005). During the declining phase of solar activity (for example, year of 2005) the Kp-index variability exhibits additionally strong oscillations with periods 13.5- and 9-days. Similar oscillations are found in rTEC as well but they do not follow the geomagnetic forcing as faithfully as those associated with F10.7. During solar minimum the quasi periodic rTEC variability is shaped mainly by the recurrent geomagnetic activity. An attempt is made to investigate the latitudinal dependence of the ∼9-27-day rTEC response over Japan as well as the phase relationship between the forcing and response. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017641

Scopus

Polarization of Pc1/EMIC waves and related proton auroras observed at subauroral latitudes Reviewed

R. Nomura, K. Shiokawa, K. Sakaguchi, Y. Otsuka, M. Connors

Journal of Geophysical Research: Space Physics   Vol. 117 ( 2 )   2012

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We have investigated the polarization of Pc1 geomagnetic pulsations and related proton auroras at subauroral latitudes, using an induction magnetometer and an all-sky camera at Athabasca, Canada (54.7°N, 246.7°E, magnetic latitude (mlat) 61.7°N). Isolated proton auroras often appear in association with Pc1 pulsations, because of proton scattering by electromagnetic ion cyclotron (EMIC) waves in the magnetosphere. We used the proton aurora as a proxy for the location and size of the Pc1 ionospheric source. For 27 Pc1 events with simultaneously observed proton auroras over 4years from September 7, 2005 to September 6, 2009, we calculated the distances between the Pc1 ionospheric source and the observation site, normalized by the scale size of the source. We tried three different definitions of the scale size of Pc1 ionospheric source to calculate the normalized distances. We found that the rotation angle θ between the Pc1 polarization ellipse and the direction to the proton aurora changes from 90° to 0° as the normalized distance increases. For the definition of the scale sizes that gives most clear θ-transition from 90° to 0°, the transition occurs at the normalized distance ∼2.0-4.0, while it was ∼1.0-2.0 by the model calculation of Fujita and Tamao (1988). The averaged major axes tend to point toward the proton aurora at larger distances. The difference of the transition location may imply that the Pc1 ionospheric sources are larger than the isolated proton auroras, or that the inhomogenuity of the ionospheric conductivity by the proton precipitation affects the transition distances. Copyright 2012 by the American Geophysical Union.

DOI： 10.1029/2011JA017241

Scopus

On post-midnight field-aligned irregularities observed with a 30.8-MHz radar at a low latitude: Comparison with F-layer altitude near the geomagnetic equator Reviewed

M. Nishioka, Y. Otsuka, K. Shiokawa, T. Tsugawa, Null Effendy, P. Supnithi, T. Nagatsuma, K. T. Murata

Journal of Geophysical Research: Space Physics   Vol. 117 ( 8 )   2012

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We investigated the relationship between post-midnight F-region field aligned irregularities (FAIs) and F-layer altitude by analyzing data of a 30.8-MHz radar installed 5at Kototabang, Indonesia (0.2S, 100.3E
geomagnetic latitude 10.4S) and an ionosonde installed at Chumphon, Thailand (10.7N, 99.4E
geomagnetic latitude 3.3N). Chumphon is located near the geomagnetic equator on approximately the same meridian as Kototabang. Case studies show that the altitude of the F-layer rose at Chumphon a half hour before the post-midnight FAIs appeared at Kototabang. The Doppler velocity of the E-region FAIs observed simultaneously by the 30.8-MHz radar was downward, indicating that the F-layer uplift was not caused by the electric field. We also investigated seasonal variations of the post-midnight FAI occurrence and the F-layer altitude. Both the post-midnight FAIs and the uplift of the F-layer were frequently seen around midnight between May and August. The seasonal variation of the midnight F-layer uplift around the geomagnetic equator coincided with that of the post-midnight FAI occurrence at Kototabang. These results suggest that the uplift of the F-layer would play an important role in the generation of post-midnight FAIs. We evaluated the linear growth rate of the Rayleigh-Taylor instability based on the altitude of the F-layer observed at Chumphon. The result shows that the uplift of the F-layer can enhance the growth rate because gravity-driven eastward electric current increases. Therefore, we interpret that the observed FAIs were accompanied by plasma bubble, the growth rate of which was reinforced by the uplifted F-layer. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017692

Scopus

Simultaneous observations at Darwin of equatorial bubbles by ionosonde-based range/time displays and airglow imaging Reviewed

K. J. W. Lynn, Y. Otsuka, K. Shiokawa

GEOPHYSICAL RESEARCH LETTERS   Vol. 38   2011.12

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Airglow observations of ionospheric electron density depletions made at Darwin, Australia have demonstrated that the tree-like structure of bubbles developed at the magnetic equator are mapped along magnetic field lines with considerable accuracy to the base of the ionosphere at higher latitudes. Ionosonde range-time displays made at Darwin and other equatorial sites in the Australian region show characteristic approaching and receding echoes which converge on a typical spread-F event. These off-angle echoes have often been referred to in the literature as satellite traces and associated with spread F with little recognition of their true significance. All four optical depletions previously reported in the literature as being seen at Darwin are found in this paper to be accompanied by such typical off-angle/spread F events. The zonal drift velocity of the moving reflectors can be measured from the speed at which such echoes approach and recede. Since digital ionosondes in equatorial sites have existed for many years, existing ionogram data, when suitably processed into range-time displays, may allow the occurrence of such events over several sunspot cycles to be found. A question remains as to whether all or only some of such equatorial range-time events correspond to electron density depletions. Citation: Lynn, K. J. W., Y. Otsuka, and K. Shiokawa (2011), Simultaneous observations at Darwin of equatorial bubbles by ionosonde-based range/time displays and airglow imaging, Geophys. Res. Lett., 38, L23101, doi:10.1029/2011GL049856.

DOI： 10.1029/2011GL049856

Web of Science

On postmidnight low-latitude ionospheric irregularities during solar minimum: 1. Equatorial Atmosphere Radar and GPS-TEC observations in Indonesia Reviewed

T. Yokoyama, M. Yamamoto, Y. Otsuka, M. Nishioka, T. Tsugawa, S. Watanabe, R. F. Pfaff

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.11

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Using the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36 degrees S dip latitude), it is shown that postmidnight irregularities during solar minimum are morphologically different from those detected during solar maximum and are quite similar to those observed with the middle and upper atmosphere (MU) radar in midlatitudes (29.3 degrees N dip latitude). Utilizing the rapid beam-steering capability of the EAR, the spatial structure of the postmidnight irregularities is clearly presented for the first time. It is found that they usually propagate westward and can be categorized into two types. One shows sharp upwelling plumes near local midnight, which should not be a mere passage of fossil plasma bubbles. The other has successive tilted structures which have the same orientation as medium-scale traveling ionospheric disturbances typically observed at midlatitudes. We suggest that the convergence of the equatorward thermospheric wind which is believed to be responsible for the midnight temperature maximum may be an important factor to produce a preferable condition for the upwelling plumes in the postmidnight sector. The displacement between geographic and magnetic equators may also be important for seasonal/longitudinal variation of the postmidnight irregularities.

DOI： 10.1029/2011JA016797

Web of Science

On postmidnight low-latitude ionospheric irregularities during solar minimum: 2. C/NOFS observations and comparisons with the Equatorial Atmosphere Radar Reviewed

T. Yokoyama, R. F. Pfaff, P. A. Roddy, M. Yamamoto, Y. Otsuka

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.11

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A detailed comparison between the observations of the Communication/Navigation Outage Forecasting System (C/NOFS) satellite and the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36 degrees S dip latitude) on the postmidnight irregularities is presented. The zonal and meridional E x B drift velocities measured by the vector electric field instrument on the C/NOFS are consistent with the westward propagation of backscatter echoes and the line-of-sight Doppler velocities observed with the EAR, respectively. The plasma density depletions are observed in the postmidnight sector for several consecutive orbits, which suggests the depletions grow slowly during the premidnight period and reach the spacecraft altitude around local midnight. The convergence of the equatorward wind which could be responsible for the midnight temperature maximum may produce a preferable condition for the growth of the Rayleigh-Taylor instability around midnight. Electric field fluctuations of medium-scale traveling ionospheric disturbances may play an important role in seeding the instability. Both equatorial and midlatitude-type plasma instabilities could be operational at the EAR latitude sector, which together would foster a high occurrence of postmidnight irregularities during solar minimum.

DOI： 10.1029/2011JA016798

Web of Science

Equatorial electrodynamics and neutral background in the Asian sector during the 2009 stratospheric sudden warming Reviewed

Huixin Liu, Mamoru Yamamoto, S. Tulasi Ram, Takuya Tsugawa, Yuichi Otsuka, Claudia Stolle, Eelco Doornbos, Kiyohumi Yumoto, Tsutomu Nagatsuma

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.8

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Using ground observations of total electron content (TEC) and equatorial electrojet (EEJ) in the Asian sector, along with plasma and neutral densities obtained from the CHAMP satellite, we investigate the ionospheric electrodynamics and neutral background in this longitude sector during the major stratospheric sudden warming (SSW) in January 2009. Our analysis reveals the following prominent features. First, the TEC response in tropical regions is strongly latitude dependent, with monotonic depletion at the dip equator but a semidiurnal perturbation at low latitudes. Second, the TEC semidiurnal perturbation possesses a significant hemispheric asymmetry in terms of onset date and magnitude. It starts on the same day as the SSW peak in the Northern Hemisphere but 2 days later in the Southern Hemisphere. Its magnitude is twice as strong in the north than in the south. Third, strong counter electrojet occurs in the afternoon, following the strengthening of the eastward EEJ in the morning. Fourth, semidiurnal perturbation in both TEC and EEJ possesses a phase shift, at a rate of about 0.7 h/day. Comparisons with results reported in the Peruvian sector reveal clear longitude dependence in the amplitude and hemispheric asymmetry of the semidiurnal perturbation. Finally, thermospheric density undergoes similar to 25% decrease at low latitudes in the afternoon local time sector during the SSW, indicating significant cooling effects in the tropical upper thermosphere.

DOI： 10.1029/2011JA016607

Web of Science

New aspects of thermospheric and ionospheric storms revealed by CHAMP Reviewed

N. Balan, M. Yamamoto, J. Y. Liu, Y. Otsuka, H. Liu, H. Luehr

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.7

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The neutral mass density N and electron density Ne at 400 km height measured by CHAMP during nine intense geomagnetic storms bring out some new aspects of the thermospheric and ionospheric storms. The thermospheric storms (increase of N) develop with the onset of the main phases (MP) of the geomagnetic storms and reach their peak phases before or by the end of the MPs. The ionospheric storms (change of Ne) in general undergo an initial negative phase (with the equatorial ionization anomaly (EIA) crests shifting poleward) before turning positive, and the positive storms reach their peak strengths (or phases) centered at +/- 25 degrees-30 degrees magnetic latitudes; in some (4) cases the positive storms develop without an initial negative phase and with the EIA crests shifting equatorward; in all cases the positive storms reach their peak phases before the end of the MPs and turn to conventional negative storms by the end of the MPs. The observations agree with the different aspects of a physical mechanism of the positive storms. The observations also reveal that the Halloween storms of 30 October 2003 with a short MP without fluctuations produced the strongest positive ionospheric storms through impulsive response, and there is strong equinoctial asymmetry in the ionosphere and thermosphere during geomagnetic storms.

DOI： 10.1029/2010JA016399

Web of Science

Decay of polar cap patch Reviewed

K. Hosokawa, J. I. Moen, K. Shiokawa, Y. Otsuka

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.5

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We report an event in which a polar cap patch was detected with an all-sky imager (ASI) at Resolute Bay, Canada (74.73 degrees N, 265.07 degrees E; AACGM latitude 82.9 degrees), on the nightside. The patch stopped its antisunward motion associated with a northward turning of interplanetary magnetic field and stayed within the field of view of the ASI for more than 1 h. When the patch stagnated, its luminosity decreased gradually, which allows us to investigate how the patch plasma decayed in a quantitative manner. The decay of the patch can be quantitatively explained by the loss through recombinations of O(+) with ambient N(2) and O(2) molecules, if we assume the altitude of the optical patch to be around 295 km. The derived altitude of the patch around 295 km is much higher than the nominal value at 235 km obtained from the MSIS-E90 and IRI-2007 models, indicating that climatological models such as IRI are not suitable for describing the actual density profile of patches. This is probably because the loss process was much faster in the lower-altitude part of the patch; thus, the peak altitude of the patch increased as it traveled across the polar cap because of rapid recombination at the bottomside of the F region. This suggests that we should employ higher emission altitude when we investigate optical patches transported deep into the nightside polar cap. Such information is important when we compare the optical data with other instruments such as coherent radars and GPS scintillation measurements by mapping the all-sky image on the geographic coordinate system with an assumption of the patch emission altitude.

DOI： 10.1029/2010JA016297

Web of Science

A statistical study of the response of the dayside equatorial F-2 layer to the main phase of intense geomagnetic storms as an indicator of penetration electric field Reviewed

N. Balan, M. Yamamoto, V. Sreeja, I. S. Batista, K. J. W. Lynn, M. A. Abdu, S. Ravindran, T. Kikuchi, Y. Otsuka, K. Shokawa, S. Alex

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.3

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The response of the dayside equatorial F-2 layer to the main phases of the 22 intense geomagnetic storms (Dst &lt; -150 nT) in 1998-2008 is investigated using the digital ionosonde data from the equatorial stations in Brazilian, Indian, and Australian longitudes together with equatorial electrojet strength and IMF B-z; the storms include 15 superstorms (Dst &lt; -200 nT). The observations show that there is a period during all MPs when the F-2 layer peak rises (and falls) rapidly with large peak electron density (Nmax) reduction, the rise velocity strongly correlates with the intensity (Dst) of the storms, and the duration of the Nmax reduction corresponds to that of strong eastward electrojet when IMF Bz remains highly negative. The observations indicate the occurrence of strong eastward prompt penetration electric fields (PPEF) during the rapid F-2 layer response. The PPEF drives the F-2 layer peak rapidly upward, which reduces Nmax due to vertical expansion and diffusion. The results therefore suggest that the rapid F-2 layer response (rapid rise (and fall) of peak height (hmax) with large Nmax reduction) observed by ionosondes can be used to detect the occurrence of the daytime eastward PPEF during intense geomagnetic storms irrespective of season and level of solar activity. The data also show two rare events of strong daytime westward electric fields due to disturbance dynamo and/or prompt penetration. The results are important when radars are not available to monitor the occurrence of the PPEF.

DOI： 10.1029/2010JA016001

Web of Science

First satellite-imaging observation of medium-scale traveling ionospheric disturbances by FORMOSAT-2/ISUAL Reviewed

Toru Adachi, Yuichi Otsuka, Masashi Yamaoka, Mamoru Yamamoto, Kazuo Shiokawa, Alfred B. Chen, Rue-Ron Hsu

GEOPHYSICAL RESEARCH LETTERS   Vol. 38   2011.2

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On the night of May 16, 2007, a satellite limb imager of FORMOSAT-2/ISUAL observed wave-like structures of the 630-nm airglow simultaneously with an all-sky imager deployed at Darwin in Australia. The height of the airglow layer was estimated as 220 km, and the structures were aligned in the northeast-southwest orientation with a wavelength of similar to 300 km and propagated toward the northwest with a phase velocity of similar to 100 m s(-1), showing typical characteristics of the nighttime medium-scale traveling ionospheric disturbances (MSTIDs). We conclude that ISUAL for the first time succeeded in observing the airglow layer altitude and airglow structures modulated by MSTID from space. Such a satellite limb airglow imaging could be a new tool to characterize ionospheric irregularities on a global level. Citation: Adachi, T., Y. Otsuka, M. Yamaoka, M. Yamamoto, K. Shiokawa, A. B. Chen, and R.-R. Hsu (2011), First satellite-imaging observation of medium-scale traveling ionospheric disturbances by FORMOSAT-2/ISUAL, Geophys. Res. Lett., 38, L04101, doi:10.1029/2010GL046268.

DOI： 10.1029/2010GL046268

Web of Science

Vertical connection from the tropospheric activities to the ionospheric longitudinal structure simulated by a new Earth's whole atmosphere-ionosphere coupled model Reviewed

H. Jin, Y. Miyoshi, H. Fujiwara, H. Shinagawa, K. Terada, N. Terada, M. Ishii, Y. Otsuka, A. Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 116   2011.1

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This paper introduces a new Earth's atmosphere-ionosphere coupled model that treats seamlessly the neutral atmospheric region from the troposphere to the thermosphere as well as the thermosphere-ionosphere interaction including the electrodynamics self-consistently. The model is especially useful for the study of vertical connection between the meteorological phenomena and the upper atmospheric behaviors. As an initial simulation using the coupled model, we have carried out a 30 day consecutive run in September. The result reveals that the longitudinal structure of the F-region ionosphere varies on a day-to-day basis in a highly complex way and that a four-peak structure of the daytime equatorial ionization anomaly (EIA) similar to the recent observations appears as an averaged feature. The simulation reproduces and thus confirms the vertical coupling processes proposed so far with respect to the formation of the averaged EIA longitudinal structure; the excitation of solar nonmigrating tides in the troposphere, their propagation through the middle atmosphere, and the modulation of ionospheric dynamo, which in turn affects EIA generation. The simulation result indicates that not only the ionospheric averaged longitudinal structure but also the day-to-day variation can be modulated significantly by the lower atmospheric effect.

DOI： 10.1029/2010JA015925

Web of Science

Statistical Study of Medium-Scale Traveling Ionospheric Disturbances Observed with a GPS Receiver Network in Japan Reviewed

Otsuka, Y, Kotake, N, Shiokawa, K, Ogawa, T, Tsugawa, T, Saito, A

AERONOMY OF THE EARTH'S ATMOSPHERE AND IONOSPHERE   Vol. 2 ( 3 ) page： 291 - 299   2011

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DOI： 10.1007/978-94-007-0326-1_21

Web of Science

A statistical study of the response of the dayside equatorial F2 layer to the main phase of intense geomagnetic storms as an indicator of penetration electric field Reviewed

N. Balan, M. Yamamoto, V. Sreeja, I. S. Batista, K. J.W. Lynn, M. A. Abdu, S. Ravindran, T. Kikuchi, Y. Otsuka, K. Shokawa, S. Alex

Journal of Geophysical Research: Space Physics   Vol. 116 ( 3 )   2011

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The response of the dayside equatorial F2 layer to the main phases of the 22 intense geomagnetic storms (Dst &lt
-150 nT) in 1998-2008 is investigated using the digital ionosonde data from the equatorial stations in Brazilian, Indian, and Australian longitudes together with equatorial electrojet strength and IMF Bz
the storms include 15 superstorms (Dst &lt
-200 nT). The observations show that there is a period during all MPs when the F2 layer peak rises (and falls) rapidly with large peak electron density (Nmax) reduction, the rise velocity strongly correlates with the intensity (Dst) of the storms, and the duration of the Nmax reduction corresponds to that of strong eastward electrojet when IMF Bz remains highly negative. The observations indicate the occurrence of strong eastward prompt penetration electric fields (PPEF) during the rapid F2 layer response. The PPEF drives the F2 layer peak rapidly upward, which reduces Nmax due to vertical expansion and diffusion. The results therefore suggest that the rapid F2 layer response (rapid rise (and fall) of peak height (hmax) with large Nmax reduction) observed by ionosondes can be used to detect the occurrence of the daytime eastward PPEF during intense geomagnetic storms irrespective of season and level of solar activity. The data also show two rare events of strong daytime westward electric fields due to disturbance dynamo and/or prompt penetration. The results are important when radars are not available to monitor the occurrence of the PPEF. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA016001

Scopus

Acoustic resonance and plasma depletion detected by GPS total electron content observation after the 2011 off the Pacific coast of Tohoku Earthquake Reviewed

A. Saito, T. Tsugawa, Y. Otsuka, M. Nishioka, T. Iyemori, M. Matsumura, S. Saito, C. H. Chen, Y. Goi, N. Choosakul

EARTH PLANETS AND SPACE   Vol. 63 ( 7 ) page： 863 - 867   2011

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Two-dimensional structures of the ionospheric variations generated by the acoustic resonance between the ground surface and the lower thermosphere was observed for the first time near the epicenter after the M 9.0 Tohoku earthquake on March 11, 2011. A short-period oscillation of total electron content was observed by a GPS receiver array after the earthquake for four hours in the vicinity of the epicenter. It was centered in the east of the epicenter where the tsunami was estimated to commence. The frequency of the dominant mode of the oscillation was 4.5 mHz, 222 seconds of period, while there were minor oscillations whose frequency were 3.7 mHz and 5.3 mHz. These periods are consistent with the periods of the acoustic resonance between the ground surface and the lower thermosphere, predicted by a numerical model. The amplitude of the TEC oscillation showed a gradual change of the amplitude. The two-dimensional distributions of TEC variations generated by this resonance had wave frontal structures that extended from northwest to southeast. The resonant oscillation of the TEC was accompanied by a depletion of TEC whose duration was about 60 minutes. The area of this depletion also centered on the epicenter.

DOI： 10.5047/eps.2011.06.034

Web of Science

Decay of polar cap patch Reviewed

K. Hosokawa, J. I. Moen, K. Shiokawa, Y. Otsuka

Journal of Geophysical Research: Space Physics   Vol. 116 ( 5 )   2011

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We report an event in which a polar cap patch was detected with an all-sky imager (ASI) at Resolute Bay, Canada (74.73° N, 265.07° E
AACGM latitude 82.9°), on the nightside. The patch stopped its antisunward motion associated with a northward turning of interplanetary magnetic field and stayed within the field of view of the ASI for more than 1 h. When the patch stagnated, its luminosity decreased gradually, which allows us to investigate how the patch plasma decayed in a quantitative manner. The decay of the patch can be quantitatively explained by the loss through recombinations of O+ with ambient N2 and O2 molecules, if we assume the altitude of the optical patch to be around 295 km. The derived altitude of the patch around 295 km is much higher than the nominal value at 235 km obtained from the MSIS-E90 and IRI-2007 models, indicating that climatological models such as IRI are not suitable for describing the actual density profile of patches. This is probably because the loss process was much faster in the lower-altitude part of the patch
thus, the peak altitude of the patch increased as it traveled across the polar cap because of rapid recombination at the bottomside of the F region. This suggests that we should employ higher emission altitude when we investigate optical patches transported deep into the nightside polar cap. Such information is important when we compare the optical data with other instruments such as coherent radars and GPS scintillation measurements by mapping the all-sky image on the geographic coordinate system with an assumption of the patch emission altitude. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA016297

Scopus

Equatorial electrodynamics and neutral background in the Asian sector during the 2009 stratospheric sudden warming Reviewed

Huixin Liu, Mamoru Yamamoto, S. Tulasi Ram, Takuya Tsugawa, Yuichi Otsuka, Claudia Stolle, Eelco Doornbos, Kiyohumi Yumoto, Tsutomu Nagatsuma

Journal of Geophysical Research: Space Physics   Vol. 116 ( 8 )   2011

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Using ground observations of total electron content (TEC) and equatorial electrojet (EEJ) in the Asian sector, along with plasma and neutral densities obtained from the CHAMP satellite, we investigate the ionospheric electrodynamics and neutral background in this longitude sector during the major stratospheric sudden warming (SSW) in January 2009. Our analysis reveals the following prominent features. First, the TEC response in tropical regions is strongly latitude dependent, with monotonic depletion at the dip equator but a semidiurnal perturbation at low latitudes. Second, the TEC semidiurnal perturbation possesses a significant hemispheric asymmetry in terms of onset date and magnitude. It starts on the same day as the SSW peak in the Northern Hemisphere but 2 days later in the Southern Hemisphere. Its magnitude is twice as strong in the north than in the south. Third, strong counter electrojet occurs in the afternoon, following the strengthening of the eastward EEJ in the morning. Fourth, semidiurnal perturbation in both TEC and EEJ possesses a phase shift, at a rate of about 0.7 h/day. Comparisons with results reported in the Peruvian sector reveal clear longitude dependence in the amplitude and hemispheric asymmetry of the semidiurnal perturbation. Finally, thermospheric density undergoes ∼25% decrease at low latitudes in the afternoon local time sector during the SSW, indicating significant cooling effects in the tropical upper thermosphere. Copyright © 2011 by the American Geophysical Union.

DOI： 10.1029/2011JA016607

Scopus

Imaging observation of the Earth's mesosphere, thermosphere and ionosphere by VISI of ISS-IMAP on the International Space Station Reviewed

Sakanoi, T, Akiya, Y, Yamazaki, A, Otsuka, Y, Saito, A, Yoshikawa, I

IEEJ Transactions on Fundamentals and Materials   Vol. 131 ( 12 ) page： 983 - 988   2011

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We present the scientific targets, instrumental design, principle of measurement, and development of the Visible and near-Infrared Spectral Imager (VISI) which will be launched onto International Space Station (ISS) in January 2012. VISI is equipped with fast optics including a grism, and a high-sensitivity CCD to obtain airglow emissions at wavelengths of O (630 nm), OH Meinel band (650 nm) and O₂ (0-0) atmospheric band (762 nm). VISI has two field-of-views (FOVs), 45 degrees forward and 45 degrees backward to nadir, to subtract background contaminations. The data acquisition of VISI will be continuously performed with short exposure cycle less than several seconds when ISS is orbiting in the night side of the earth, and then provide the seamless line scanning images of the airglows with a spatial resolution better than 50 km. After manufacturing VISI, we checked its performance though optical and electric function tests as well as environmental tests that are necessary to be launched.

DOI： 10.1541/ieejfms.131.983

CiNii Books

Other Link： https://jlc.jst.go.jp/DN/JALC/00383579674?from=CiNii

New aspects of thermospheric and ionospheric storms revealed by CHAMP Reviewed

N. Balan, M. Yamamoto, J. Y. Liu, Y. Otsuka, H. Liu, H. Lühr

Journal of Geophysical Research: Space Physics   Vol. 116 ( 7 )   2011

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The neutral mass density N and electron density Ne at 400 km height measured by CHAMP during nine intense geomagnetic storms bring out some new aspects of the thermospheric and ionospheric storms. The thermospheric storms (increase of N) develop with the onset of the main phases (MP) of the geomagnetic storms and reach their peak phases before or by the end of the MPs. The ionospheric storms (change of Ne) in general undergo an initial negative phase (with the equatorial ionization anomaly (EIA) crests shifting poleward) before turning positive, and the positive storms reach their peak strengths (or phases) centered at 25-30 magnetic latitudes
in some (4) cases the positive storms develop without an initial negative phase and with the EIA crests shifting equatorward
in all cases the positive storms reach their peak phases before the end of the MPs and turn to conventional negative storms by the end of the MPs. The observations agree with the different aspects of a physical mechanism of the positive storms. The observations also reveal that the Halloween storms of 30 October 2003 with a short MP without fluctuations produced the strongest positive ionospheric storms through impulsive response, and there is strong equinoctial asymmetry in the ionosphere and thermosphere during geomagnetic storms. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA016399

Scopus

Motion of polar cap arcs Reviewed

Hosokawa, K, J. I. Moen, K. Shiokawa, Y. Otsuka

Journal of Geophysical Research   Vol. 116   page： doi:10.1029/2010JA015906   2011

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Long-distance propagation of ionospheric disturbance generated by the 2011 off the Pacific coast of Tohoku Earthquake Reviewed

C. H. Chen, A. Saito, C. H. Lin, J. Y. Liu, H. F. Tsai, T. Tsugawa, Y. Otsuka, M. Nishioka, M. Matsumura

EARTH PLANETS AND SPACE   Vol. 63 ( 7 ) page： 881 - 884   2011

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Propagation of the initial ionospheric total electron content (TEC) disturbances generated by the 2011 off the Pacific coast of Tohoku Earthquake at 05: 46: 23 UT on March 11, 2011, was investigated with ground-based Global Positioning System (GPS) receivers in the east-Asian region. It was found that the initial ionospheric disturbance formed a zonal wave front after the earthquake occurrence. Four zonal wave fronts of this initial ionospheric disturbance were observed to travel southward from Japan to Taiwan with a velocity of about 1,0001,700 m/s. This study further found that the direction of the wave vector rotated from the south-southwest to the south-southeast as it traveled from Japan to Taiwan. The meridional propagation of the coseismic ionospheric disturbances is consistent with those observed after previous intense earthquakes. The temporal evolutions of initial ionospheric disturbances, after the earthquake, near the epicenter was observed in two-dimensions. The directivity of the disturbances was caused by a geomagnetic field effect.

DOI： 10.5047/eps.2011.06.026

Web of Science

ISS-IMAPミッションによって明らかになる超高層大気の姿

斎藤昭則, 山崎敦, 阿部琢美, 鈴木睦, 坂野井健, 藤原均, 吉川一朗, 大塚雄一, 田口真, 山本衛, 中村卓司, 江尻省, 菊池雅行, 河野英昭, 石井守, 久保田実, 星野尾一明, 坂野井和代

第25回大気圏シンポジウム     2011

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Ionospheric multiple stratifications and irregularities induced by the 2011 off the Pacific coast of Tohoku Earthquake Reviewed

Takashi Maruyama, Takuya Tsugawa, Hisao Kato, Akinori Saito, Yuichi Otsuka, Michi Nishioka

EARTH PLANETS AND SPACE   Vol. 63 ( 7 ) page： 869 - 873   2011

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A strong earthquake with a magnitude of 9.0 occurred at 1446: 23 JST on March 11, 2011, in Japan. Ionospheric disturbances were detected at 1500 JST at four ionosonde stations. An irregular distortion of echo trace was observed at Kokubunji, which is the nearest station to the epicenter and is 440 km from it. Multiple-cusp-type trace indicating extra stratification was observed at Wakkanai and Yamagawa, which are 870 and 1410 km away from the epicenter. A small wavy fluctuation was observed at Okinawa 1910 km away from the epicenter. The real height analysis of the ionograms showed a vertical structure with a scale size of 20 similar to 30 km.

DOI： 10.5047/eps.2011.06.008

Web of Science

Ionospheric disturbances detected by GPS total electron content observation after the 2011 off the Pacific coast of Tohoku Earthquake Reviewed

T. Tsugawa, A. Saito, Y. Otsuka, M. Nishioka, T. Maruyama, H. Kato, T. Nagatsuma, K. T. Murata

EARTH PLANETS AND SPACE   Vol. 63 ( 7 ) page： 875 - 879   2011

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All the details of ionospheric disturbances following the 2011 Tohoku Earthquake were first revealed by the high-resolution GPS total electron content observation in Japan. The initial ionospheric disturbance appeared as sudden depletions following small impulsive TEC enhancements similar to 7 minutes after the earthquake onset, near the epicenter. Then, concentric waves appeared to propagate in the radial direction with a velocity of 138-3,457 m/s. Zonally-extended enhancements of the TEC also appeared in the west of Japan. In the vicinity of the epicenter, short-period oscillations with a period of similar to 4 minutes were observed. This paper focuses on the concentric waves. The concentric pattern indicates that they had a point source. The center of these structures, termed the "ionospheric epicenter", was located about 170 km from the epicenter in the southeast direction. According to the propagation characteristics, these concentric waves could be caused by atmospheric waves classified into three types: acoustic waves generated from a propagating Rayleigh wave, acoustic waves from the ionospheric epicenter, and atmospheric gravity waves from the ionospheric epicenter. The amplitude of the concentric waves was not uniform and was dependent on the azimuth of their propagation direction, which could not be explained by previously-proposed theory.

DOI： 10.5047/eps.2011.06.035

Web of Science

Numerical simulations of atmospheric waves excited by the 2011 off the Pacific coast of Tohoku Earthquake Reviewed

M. Matsumura, A. Saito, T. Iyemori, H. Shinagawa, T. Tsugawa, Y. Otsuka, M. Nishioka, C. H. Chen

EARTH PLANETS AND SPACE   Vol. 63 ( 7 ) page： 885 - 889   2011

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Numerical simulations are performed to simulate atmospheric perturbations observed at ionospheric heights just after the 2011 off the Pacific coast of Tohoku Earthquake. A time-dependent, two-dimensional, nonlinear, non-hydrostatic, compressible and neutral, numerical model is developed to reproduce the atmospheric perturbations. An impulsive upward surface motion is assumed as the source of the perturbations. Simulated atmospheric perturbations at 300-km altitude show remarkable agreement with oscillations observed in the ionospheric total electron content (TEC) when the source width is about 250 km. In the vicinity of the source, the acoustic resonance modes between the ground surface and the lower thermosphere are dominant. They have three dominant frequencies for the interval between 20 and 60 min after the impulsive input. The perturbation with the maximum amplitude has a frequency of 4.4 mHz. The other dominant modes have frequencies of 3.6 and 5.1 mHz. The beats between the dominant modes are also seen. In the distance, the gravity modes are dominant. The horizontal phase velocities are about 220 to 300 m/s, and the horizontal wavelengths are about 200 to 400 km. The good agreement between the simulation and the observations indicates that ionospheric oscillations generated by the earthquake are mainly due to the motion of the neutral atmosphere.

DOI： 10.5047/eps.2011.07.015

Web of Science

Propagation of large amplitude ionospheric disturbances with velocity dispersion observed by the SuperDARN Hokkaido radar after the 2011 off the Pacific coast of Tohoku Earthquake Reviewed

Nozomu Nishitani, Tadahiko Ogawa, Yuichi Otsuka, Keisuke Hosokawa, Tomoaki Hori

EARTH PLANETS AND SPACE   Vol. 63 ( 7 ) page： 891 - 896   2011

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Ionospheric responses to the 2011 off the Pacific coast of Tohoku Earthquake are studied using the SuperDARN Hokkaido radar, which is located at (43.5 degrees N, 143.6 degrees E) and which monitors the ionosphere over a wide horizontal area. The radar observed an oscillation of the vertical motion of the ionosphere with a period of about 1 to 2 min. The disturbance propagated northward, away from the epicenter with the velocity of about 6.2, 4.5, 3.9 and 3.5 km/s. The latter three values are basically consistent with the propagation of the Earth&apos;s surface waves reported in several previous studies. The propagation velocities decreased with time, which has not been reported in previous studies for this propagation velocity range. The peak-to-peak amplitudes of Doppler velocities of ground/sea scatter echoes observed by the radar were up to 200 m/s, which is considerably larger than previously-reported values using HF Doppler measurements, although they are not extremely large for this historical earthquake (M = 9.0). This is the first time that ionospheric data have been obtained with high temporal (8 s) and spatial (22.5 km) resolutions following a giant earthquake, which enables us to discuss the detailed characteristics of the propagation of coseismic ionospheric disturbances.

DOI： 10.5047/eps.2011.07.003

Web of Science

On postmidnight low-latitude ionospheric irregularities during solar minimum: 2. C/NOFS observations and comparisons with the Equatorial Atmosphere Radar Reviewed

T. Yokoyama, R. F. Pfaff, P. A. Roddy, M. Yamamoto, Y. Otsuka

Journal of Geophysical Research: Space Physics   Vol. 116 ( 11 )   2011

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A detailed comparison between the observations of the Communication/ Navigation Outage Forecasting System (C/NOFS) satellite and the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36S dip latitude) on the postmidnight irregularities is presented. The zonal and meridional E × B drift velocities measured by the vector electric field instrument on the C/NOFS are consistent with the westward propagation of backscatter echoes and the line-of-sight Doppler velocities observed with the EAR, respectively. The plasma density depletions are observed in the postmidnight sector for several consecutive orbits, which suggests the depletions grow slowly during the premidnight period and reach the spacecraft altitude around local midnight. The convergence of the equatorward wind which could be responsible for the midnight temperature maximum may produce a preferable condition for the growth of the Rayleigh-Taylor instability around midnight. Electric field fluctuations of medium-scale traveling ionospheric disturbances may play an important role in seeding the instability. Both equatorial and midlatitude-type plasma instabilities could be operational at the EAR latitude sector, which together would foster a high occurrence of postmidnight irregularities during solar minimum. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2011JA016798

Scopus

On postmidnight low-latitude ionospheric irregularities during solar minimum: 1. Equatorial Atmosphere Radar and GPS-TEC observations in Indonesia Reviewed

T. Yokoyama, M. Yamamoto, Y. Otsuka, M. Nishioka, T. Tsugawa, S. Watanabe, R. F. Pfaff

Journal of Geophysical Research: Space Physics   Vol. 116 ( 11 )   2011

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Using the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36S dip latitude), it is shown that postmidnight irregularities during solar minimum are morphologically different from those detected during solar maximum and are quite similar to those observed with the middle and upper atmosphere (MU) radar in midlatitudes (29.3N dip latitude). Utilizing the rapid beam-steering capability of the EAR, the spatial structure of the postmidnight irregularities is clearly presented for the first time. It is found that they usually propagate westward and can be categorized into two types. One shows sharp upwelling plumes near local midnight, which should not be a mere passage of fossil plasma bubbles. The other has successive tilted structures which have the same orientation as medium-scale traveling ionospheric disturbances typically observed at midlatitudes. We suggest that the convergence of the equatorward thermospheric wind which is believed to be responsible for the midnight temperature maximum may be an important factor to produce a preferable condition for the upwelling plumes in the postmidnight sector. The displacement between geographic and magnetic equators may also be important for seasonal/longitudinal variation of the postmidnight irregularities. © 2011 by the American Geophysical Union.

DOI： 10.1029/2011JA016797

Scopus

Vertical connection from the tropospheric activities to the ionospheric longitudinal structure simulated by a new Earth's whole atmosphere-ionosphere coupled model Reviewed

H. Jin, Y. Miyoshi, H. Fujiwara, H. Shinagawa, K. Terada, N. Terada, M. Ishii, Y. Otsuka, A. Saito

Journal of Geophysical Research: Space Physics   Vol. 116 ( 1 )   2011

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This paper introduces a new Earth's atmosphere-ionosphere coupled model that treats seamlessly the neutral atmospheric region from the troposphere to the thermosphere as well as the thermosphere-ionosphere interaction including the electrodynamics self-consistently. The model is especially useful for the study of vertical connection between the meteorological phenomena and the upper atmospheric behaviors. As an initial simulation using the coupled model, we have carried out a 30 day consecutive run in September. The result reveals that the longitudinal structure of the F-region ionosphere varies on a day-to-day basis in a highly complex way and that a four-peak structure of the daytime equatorial ionization anomaly (EIA) similar to the recent observations appears as an averaged feature. The simulation reproduces and thus confirms the vertical coupling processes proposed so far with respect to the formation of the averaged EIA longitudinal structure
the excitation of solar nonmigrating tides in the troposphere, their propagation through the middle atmosphere, and the modulation of ionospheric dynamo, which in turn affects EIA generation. The simulation result indicates that not only the ionospheric averaged longitudinal structure but also the day-to-day variation can be modulated significantly by the lower atmospheric effect. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA015925

Scopus

Dynamic temporal evolution of polar cap tongue of ionization during magnetic storm Reviewed

K. Hosokawa, T. Tsugawa, K. Shiokawa, Y. Otsuka, N. Nishitani, T. Ogawa, M. R. Hairston

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.12

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During a magnetic storm on 14-16 December 2006, a polar cap tongue of ionization (TOI) was detected by an all-sky imager (ASI) at Resolute Bay, Canada (74.73 degrees N, 265.07 degrees E). We investigate the temporal evolution and spatial structure of the TOI in detail by combining the optical data with other observations (e. g., solar wind, GPS total electron content, SuperDARN, and DMSP and NOAA POES satellites). The TOI was observed as a bright and elongated 630 nm airglow plume for 4 h during the main phase of the storm. This interval corresponded to a period of prolonged stable large-amplitude southward IMF during a coronal mass ejection (CME). One to one and a half hours before the appearance of TOI, the polar cap boundary expanded rapidly far equatorward, and a positive ionospheric storm occurred. This implies that both the "expansion of the high-latitude plasma convection" and "build up of the source plasma in the midlatitudes" are necessary conditions for the formation of a TOI. Because both of them were triggered by a major southward turning of the IMF, the prolonged large-amplitude southward IMF orientation in the trailing part of the CME was primarily responsible for the generation of TOI. After its appearance, the TOI exhibited dynamic motion in the dawn to dusk direction. Simultaneous SuperDARN data suggest that a longitudinal progression of subauroral polarization stream controlled this dynamic motion. The optical TOI was found to be a continuous stream elongated in the noon-midnight direction although it contained some mesoscale patterns. Absence of large-scale temporal changes in the cusp plasma flow during the stable IMF period allowed the TOI to remain continuous without being broken into polar cap patches. The mesoscale structures within the TOI were probably produced by small-scale velocity fluctuations in the cusp plasma flow. The TOI as visualized with the all-sky airglow imager was found to be much more dynamic and much more complicated than we ever thought. The current study indicates that such a behavior of the TOI was presumably caused by a combination of temporal variations in the global-scale plasma circulation system, expansion and contraction of the polar cap area, and plasma density changes in the dayside low to midlatitudes.

DOI： 10.1029/2010JA015848

Web of Science

Midnight latitude-altitude distribution of 630 nm airglow in the Asian sector measured with FORMOSAT-2/ISUAL Reviewed

Toru Adachi, Masashi Yamaoka, Mamoru Yamamoto, Yuichi Otsuka, Huixin Liu, Chun-Chieh Hsiao, Alfred B. Chen, Rue-Ron Hsu

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.9

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The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) payload on board the FORMOSAT-2 satellite carried out the first limb imaging observation of 630 nm airglow for the purpose of studying physical processes in the F region ionosphere. For a total of 14 nights in 2006-2008, ISUAL scanned the midnight latitude-altitude distribution of 630 nm airglow in the Asian sector. On two nights of relatively active conditions (Sigma Kp = 26, 30+) we found several bright airglow regions, which were highly variable each night in terms of luminosity and location. In relatively quiet conditions (Sigma Kp = 4-20) near May/June we found two bright regions which were stably located in the midlatitude region of 40 degrees S-10 degrees S (50 degrees S-20 degrees S magnetic latitude (MLAT)) and in the equatorial region of 0 degrees-10 degrees N (10 degrees S-0 degrees MLAT). On one of the quiet nights, FORMOSAT-3/COSMIC and CHAMP simultaneously measured the plasma density in the same region where ISUAL observed airglow. The plasma density data generally show good agreement, suggesting that plasma enhancements were the primary source of these two bright airglow regions. From detailed comparison with past studies we explain that the airglow in the equatorial region was due to the midnight brightness wave produced in association with the midnight temperature maximum, while that in the midlatitude region was due to the typical plasma distribution usually formed in the midnight sector. The fact that the equatorial airglow was much brighter than the midlatitude airglow and was observed on most nights during the campaign period strongly suggests the importance of further studies on the MTM/MBW phenomenology, which is not well reproduced in the current general circulation model.

DOI： 10.1029/2009JA015147

Web of Science

Large-scale traveling ionospheric disturbance observed by superDARN Hokkaido HF radar and GPS networks on 15 December 2006 Reviewed

Hayashi, H, Nishitani, N, Ogawa, T, Otsuka, Y, Tsugawa, T, Hosokawa, K, Saito, A

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.6

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DOI： 10.1029/2009JA014297

Large-scale traveling ionospheric disturbance observed by superDARN Hokkaido HF radar and GPS networks on 15 December 2006 Reviewed

H. Hayashi, N. Nishitani, T. Ogawa, Y. Otsuka, T. Tsugawa, K. Hosokawa, A. Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

On 15 December 2006, during the main phase of a relatively large storm, Doppler velocity data from the Super Dual Aural Radar Network (SuperDARN) Hokkaido radar, together with total electron content (TEC) data from the GPS Earth Observation Network (GEONET), recorded daytime large-scale traveling ionospheric disturbances (LSTIDs). We studied two disturbances propagating southward and one disturbance propagating northward between 0000 and 0600 UT on 15 December 2006. The former disturbances were LSTIDs typical of those reported in many previous studies, whereas the latter was confirmed as an LSTID propagating from the Southern into the Northern Hemisphere, reported in a few past studies. From comparisons of SuperDARN Hokkaido radar Doppler velocity and GEONET TEC, we found a positive correlation between downward ionospheric motion and increasing TEC. This relationship is consistent with results of model calculation. This is the first observation of LSTIDs ranging from high to low latitude combining simultaneous SuperDARN HF radar and GPS network observations.

DOI： 10.1029/2009JA014297

Web of Science

Nighttime-like quasi periodic echoes induced by a partial solar eclipse Reviewed

Smitha V. Thampi, Mamoru Yamamoto, Huixin Liu, Susumu Saito, Yuichi Otsuka, Amit Kumar Patra

GEOPHYSICAL RESEARCH LETTERS   Vol. 37   2010.5

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The first observations of solar eclipse induced mid-latitude plasma irregularities using the middle and upper atmosphere radar (MU radar) at Shigaraki (34.85 degrees N, 136.1 degrees E, 25.0 degrees N geomagnetic) are presented. The observations were done during the partial solar eclipse on 22 July, 2009. The observations show that the sudden withdrawal of solar radiation could deplete the background E-region densities, thereby unmasking the long-lived metallic ions within the strong and patchy Sporadic E-layers. As a result of this, Quasi-Periodic (QP) echoes were generated, which were detected by the MU radar. These echoes resemble the normal post-sunset QP echoes observed over mid-latitudes as revealed by the multi-channel interfereometry imaging. This example shows that over mid-latitudes E-region plasma irregularities can be generated during a partial solar eclipse, revealing a hitherto unobserved aspect of mid-latitude ionospheric responses to eclipses. Citation: Thampi, S. V., M. Yamamoto, H. Liu, S. Saito, Y. Otsuka, and A. K. Patra (2010), Nighttime-like quasi periodic echoes induced by a partial solar eclipse, Geophys. Res. Lett., 37, L09107, doi: 10.1029/2010GL042855.

DOI： 10.1029/2010GL042855

Web of Science

Longitudinal development of low-latitude ionospheric irregularities during the geomagnetic storms of July 2004 Reviewed

Guozhu Li, Baiqi Ning, Lianhuan Hu, Libo Liu, Xinan Yue, Weixing Wan, Biqiang Zhao, K. Igarashi, Minoru Kubota, Yuichi Otsuka, J. S. Xu, J. Y. Liu

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115 ( 4 )   2010.4

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During the period 22-28 July 2004, three geomagnetic storms occurred due to a sequence of coronal mass ejections. In this paper we present and discuss the ionospheric observations from a set of in situ satellites and ground-based GPS total electron content and scintillation receivers, a VHF radar, and two chains of ionosondes (similar to 300 degrees E and similar to 120 degrees E, respectively) that provide the evolutionary characteristics of equatorial and low-latitude ionospheric irregularities versus longitude during these storm periods. It is found that the irregularities occurred over a wide longitudinal range, extending from around 300 degrees E to 120 degrees E on storm days 25 and 27 July 2004. On 25 July plasma bubbles (PBs) began premidnight in America and postmidnight in Southeast Asia. On 27 July the occurrence of irregularities followed the sunset terminator and was observed sequentially after sunset from American to Southeast Asian longitudes. Past studies have reported that storm-time low-latitude ionospheric irregularities are mostly confined to a narrower longitude range, &lt;90 degrees, after sunset hours and are associated with the prompt penetration of eastward electric fields (PPEFs) into low latitudes. In June solstice months the occurrence of range-type spread F or PBs is very low in Southeast Asian and South American sectors. In contrast, the present results indicate that geomagnetic storms triggered the wide longitudinal development of PBs. In the American sector this was probably due to the effects of PPEFs on both storm days. However, in the Southeast Asian sector the PBs on the 2 days probably arose from disturbance dynamo electric field (DDEF), PPEF, and gravity wave seeding effects. This study further shows that under complex storm conditions, besides the long duration or multiple penetrations, the combined effects of PPEFs and DDEFs could result in a wide longitude extent of ionospheric irregularities at times.

DOI： 10.1029/2009JA014830

Web of Science

Scopus

A physical mechanism of positive ionospheric storms at low latitudes and midlatitudes Reviewed

Balan N, Shiokawa K, Otsuka Y, Kikuchi T, Lekshmi D. Vijaya, Kawamura S, Yamamoto M, Bailey G. J

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.2

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DOI： 10.1029/2009JA014515

Web of Science

Reorganization of polar cap patches through shears in the background plasma convection Reviewed

K. Hosokawa, J. -P. St-Maurice, G. J. Sofko, K. Shiokawa, Y. Otsuka, T. Ogawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 115   2010.1

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On the night of December 20, 2006, 630 nm airglow images obtained by an all-sky camera at Resolute Bay, Canada (74.73 degrees N, 265.07 degrees E; altitude adjusted corrected geomagnetic (AACGM) latitude 82.9 degrees) showed the passage of successive polar cap patches. Shortly after convection came to a temporary halt, one of the patches was reorganized into two substructures in approximately 8 min. The two-dimensional background ionospheric convection pattern measured using the newly deployed PolarDARN radar at Rankin Inlet (62.82 degrees N, 93.11 degrees W; AACGM latitude 72.96 degrees) showed that a velocity shear of approximately 120 m s(-1)/340 km suddenly appeared in the vicinity of the patch at the time of reorganization. A qualitative analysis of the relationship between the magnitude of the velocity shear and the distance between the divided patches indicates that the shear in the background plasma convection velocity significantly contributed to the reorganization of the patch. This shear structure appeared soon after a southward turning of the interplanetary magnetic field (IMF) and was probably associated with the reconfiguration of the convection pattern from a pre-existing northward-oriented IMF pattern to a southward-oriented one. The present observations indicate that the reconfiguration/deformation of patches because of a shear in the background convection field, especially reorganization of patches into smaller substructures, may play an important role in the rapid structuring of patches.

DOI： 10.1029/2009JA014599

Web of Science

A physical mechanism of positive ionospheric storms at low latitudes and midlatitudes Reviewed

N. Balan, K. Shiokawa, Y. Otsuka, T. Kikuchi, D. Vijaya Lekshmi, S. Kawamura, M. Yamamoto, G. J. Bailey

Journal of Geophysical Research: Space Physics   Vol. 115 ( 2 )   2010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

[1] A physical mechanism of the positive ionospheric storms at low latitudes and midlatitudes is presented through multi-instrument observations, theoretical modeling, and basic principles. According to the mechanism, an equatorward neutral wind is required to produce positive ionospheric storms. The mechanical effects of the wind (1) reduce (or stop) the downward diffusion of plasma along the geomagnetic field lines, (2) raise the ionosphere to high altitudes of reduced chemical loss, and hence (3) accumulate the plasma at altitudes near and above the ionospheric peak centered at around ±30° magnetic latitudes. Daytime eastward prompt penetration electric field (PPEF), if it occurs, also shifts the equatorial ionization anomaly crests to higher than normal latitudes, up to approximately ±30° latitudes. The positive ionospheric storms are most likely in the longitudes where the onset of the geomagnetic storms falls in the ionization production dominated morning-noon local time sector when the plasma accumulation due to the mechanical effects of the wind largely exceeds the plasma loss due to the chemical effect of the wind. The mechanism agrees with the multi-instrument observations made during the supergeomagnetic storm of 7-8 November 2004, with 18 h long initial phase (IP) and 10 h long main phase (MP). The observations, which are mainly in the Japanese-Australian longitudes where the MP onset was in the morning (0600 LT, 2100 UT), show (1) strong positive ionospheric storms (in Ne, Nmax, hmax, Global Positioning System-total electron content (GPS-TEC), and 630 nm airglow intensity) in both Northern and Southern hemispheres started at the morning (0600 LT) MP onset and lasted for a day, (2) repeated occurrence of strong eastward PPEF events penetrated after the MP onset and superposed with westward electric field started before the MP onset, and (3) storm time equatorward neutral winds (inferred from 1 and 2). Repeated occurrence of an unusually strong F3 layer with large density depletions around the equator was also observed during the morning-noon MP. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2009JA014515

Scopus

Dynamic temporal evolution of polar cap tongue of ionization during magnetic storm Reviewed

K. Hosokawa, T. Tsugawa, K. Shiokawa, Y. Otsuka, N. Nishitani, T. Ogawa, M. R. Hairston

Journal of Geophysical Research: Space Physics   Vol. 115 ( 12 )   2010

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During a magnetic storm on 14-16 December 2006, a polar cap tongue of ionization (TOI) was detected by an all-sky imager (ASI) at Resolute Bay, Canada (74.73°N, 265.07°E). We investigate the temporal evolution and spatial structure of the TOI in detail by combining the optical data with other observations (e.g., solar wind, GPS total electron content, SuperDARN, and DMSP and NOAA POES satellites). The TOI was observed as a bright and elongated 630 nm airglow plume for 4 h during the main phase of the storm. This interval corresponded to a period of prolonged stable large-amplitude southward IMF during a coronal mass ejection (CME). One to one and a half hours before the appearance of TOI, the polar cap boundary expanded rapidly far equatorward, and a positive ionospheric storm occurred. This implies that both the "expansion of the high-latitude plasma convection" and "build up of the source plasma in the midlatitudes" are necessary conditions for the formation of a TOI. Because both of them were triggered by a major southward turning of the IMF, the prolonged large-amplitude southward IMF orientation in the trailing part of the CME was primarily responsible for the generation of TOI. After its appearance, the TOI exhibited dynamic motion in the dawn to dusk direction. Simultaneous SuperDARN data suggest that a longitudinal progression of subauroral polarization stream controlled this dynamic motion. The optical TOI was found to be a continuous stream elongated in the noon-midnight direction although it contained some mesoscale patterns. Absence of large-scale temporal changes in the cusp plasma flow during the stable IMF period allowed the TOI to remain continuous without being broken into polar cap patches. The mesoscale structures within the TOI were probably produced by small-scale velocity fluctuations in the cusp plasma flow. The TOI as visualized with the all-sky airglow imager was found to be much more dynamic and much more complicated than we ever thought. The current study indicates that such a behavior of the TOI was presumably caused by a combination of temporal variations in the global-scale plasma circulation system, expansion and contraction of the polar cap area, and plasma density changes in the dayside low to midlatitudes. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2010JA015848

Scopus

Large-scale traveling ionospheric disturbance observed by superDARN Hokkaido HF radar and GPS networks on 15 December 2006 Reviewed

H. Hayashi, N. Nishitani, T. Ogawa, Y. Otsuka, T. Tsugawa, K. Hosokawa, A. Saito

Journal of Geophysical Research: Space Physics   Vol. 115 ( 6 )   2010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

On 15 December 2006, during the main phase of a relatively large storm, Doppler velocity data from the Super Dual Aural Radar Network (SuperDARN) Hokkaido radar, together with total electron content (TEC) data from the GPS Earth Observation Network (GEONET), recorded daytime large-scale traveling ionospheric disturbances (LSTIDs). We studied two disturbances propagating southward and one disturbance propagating northward between 0000 and 0600 UT on 15 December 2006. The former disturbances were LSTIDs typical of those reported in many previous studies, whereas the latter was confirmed as an LSTID propagating from the Southern into the Northern Hemisphere, reported in a few past studies. From comparisons of SuperDARN Hokkaido radar Doppler velocity and GEONET TEC, we found a positive correlation between downward ionospheric motion and increasing TEC. This relationship is consistent with results of model calculation. This is the first observation of LSTIDs ranging from high to low latitude combining simultaneous SuperDARN HF radar and GPS network observations. Copyright © 2010 by the American Geophysical Union.

DOI： 10.1029/2009JA014297

Scopus

Midnight latitude-altitude distribution of 630 nm airglow in the Asian sector measured with FORMOSAT-2/ISUAL Reviewed

Toru Adachi, Masashi Yamaoka, Mamoru Yamamoto, Yuichi Otsuka, Huixin Liu, Chun-Chieh Hsiao, Alfred B. Chen, Rue-Ron Hsu

Journal of Geophysical Research: Space Physics   Vol. 115 ( 9 )   2010

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The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) payload on board the FORMOSAT-2 satellite carried out the first limb imaging observation of 630 nm airglow for the purpose of studying physical processes in the F region ionosphere. For a total of 14 nights in 2006-2008, ISUAL scanned the midnight latitude-altitude distribution of 630 nm airglow in the Asian sector. On two nights of relatively active conditions (ΣKp = 26, 30+) we found several bright airglow regions, which were highly variable each night in terms of luminosity and location. In relatively quiet conditions (ΣKp = 4-20) near May/June we found two bright regions which were stably located in the midlatitude region of 40°S-10°S (50°S-20°S magnetic latitude (MLAT)) and in the equatorial region of 0°-10°N (10°S-0° MLAT). On one of the quiet nights, FORMOSAT-3/COSMIC and CHAMP simultaneously measured the plasma density in the same region where ISUAL observed airglow. The plasma density data generally show good agreement, suggesting that plasma enhancements were the primary source of these two bright airglow regions. From detailed comparison with past studies we explain that the airglow in the equatorial region was due to the midnight brightness wave produced in association with the midnight temperature maximum, while that in the midlatitude region was due to the typical plasma distribution usually formed in the midnight sector. The fact that the equatorial airglow was much brighter than the midlatitude airglow and was observed on most nights during the campaign period strongly suggests the importance of further studies on the MTM/MBW phenomenology, which is not well reproduced in the current general circulation model. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2009JA015147

Scopus

Lower-thermospheric wind fluctuations measured with an FPI during pulsating aurora at Tromso, Norway Reviewed

S. Oyama, K. Shiokawa, J. Kurihara, T. T. Tsuda, S. Nozawa, Y. Ogawa, Y. Otsuka, B. J. Watkins

ANNALES GEOPHYSICAE   Vol. 28 ( 10 ) page： 1847 - 1857   2010

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Simultaneous observations were conducted with a Fabry-Perot Interferometer (FPI) at a wavelength of 557.7 nm, an all-sky camera at a wavelength of 557.7 nm, and the European Incoherent Scatter (EISCAT) UHF radar during the Dynamics and Energetics of the Lower Thermosphere in Aurora 2 (DELTA-2) campaign in January 2009. This paper concentrated on two events during periods of pulsating aurora. The lower-thermospheric wind velocity measured with the FPI showed obvious fluctuations in both vertical and horizontal components. Of particular interest is that the location of the fluctuations was found in a darker area that appeared within the pulsating aurora. During the same time period, the EISCAT radar observed sporadic enhancements in the F-region backscatter echo power, which suggests the presence of low-energy electron (1 keV or lower) precipitation coinciding with increase in amplitude of the electromagnetic wave (at the order of 10 Hz or higher). While we have not yet identified the dominant mechanism causing the fluctuations in FPI-derived wind velocity during the pulsating aurora, the frictional heating energy dissipated by the electric-field perturbations may be responsible for the increase in ionospheric thermal energy thus modifying the local wind dynamics in the lower thermosphere.

DOI： 10.5194/angeo-28-1847-2010

Web of Science

Reorganization of polar cap patches through shears in the background plasma convection Reviewed

K. Hosokawa, J. P. St-Maurice, G. J. Sofko, K. Shiokawa, Y. Otsuka, T. Ogawa

Journal of Geophysical Research: Space Physics   Vol. 115 ( 1 )   2010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

On the night of December 20, 2006, 630 nm airglow images obtained by an all-sky camera at Resolute Bay, Canada (74.73°N, 265.07°E
altitude adjusted corrected geomagnetic (AACGM) latitude 82.9°) showed the passage of successive polar cap patches. Shortly after convection came to a temporary halt, one of the patches was reorganized into two substructures in approximately 8 min. The two-dimensional background ionospheric convection pattern measured using the newly deployed PolarDARN radar at Rankin Inlet (62.82°N, 93.11°W
AACGM latitude 72.96°) showed that a velocity shear of approximately 120 m s-1/340 km suddenly appeared in the vicinity of the patch at the time of reorganization. A qualitative analysis of the relationship between the magnitude of the velocity shear and the distance between the divided patches indicates that the shear in the background plasma convection velocity significantly contributed to the reorganization of the patch. This shear structure appeared soon after a southward turning of the interplanetary magnetic field (IMF) and was probably associated with the reconfiguration of the convection pattern from a pre-existing northward-oriented IMF pattern to a southward-oriented one. The present observations indicate that the reconfiguration/deformation of patches because of a shear in the background convection field, especially reorganization of patches into smaller substructures, may play an important role in the rapid structuring of patches. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2009JA014599

Scopus

The STEL induction magnetometer network for observation of high-frequency geomagnetic pulsations Reviewed

K. Shiokawa, R. Nomura, K. Sakaguchi, Y. Otsuka, Y. Hamaguchi, M. Satoh, Y. Katoh, Y. Yamamoto, B. M. Shevtsov, S. Smirnov, I. Poddelsky, M. Connors

EARTH PLANETS AND SPACE   Vol. 62 ( 6 ) page： 517 - 524   2010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

The Solar-Terrestrial Environment Laboratory (STEL) induction magnetometer network has been developed to investigate the propagation characteristics of high-frequency geomagnetic pulsations in the Pc1 frequency range (0.2-5 Hz). Five induction magnetometers were installed in the period 2005-2008 at Athabasca in Canada, Magadan and Paratunka in Far East Russia, and Moshiri and Sata in Japan. The sensitivity of these magnetometers is between 0.3 and 13 V/nT at turnover frequencies of 1.7-5.5 Hz. GPS time pulses are used for accurate triggering of the 64-Hz data sampling. We show examples of PiB/Pc1 magnetic pulsations observed at these five stations, as well as the harmonic structure of ionospheric Alfven resonators observed at Moshiri. We found that the Pc1 packets are slightly modulated as they propagate from high to low latitudes in the ionospheric duct. These network observations are expected to contribute to our understanding of Pc1-range magnetic pulsations and their interaction with relativistic electrons by combining the obtained results with future satellite missions that observe radiation belt particles.

DOI： 10.5047/eps.2010.05.003

Web of Science

国際宇宙ステーションJEM曝露部からの超高層大気撮像観測計画ISS-IMAP

齊藤昭則, 阿部琢美, 山崎敦, 鈴木睦, 坂野井健, 藤原均, 吉川一朗, 大塚雄一, 田口真, 山本衛, 中村卓司, 菊池雅行, 江尻省, 河野英昭, 石井守, 久保田実, 星野尾一明, 坂野井和代

第10回宇宙科学シンポジウム講演集     2010

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Thermospheric temperature and density variations Reviewed

Hitoshi Fujiwara, Yasunobu Miyoshi, Hidekatsu Jin, Hiroyuki Shinagawa, Yuichi Otsuka, Akinori Saito, Mamoru Ishii

SOLAR AND STELLAR VARIABILITY: IMPACT ON EARTH AND PLANETS   ( 264 ) page： 310 - +   2010

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：CAMBRIDGE UNIV PRESS  

The thermosphere is the transition region from the atmosphere to space. Both the solar ultraviolet radiation and the solar wind energy inputs have caused significant thermospheric variations from past to present. In order to understand thermospheric/ionospheric disturbances in association with changes in solar activity, observational and modelling efforts have been made by many researchers. Recent satellite observations, e.g., the satellite CHAMP, have revealed mass density variations in the upper thermosphere. The thermospheric temperature, wind, and composition variations have been also investigated with general/global circulation models (GCMs) which include forcings due to the solar wind energy inputs and the lower atmospheric effects. In particular, we have developed a GCM which covers all the atmospheric regions, troposphere, stratosphere, mesosphere, and thermosphere, to describe variations of the thermospheric temperature and density caused by both effects from the lower atmosphere and the magnetosphere. GCM simulations represent global and localized temperature and density structures, winch vary from hour to hour, depending on forcings due to the lower atmosphere, solar and geomagnetic activities. This modelling attempt will enable us to describe the thermospheric weather influenced by solar activity in cooperation with ground-based and satellite observations.

DOI： 10.1017/S1743921309992857

Web of Science

Ionospheric and geomagnetic disturbances during the 2005 Sumatran earthquakes Reviewed

Alina Marie Hasbi, Mohammed Awad Momani, Mohd Alauddin Mohd Ali, Norbahiah Misran, Kazuo Shiokawa, Yuichi Otsuka, Kiyohumi Yumoto

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 71 ( 17-18 ) page： 1992 - 2005   2009.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This paper investigates the ionospheric and geomagnetic responses during the 28 March 2005 and 14 May 2005 Sumatran earthquakes using GPS and magnetometer stations located in the near zone of the epicenters. These events occurred during low solar and geomagnetic activity. TEC oscillations with periods of 5-10 min were observed about 10-24 min after the earthquakes and have horizontal propagation velocities of 922-1259 m/s. Ionospheric disturbances were observed at GPS stations located to the northeast of the epicenters, while no significant disturbances were seen relatively east and south of the epicenters. The magnetic field measurements show rapid fluctuations of 4-5 s shortly after the earthquake, followed by a Pc5 pulsation of 4.8 min about 11 min after the event. The correlation between the ionospheric and geomagnetic responses shows a good agreement in the period and time lag of the peak disturbance arrival, i.e. about 11-13 min after the earthquake. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2009.09.004

Web of Science

On the gravity wave-driven instability of E layer at mid-latitude Reviewed

S. Shalimov, T. Ogawa, Y. Otsuka

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 71 ( 17-18 ) page： 1943 - 1947   2009.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The plasma instability process during internal gravity wave propagation through the ionospheric E region is considered. The growth rate of the instability has been found and it has been shown that it depends on perturbation wavelength, gravity wave parameters and direction of propagation. The conditions for the instability are favorable when the vorticity of the associated neutral motion becomes antiparallel to the geomagnetic field. In the proposed instability mechanism plasma irregularities could seed the large-scale sporadic E layer structuring because they are generated in situ as a part of the same neutral wind structure that serves to initiate the formation of the layer. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2009.08.004

Web of Science

First observations of large-scale wave structure and equatorial spread F using CERTO radio beacon on the C/NOFS satellite Reviewed

Smitha V. Thampi, Mamoru Yamamoto, Roland T. Tsunoda, Yuichi Otsuka, Takuya Tsugawa, Jyunpei Uemoto, Mamoru Ishii

GEOPHYSICAL RESEARCH LETTERS   Vol. 36   2009.9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

First observations of large-scale wave structure (LSWS) and the subsequent development of equatorial spread F (ESF), using total electron content (TEC) derived from the ground based reception of beacon signals from the CERTO (Coherent Electromagnetic Radio Tomography) radio beacon on board C/NOFS (Communications/Navigation Outage Forecasting System) satellite, are presented. Selected examples of TEC variations, using measurements made during January 2009 from Bac Lieu, Vietnam (9.2 degrees N, 105.6 degrees E geographic, 1.7 degrees N magnetic dip latitude) are presented to illustrate two key findings: (1) LSWS appears to play a more important role in the development of ESF than the post-sunset rise (PSSR) of the F-layer, and (2) LSWS can appear well before E region sunset. Other findings, that LSWS does not have significant zonal drift in the initial stages of growth, and can have zonal wavelengths of several hundred kilometers, corroborate earlier reports. Citation: Thampi, S. V., M. Yamamoto, R. T. Tsunoda, Y. Otsuka, T. Tsugawa, J. Uemoto, and M. Ishii (2009), First observations of large-scale wave structure and equatorial spread F using CERTO radio beacon on the C/NOFS satellite, Geophys. Res. Lett., 36, L18111, doi: 10.1029/2009GL039887.

DOI： 10.1029/2009GL039887

Web of Science

Coordinated observations of nighttime medium-scale traveling ionospheric disturbances in 630-nm airglow and HF radar echoes at midlatitudes Reviewed

Suzuki S, Hosokawa K, Otsuka Y, Shiokawa K, Ogawa T, Nishitani N, Shibata T. F, Koustov A. V, Shevtsov B. M

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   page： A07312   2009.7

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DOI： 10.1029/2008JA013963

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First simultaneous observations of daytime MSTIDs over North America using GPS-TEC and DEMETER satellite data Reviewed

Tatsuo Onishi, Takuya Tsugawa, Yuichi Otsuka, Jean-Jacques Berthelier, Jean-Pierre Lebreton

GEOPHYSICAL RESEARCH LETTERS   Vol. 36   2009.6

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We present simultaneous observations of daytime medium-scale traveling ionospheric disturbances (MSTIDs) over North America, using measurements of the Total Electron Content (TEC) by the US GPS network and ionospheric plasma data from the DEMETER microsatellite. Several events show latitudinal variations of the plasma parameters at satellite altitude corresponding to the MSTID structures revealed on 2D TEC maps. In a case study with a very well defined MSTID, quasi periodic variations of the plasma density and of the ion velocity parallel to the Earth&apos;s magnetic field are observed along the satellite orbit that match the signature of the MSTID on the TEC maps. We believe it is the first simultaneous observation of parallel plasma motion in the topside ionosphere and propagating MSTID structures in the F-region. An initial analysis of this event is performed in the light of results from simple model of ionospheric disturbances associated with Atmospheric Gravity Waves (AGWs). Citation: Onishi, T., T. Tsugawa, Y. Otsuka, J.-J. Berthelier, and J.-P. Lebreton (2009), First simultaneous observations of daytime MSTIDs over North America using GPS-TEC and DEMETER satellite data, Geophys. Res. Lett., 36, L11808, doi:10.1029/2009GL038156.

DOI： 10.1029/2009GL038156

Web of Science

Spatial relationship of nighttime medium-scale traveling ionospheric disturbances and F region field-aligned irregularities observed with two spaced all-sky airglow imagers and the middle and upper atmosphere radar Reviewed

Y. Otsuka, K. Shiokawa, T. Ogawa, T. Yokoyama, M. Yamamoto

Journal of Geophysical Research: Space Physics   Vol. 114 ( 5 )   2009.5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We report simultaneous observations of medium-scale traveling ionospheric disturbances (MSTIDs) and field-aligned irregularities (FAls) in the F region using two all-sky airglow imagers and the middle and upper atmosphere (MU) radar. MSTIDs propagating southwestward were observed simultaneously in 630-nm airglow images over Sakata (39.0°N, 139.9°E) and Shigaraki (34.90N, 136.1°E), Japan, on the night of 16 June 2004. By using all-sky images over both sites, we estimated the altitude of the airglow layer to be 260 km by the triangulation method. During the MSTID event, FAIs in the F region were observed by making multibeam measurements with the MU radar at Shigaraki. In order to investigate the spatial relationship between the MSTIDs and FAIs, the FAIs were mapped onto the 630-nm airglow layer (altitude, 260 km) along the geomagnetic field lines. We found that FAIs with an intense (weak) signal-to-noise ratio coincided with the airglow depletion (enhancement) caused by the MSTIDs. FAI velocity obtained from a combination of the Doppler velocities on the three radar beams oscillated in the northwest-southeast direction, with an amplitude of approximately 82 m/s. The FAI velocity was northwestward (southeastward) at the airglow depletion (enhancement). The directions of the FAI velocity were consistent with those of the ExB drifts caused by the polarized electric fields associated with the MSTIDs. The northeastward polarized electric field at the airglow depletion region strengthened the background eastward effective electric field and drove the gradient drift instability generating FAIs. This might be the reason why the FAIs preferred to occur at the airglow depletion region. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013902

Scopus

VHF radar observations of nighttime F-region field-aligned irregularities over Kototabang, Indonesia Reviewed

Otsuka, Y., T. Ogawa, and Effendy

Earth Planets Space   Vol. 61   page： 431-437   2009.5

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We report the response of the ionosphere to the large erthquake that occurred in West Sumatra, Indonesia, at 0058 UT on December 26, 2004. We have analyzed Global Positioning System (GPS) data obtained at two sites in Sumatra and at three sites in Thailand to investigate total electron content (TEC) variations. Between 14 and 40 min after the earthquake,
TEC enhancements of 1.6--6.9 TEC units (TECU) were observed
at subionospheric points located 360-2000 km north of the epicenter. From the time delays of the observed TEC enhancements, we find that the TEC enhancements propagated northward from the epicenter. The time delays between the earthquake and rapid increases in TEC, which occurred near the epicenter, are consistent with the idea that acoustic waves generated by the earthquake propagated into the ionosphere at the speed of sound to cause the TEC variations.
A small TEC enhancement of 0.6 TECU was observed south of the epicenter, while no TEC enhancements were seen east of the epicenter. From a model calculation, we find that this directivity of the TEC variations with respect to the azimuth from the epicenter could be caused partially by the directivity in the response of the electron density variation
to the acoustic waves in the neutral atmosphere.

Spatial relationship of nighttime medium-scale traveling ionospheric disturbances and F region field-aligned irregularities observed with two spaced all-sky airglow imagers and the middle and upper atmosphere radar Reviewed

Otsuka, Y., K. Shiokawa, T. Ogawa, T. Yokoyama, and M. Yamamoto

J. Geophys. Res.   Vol. 114   page： A05302, doi:10.1029/2008JA013902   2009.5

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We report simultaneous observations of medium-scale traveling ionospheric disturbances (MSTIDs) and field-aligned irregularities (FAIs) in the F region by using two all-sky airglow imagers and the MU radar. MSTIDs propagating southwestward were observed simultaneously in 630-nm airglow images over Sakata (39.0N, 139.9E) and Shigaraki (34.9N, 136.1E), Japan on the night of June 16, 2004. By using all-sky images over both sites, we estimated the altitude of the airglow layer to be 260 km by the triangulation method.
During the MSTID event, FAIs in the F region were observed
by making multibeam measurements with the MU radar at Shigaraki. In order to investigate the spatial relationship between the MSTIDs and FAIs, the FAIs were mapped onto the 630-nm airglow layer (altitude: 260 km) along the geomagnetic field lines. We found that FAIs with intense (weak) signal-to-noise ratio (SNR) coincided with the airglow depletion (enhancement) caused by the MSTIDs. FAI velocity obtained from a combination of the Doppler velocities on the three radar beams oscillated in the NW-SE direction with an amplitude of approximately 82 m/s. The FAI velocity was northwestward (southeastward) at the airglow depletion (enhancement). The directions of the FAI velocity
were consistent with those of the ExB drifts caused by the polarized electric fields associated with the MSTIDs.
The northeastward polarized electric field at the airglow depletion region strengthened the background eastward
effective electric field, and drove the gradient drift instability generating FAIs. This might be the reason why the FAIs preferred to occur at the airglow depletion region.

Spatial relationship of nighttime medium-scale traveling ionospheric disturbances and F region field-aligned irregularities observed with two spaced all-sky airglow imagers and the middle and upper atmosphere radar Reviewed

Y. Otsuka, K. Shiokawa, T. Ogawa, T. Yokoyama, M. Yamamoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report simultaneous observations of medium-scale traveling ionospheric disturbances (MSTIDs) and field-aligned irregularities (FAIs) in the F region using two all-sky airglow imagers and the middle and upper atmosphere (MU) radar. MSTIDs propagating southwestward were observed simultaneously in 630-nm airglow images over Sakata (39.0 degrees N, 139.9 degrees E) and Shigaraki (34.9 degrees N, 136.1 degrees E), Japan, on the night of 16 June 2004. By using all-sky images over both sites, we estimated the altitude of the airglow layer to be 260 km by the triangulation method. During the MSTID event, FAIs in the F region were observed by making multibeam measurements with the MU radar at Shigaraki. In order to investigate the spatial relationship between the MSTIDs and FAIs, the FAIs were mapped onto the 630-nm airglow layer (altitude, 260 km) along the geomagnetic field lines. We found that FAIs with an intense (weak) signal-to-noise ratio coincided with the airglow depletion (enhancement) caused by the MSTIDs. FAI velocity obtained from a combination of the Doppler velocities on the three radar beams oscillated in the northwest-southeast direction, with an amplitude of approximately 82 m/s. The FAI velocity was northwestward (southeastward) at the airglow depletion (enhancement). The directions of the FAI velocity were consistent with those of the E x B drifts caused by the polarized electric fields associated with the MSTIDs. The northeastward polarized electric field at the airglow depletion region strengthened the background eastward effective electric field and drove the gradient drift instability generating FAIs. This might be the reason why the FAIs preferred to occur at the airglow depletion region.

DOI： 10.1029/2008JA013902

Web of Science

Motion of polar cap patches: A statistical study with all-sky airglow imager at Resolute Bay, Canada Reviewed

Hosokawa, K, Kashimoto, T, Suzuki, S, Shiokawa, K, Otsuka, Y, Ogawa, T

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.4

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DOI： 10.1029/2008JA014020

Motion of polar cap patches: A statistical study with all-sky airglow imager at Resolute Bay, Canada Reviewed

Hosokawa K, Kashimoto T, Suzuki S, Shiokawa K, Otsuka Y, Ogawa T

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.4

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DOI： 10.1029/2008JA014020

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Unusually elongated, bright airglow plume in the polar cap F region: Is it a tongue of ionization? Reviewed

K. Hosokawa, T. Tsugawa, K. Shiokawa, Y. Otsuka, T. Ogawa, M. R. Hairston

GEOPHYSICAL RESEARCH LETTERS   Vol. 36   2009.4

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We report an event of unusually elongated, bright airglow plume, which is considered as an optical manifestation of tongue of ionization (TOI) in the central polar cap. This optical structure was detected with an all-sky airglow imager at Resolute Bay (74.73 degrees N, 265.07 degrees E) during a large magnetic storm on December 15, 2006. The absolute optical intensity of the plume was approximate to 1 kR, which is much brighter than that of non-stormtime polar cap patches. Two-dimensional imaging capability of the all-sky imager demonstrates that some meso-scale structures (approximate to 250-600 km) were embedded within the plume. Simultaneous ion density and drift measurements with the DMSP spacecraft strongly suggest that the plume was extending from the dayside as a narrow stream of dense plasma and thus is an optical manifestation of polar cap TOI. The DMSP data also implies that the possible source of the plume is a narrow stream of storm enhanced density (SED) transported from lower latitudes. The DMSP auroral particle observation demonstrates that the polar cap extremely expanded equatorward during this interval. This extreme expansion allowed the anti-sunward convection to capture plasmas within the SED and deliver them deep into the polar cap as a luminous airglow plume. This observation claims that the plasma transport from the dayside lower latitudes plays an important role in controlling the plasma environment in the polar cap ionosphere during magnetic storms. Citation: Hosokawa, K., T. Tsugawa, K. Shiokawa, Y. Otsuka, T. Ogawa, and M. R. Hairston (2009), Unusually elongated, bright airglow plume in the polar cap F region: Is it a tongue of ionization?, Geophys. Res. Lett., 36, L07103, doi:10.1029/2009GL037512.

DOI： 10.1029/2009GL037512

Web of Science

Medium-scale traveling ionospheric disturbances observed with the SuperDARN Hokkaido radar, all-sky imager, and GPS network and their relation to concurrent sporadic E irregularities Reviewed

Ogawa T, Nishitani N, Otsuka Y, Shiokawa K, Tsugawa T, Hosokawa K

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.3

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DOI： 10.1029/2008JA013893

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Medium-scale traveling ionospheric disturbances observed with the SuperDARN Hokkaido radar, all-sky imager, and GPS network and their relation to concurrent sporadic E irregularities Reviewed

T. Ogawa, N. Nishitani, Y. Otsuka, K. Shiokawa, T. Tsugawa, K. Hosokawa

Journal of Geophysical Research: Space Physics   Vol. 114 ( 3 )   2009.3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We present midlatitude medium-scale traveling ionospheric disturbances (MSTIDs) observed with a Super Dual Auroral Radar Network (SuperDARN) HF radar at around 10 MHz in Hokkaido, Japan, in combination with a 630-nm all-sky imager and a GPS network (GEONET) that provides total electron content (TEC) data. MSTIDs propagating southward from high latitudes are detected at first with the HF radar and then with the imager and GEONET. We analyze two MSTID events, one in winter (event 1) and the other in summer (event 2), to find that MSTIDs appear simultaneously, at least, at 55°-25°N. It is shown that nighttime MSTIDs propagate toward the southwest over a horizontal distance of about 4000 km, and daytime MSTIDs do so toward the southeast. Daytime radar echoes are due to ground/sea surface (GS) scatter, while nighttime echoes in event 1 return from 15-m-scale F region field-aligned irregularities (FAIs) and those in event 2 are due to GS scatter. Doppler velocities of the nighttime F region FAI echoes in event 1 are negative (motion away from the radar) within strong echo regions and are positive (motion toward the radar) within weak echo regions. This fact suggests that the strong (weak) echoes return from suppressed (enhanced) airglow/TEC areas, in line with previous observations over central Japan. The nighttime MSTIDs in events 1 and 2 are often accompanied by concurrent coherent echoes from FAIs in sporadic E (Es) layers. The Es echo areas in event 2 rather coincide with suppressed airglow/TEC areas in the F region that are connected with the echo areas along the geomagnetic field, indicating the existence of E and F region coupling at night. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013893

Scopus

Relationship between polar cap patches and field-aligned irregularities as observed with an all-sky airglow imager at Resolute Bay and the PolarDARN radar at Rankin Inlet Reviewed

K. Hosokawa, K. Shiokawa, Y. Otsuka, T. Ogawa, J. P. St-Maurice, G. J. Sofko, D. A. Andre

Journal of Geophysical Research: Space Physics   Vol. 114 ( 3 )   2009.3

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Simultaneous two-dimensional observations of airglow enhancement and radar backscatter from field-aligned irregularities (FAIs) associated with polar cap patches were conducted. The spatial structure of 630 nm airglow from polar cap patches was imaged using an all-sky airglow imager at Resolute Bay, Canada, while backscatter echoes from decameter-scale FAIs were observed using the newly constructed HF Polar Dual Auroral Radar Network (PolarDARN) radar at Rankin Inlet, Canada. Both the airglow enhancement and the radar backscatter appeared within a structured region with the spatial extent of about 500-1000 km. The decameter-scale FAIs were found to extend over the entire region of airglow enhancement associated with polar cap patches, indicating that the polar patch plasma became almost fully structured soon after initiation (within approximately 2025 min). These findings imply that some rapid structuring process of the entire patch area is involved in addition to the primary gradient-drift instabilities. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013707

Scopus

Super plasma fountain and equatorial ionization anomaly during penetration electric field Reviewed

N. Balan, K. Shiokawa, Y. Otsuka, S. Watanabe, G. J. Bailey

Journal of Geophysical Research: Space Physics   Vol. 114 ( 3 )   2009.3

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Relative importance of diffusion, electric field, and neutral wind on equatorial plasma fountain and equatorial ionization anomaly (EIA) during a strong daytime eastward prompt penetration electric field (PPEF) event are evaluated using the Sheffield University Plasmasphere Ionosphere Model and the recorded PPEF during the super geomagnetic storm of 9 November 2004. The fountain rapidly develops into a super fountain during the PPEF event. The super fountain becomes strong with less poleward turning of the velocity vectors in the presence of an equatorward wind that reduces (or stops) the downward velocity component due to diffusion and raises the ionosphere to high altitudes of reduced chemical loss. The EIA crests in peak electron density and total electron content shift rapidly to higher than normal latitudes during the PPEF event. However, the crests become stronger than normal only in the presence of an equatorward wind. The results suggest that the presence of an equatorward neutral wind is required to produce a strong positive ionospheric storm during, a daytime eastward PPEF event. The equatorward neutral wind need not be a storm time wind though stronger wind can lead to stronger ionospheric storms. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013768

Scopus

Three-dimensional simulation of the coupled Perkins and es-layer instabilities in the nighttime midlatitude ionosphere Reviewed

Tatsuhiro Yokoyama, David L. Hysell, Yuichi Otsuka, Mamoru Yamamoto

Journal of Geophysical Research: Space Physics   Vol. 114 ( 3 )   2009.3

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Plasma density structures and associated irregularities in the nighttime midlatitude ionosphere are frequently observed as frontal structures elongated from northwest to southeast (NW-SE) in the Northern Hemisphere. The frontal structures and the coupling process between the E and F regions are studied with a three-dimensional numerical model, which can simulate two instability mechanisms: Perkins instability in the F-region and sporadic-E (E s)-layer instability in the E region. The fastest growth of the coupled instability occurs when the unstable conditions on NW-SE perturbation are satisfied in both regions. The perturbation of F-region integrated conductivity grows much faster than the isolated Perkins instability. The meridional component of a rotational wind shear blows an existing E5 layer southward, and the F-region structure follows the E-region drift velocity. The NW-SE structure in the E region can be formed from random perturbation regardless of the F-region condition. When the F region is unstable on the NW-SE perturbation, however, the NW-SE structure is formed in both regions with a common scale length. We conclude that (1) the Es-layer instability plays a major role in seeding NW-SE structure in the F region, and the Perkins instability is required to amplify its perturbation
(2) the rotational wind shear in the E region produces southwestward phase propagation of the NW-SE structure in both the E and F regions
and (3) the coupling process has a significant effect on the scale of the Es-layer perturbation rather than the growth rate of the Es-layer instability. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013789

Scopus

Relationship between polar cap patches and field-aligned irregularities as observed with an all-sky airglow imager at Resolute Bay and the PolarDARN radar at Rankin Inlet Reviewed

K. Hosokawa, K. Shiokawa, Y. Otsuka, T. Ogawa, J. -P. St-Maurice, G. J. Sofko, D. A. Andre

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Simultaneous two-dimensional observations of airglow enhancement and radar backscatter from field-aligned irregularities (FAIs) associated with polar cap patches were conducted. The spatial structure of 630 nm airglow from polar cap patches was imaged using an all-sky airglow imager at Resolute Bay, Canada, while backscatter echoes from decameter-scale FAIs were observed using the newly constructed HF Polar Dual Auroral Radar Network (PolarDARN) radar at Rankin Inlet, Canada. Both the airglow enhancement and the radar backscatter appeared within a structured region with the spatial extent of about 500-1000 km. The decameter-scale FAIs were found to extend over the entire region of airglow enhancement associated with polar cap patches, indicating that the polar patch plasma became almost fully structured soon after initiation (within approximately 20-25 min). These findings imply that some rapid structuring process of the entire patch area is involved in addition to the primary gradient-drift instabilities.

DOI： 10.1029/2008JA013707

Web of Science

Super plasma fountain and equatorial ionization anomaly during penetration electric field Reviewed

N. Balan, K. Shiokawa, Y. Otsuka, S. Watanabe, G. J. Bailey

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Relative importance of diffusion, electric field, and neutral wind on equatorial plasma fountain and equatorial ionization anomaly (EIA) during a strong daytime eastward prompt penetration electric field (PPEF) event are evaluated using the Sheffield University Plasmasphere Ionosphere Model and the recorded PPEF during the super geomagnetic storm of 9 November 2004. The fountain rapidly develops into a super fountain during the PPEF event. The super fountain becomes strong with less poleward turning of the velocity vectors in the presence of an equatorward wind that reduces (or stops) the downward velocity component due to diffusion and raises the ionosphere to high altitudes of reduced chemical loss. The EIA crests in peak electron density and total electron content shift rapidly to higher than normal latitudes during the PPEF event. However, the crests become stronger than normal only in the presence of an equatorward wind. The results suggest that the presence of an equatorward neutral wind is required to produce a strong positive ionospheric storm during a daytime eastward PPEF event. The equatorward neutral wind need not be a storm time wind though stronger wind can lead to stronger ionospheric storms.

DOI： 10.1029/2008JA013768

Web of Science

Three-dimensional simulation of the coupled Perkins and E-s-layer instabilities in the nighttime midlatitude ionosphere Reviewed

Tatsuhiro Yokoyama, David L. Hysell, Yuichi Otsuka, Mamoru Yamamoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 114   2009.3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Plasma density structures and associated irregularities in the nighttime midlatitude ionosphere are frequently observed as frontal structures elongated from northwest to southeast (NW-SE) in the Northern Hemisphere. The frontal structures and the coupling process between the E and F regions are studied with a three-dimensional numerical model, which can simulate two instability mechanisms: Perkins instability in the F-region and sporadic-E (E-s)-layer instability in the E region. The fastest growth of the coupled instability occurs when the unstable conditions on NW-SE perturbation are satisfied in both regions. The perturbation of F-region integrated conductivity grows much faster than the isolated Perkins instability. The meridional component of a rotational wind shear blows an existing E-s layer southward, and the F-region structure follows the E-region drift velocity. The NW-SE structure in the E region can be formed from random perturbation regardless of the F-region condition. When the F region is unstable on the NW-SE perturbation, however, the NW-SE structure is formed in both regions with a common scale length. We conclude that (1) the E-s-layer instability plays a major role in seeding NW-SE structure in the F region, and the Perkins instability is required to amplify its perturbation; (2) the rotational wind shear in the E region produces southwestward phase propagation of the NW-SE structure in both the E and F regions; and (3) the coupling process has a significant effect on the scale of the E-s-layer perturbation rather than the growth rate of the E-s-layer instability.

DOI： 10.1029/2008JA013789

Web of Science

Medium-Scale Traveling Ionospheric Disturbances and Plasma Bubbles Observed by an All-Sky Airglow Imager at Yonaguni, Japan Reviewed

Tadahiko Ogawa, Yuichi Otsuka, Kazuo Shiokawa, Takuya Tsugawa, Akinori Saito, Kazuaki Hoshinoo, Keisuke Matunaga, Minoru Kubota, Mamoru Ishii

TERRESTRIAL ATMOSPHERIC AND OCEANIC SCIENCES   Vol. 20 ( 1 ) page： 287 - 295   2009.2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CHINESE GEOSCIENCE UNION  

We report on nighttime airglow imaging observations of the low latitude ionosphere by means of a 630-m all-sky imager installed in March 2006 at Yonaguni, Japan (24.5 degrees N, 123.0 degrees E; 14.6 degrees N geomagnetic), about 100 kin east of Taiwan. The imager detected medium-scale traveling ionospheric disturbances (MSTIDs) for about 7 hours on the night of 26 May 2006. A dense GPS network in Japan also observed the same MSTID event on this night. The imager and GEONET data indicate that most of the MSTIDs propagated southwestward from the north of Japan to the south of Yonaguni and Taiwan over 4000 kin, with a southern limit of 19 degrees N (geomagnetic latitude 9 degrees N) or lower. On the night of 10 November 2006, the imager observed two weak emission bands that were embedded on the F-region anomaly crest to the south of Yonaguni. The simultaneous electron density profiles from the FORMOSAT-3/COSMIC mission demonstrate that the weak emission bands are due to density depletions in equatorial plasma bubbles. These case studies suggest that the Yonaguni imager in collaboration with other instruments is very suitable for the study of ionospheric disturbances in and around the northern F-region anomaly crest.

DOI： 10.3319/TAO.2007.12.06.02(F3C)

Web of Science

Effects observed in the ionospheric F region in the east Asian sector during the intense geomagnetic disturbances in the early part of November 2004 Reviewed

Y. Sahai, F. Becker-Guedes, P. R. Fagundes, R. De Jesus, A. J. De Abreu, Y. Otsuka, K. Shiokawa, K. Igarashi, K. Yumoto, C. S. Huang, H. T. Lan, A. Saito, F. L. Guarnieri, V. G. Pillat, J. A. Bittencourt

Journal of Geophysical Research: Space Physics   Vol. 114 ( 3 )   2009.1

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The Sun was very active in the early part of November 2004. During the period of 8-10 November 2004, intense geomagnetic disturbances with two superstorms were observed. In a companion paper (hereinafter referred to as paper 1), the effects observed in the F region during the intense geomagnetic disturbances in the early part of November 2004 in the Latin American sector were presented. In the present paper, we investigate the effects observed in the F region during the intense geomagnetic disturbances in the early part of November 2004 in the east Asian sector. We have used the ionospheric sounding observations at Ho Chi Minh City (Vietnam) and Okinawa, Yamagawa, Kokubunji, and Wakkanai (Japan) in the present investigations. Also, GPS observations in the east Asian sector (several longitude zones) have been used to study the effect in the F region during the intense geomagnetic disturbances. The ion density versus latitudinal variations obtained by the DMSP F15 satellite orbiting at about 800 km altitude in the east Asian sector and the magnetic field data obtained at several stations in the Japanese meridian are also presented. Several important features from these observations in both the sectors during this extended period of intense geomagnetic disturbances are presented. The east Asian sector showed very pronounced effects during the second superstorm, which was preceded by two storm enhancements. It should be mentioned that around the beginning of the night on 10 November, ionospheric irregularities propagating from higher midlatitude region to low-latitude region were observed in the Japanese sector. The most intense geomagnetic field H component in that sector was observed on 10 November at L = 2.8, indicating that the auroral oval and the heating got further to low latitudes and the ionospheric irregularities observed in the Japanese sector on this night are midlatitude ionospheric disturbances associated with the second superstorm. The absence of ionospheric irregularities in the Japanese sector during the 8 November superstorm suggests that the magnetosphere-ionosphere system was possibly preconditioned (primed) when the second interplanetary structure impacted the magnetosphere. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013053

Web of Science

Scopus

Characteristics of equatorial gravity waves derived from mesospheric airglow imaging observations Reviewed

S. Suzuki, K. Shiokawa, A. Z. Liu, Y. Otsuka, T. Ogawa, T. Nakamura

ANNALES GEOPHYSICAE   Vol. 27 ( 4 ) page： 1625 - 1629   2009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS GESELLSCHAFT MBH  

We present the characteristics of small-scale (&lt; 100 km) gravity waves in the equatorial mesopause region derived from OH airglow imaging observations at Kototabang (100.3 degrees E, 0.2 degrees S), Indonesia, from 2002 to 2005. We adopted a method that could automatically detect gravity waves in the airglow images using two-dimensional cross power spectra of gravity waves. The propagation directions of the waves were likely controlled by zonal filtering due to stratospheric mean winds that show a quasi-biennial oscillation (QBO) and the presence of many wave sources in the troposphere.

DOI： 10.5194/angeo-27-1625-2009

Web of Science

Equatorial GPS ionospheric scintillations over Kototabang, Indonesia and their relation to atmospheric waves from below Reviewed

Tadahiko Ogawa, Yasunobu Miyoshi, Yuichi Otsuka, Takuji Nakamura, Kazuo Shiokawa

EARTH PLANETS AND SPACE   Vol. 61 ( 4 ) page： 397 - 410   2009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

Using Global Positioning System (GPS) satellites. we have been conducting equatorial ionospheric scintillation observations at Kototabang, Indonesia since January 2003. Scintillations caused by equatorial plasma bubbles appear between 2000 and 0 100 LT in equinoctial months with a seasonal asymmetry, and their activity decreases with decreasing solar activity. A comparison between scintillation index (S-4) and Earth's brightness temperature variations suggests that file scintillation activity call be related to tropospheric disturbances over file Indian Ocean to the west of Kototabang. TO Understand better the reasons of day-to-day variability of S-4, we analyze S-4. T-bb and lower thermospheric neutral wind ((mu'(2)) over bar) data. The results show that S-4 fluctuates with periods of about 2.5, 5, 8, 14 and 25 days, possibly due to atmospheric waves from below and that similar periods are also found in the T-bb and (mu'(2)) over tilde variations. Using a general circulation model, we made numerical simulations to determine the behavior of neutral wind in the equatorial thermosphere. The results indicate the following: (1) 2- to 20-day waves dissipate rapidly above about an altitude of 125 km, and 0.5- to 3-hour waves become predominant above 100 km, (2) zonal winds above 200 km altitude are, on the whole, eastward during sunset-sunrise, (3) zonal wind patterns due to short-period (1-4 h) atmospheric gravity waves (AGWs) above 120 km altitude change day by day, exhibit wavy structures with scale lengths of about 30-1000 km and, as it whole, move eastward in about 100(-1) while changing patterns over time. These simulations suggest that the Rayleigh-Taylor instability responsible for plasma bubble generation call be seeded by AGWs with short periods of about 0.5-3 h, and that background conditions necessary for this instability are modulated by planetary-scale atmospheric waves propagating up to an altitude of about 120 km front below.

DOI： 10.1186/BF03353157

Web of Science

Observations of the F-region ionospheric irregularities in the South American sector during the October 2003 'Halloween Storms' Reviewed

Y. Sahai, F. Becker-Guedes, P. R. Fagundes, A. J. de Abreu, R. de Jesus, V. G. Pillat, J. R. Abalde, C. R. Martinis, C. Brunini, M. Gende, C. -S. Huang, X. Pi, W. L. C. Lima, J. A. Bittencourt, Y. Otsuka

ANNALES GEOPHYSICAE   Vol. 27 ( 12 ) page： 4463 - 4477   2009

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The response of the ionospheric F-region in the South American sector during the super geomagnetic storms on 29 and 30 October 2003 is studied in the present investigation.
In this paper, we present ionospheric sounding observations during the period 29-31 October 2003 obtained at Palmas (a near equatorial location) and Sao Jose dos Campos (a location under the southern crest of the equatorial ionospheric anomaly), Brazil, along with observations during the period 27-31 October 2003 from a chain of GPS stations covering the South American sector from Imperatriz, Brazil, to Rio Grande, Argentina. Also, complementary observations that include sequences of all-sky images of the OI 777.4 and 630.0 nm emissions observed at El Leoncito, Argentina, on the nights of 28-29 (geomagnetically quiet night) and 29-30 (geomagnetically disturbed night) October 2003, and ion densities observed in the South American sector by the DMSP F13, F14 and F15 satellites orbiting at about 800 km on 29 and 30 October 2003 are presented. In addition, global TEC maps derived from GPS observations collected from the global GPS network of International GPS Service (IGS) are presented, showing widespread and drastic TEC changes during the different phases of the geomagnetic disturbances. The observations indicate that the equatorial ionospheric irregularities or plasma bubbles extend to the Argentinean station Rawson (geom. Lat. 33.1 degrees S) and map at the magnetic equator at an altitude of about 2500 km.

DOI： 10.5194/angeo-27-4463-2009

Web of Science

Relative effects of electric field and neutral wind on positive ionospheric storms Reviewed

N. Balan, H. Alleyne, Y. Otsuka, D.Vijaya Lekshmi, B. G. Fejer, I. McCrea

Earth, Planets and Space   Vol. 61 ( 4 ) page： 439 - 445   2009

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Springer Berlin  

The paper studies the relative importance of penetrating eastward electric field (PEEF) and direct effects of equatorward neutral wind in leading to positive ionospheric storms at low-mid latitudes using observations and modeling. The observations show strong positive ionospheric storms in total electron content (TEC) and peak electron density (Nmax) at low-mid latitudes in Japan longitudes (≈125°E-145°E) during the first main phase (started at sunrise on 08 November) of a super double geomagnetic storm during 07-11 November 2004. The model results obtained using the Sheffield University Plasmashpere Ionosphere Model (SUPIM) show that the direct effects of storm-time equatorward neutral wind (that reduce poleward plasma flow and raise the ionosphere to high altitudes of reduced chemical loss) can be the main driver of positive ionospheric storms at low-mid latitudes except in Nmax around the equator. The equatorward wind without PEEF can also result in stronger positive ionospheric storms than with PEEF. Though PEEF on its own is unlikely to cause positive ionospheric storms, it can lead to positive ionospheric storms in the presence of an equatorward wind. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).

DOI： 10.1186/BF03353160

Scopus

Propagation characteristics of nighttime mesospheric and thermospheric waves observed by optical mesosphere thermosphere imagers at middle and low latitudes Reviewed

K. Shiokawa, Y. Otsuka, T. Ogawa

EARTH PLANETS AND SPACE   Vol. 61 ( 4 ) page： 479 - 491   2009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

We review measurements of nighttime atmospheric/ionospheric wave in the upper atmosphere in Japan. Indonesia. and Australia. using all-sky airglow imagers of optical mesosphere thermosphere imagers (OMTIs). The imagers observe two-dimensional patterns of airglow emissions from oxygen (wavelength: 557.7 nm) and hydorxyl (OH) (near-infrared band) in the mesopause region (80-100 km) and from oxygen (630.0 nm) in the thermosphere/ionosphere (200-300 km). Several statistical studies were done to investigate propagation characteristics of small-scale (less than 100 km) gravity waves in the mesopause region and medium-scale traveling ionospheric disturbances (MSTIDs. similar to 100-1,000 km) in the thermosphere/ionosphere. Clear seasonal variations of occurrence and propagation directions were reported for these waves. The propagation directions in the mesopause region are controlled by wind filtering, ducting processes and relative location to the wave sources in the troposphere. On the other hand, systematic equatorward and westward motions were observed for all seasons for nighttime MSTIDs in the midlatitude ionosphere with geomagnetic conjugacy between the northern and Southern hemispheres. Ionospheric instabilities may play important role for the generation and propagation of these MSTIDs. We also give an example of simultaneous observation of quasi-periodic southward-moving waves in the mesopause and in the thermosphere at the geographic equator, From these results, we discuss mean wind acceleration by mesospheric gravity waves and penetration of gravity waves from the mesosphere to the thermosphere.

DOI： 10.1186/BF03353165

Web of Science

The Optical Mesosphere Thermosphere Imagers (OMTIs) for network measurements of aurora and airglow Reviewed

K. Shiokawa, K. Hosokawa, K. Sakaguchi, A. Ieda, Y. Otsuka, T. Ogawa, M. Connors

FUTURE PERSPECTIVES OF SPACE PLASMA AND PARTICLE INSTRUMENTATION AND INTERNATIONAL COLLABORATIONS   Vol. 1144   page： 212 - +   2009

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

The Optical Mesosphere Thermosphere Imagers (OMTIs) currently consist of eight all-sky cooled-CCD imagers and several interferometers and spectrometers. They are making routine observations of aurora and airglow in Japan, Australia, Indonesia, and Canada. Here we show recent results of OMTIs particularly from. the two Canadian stations at Resolute Bay (RSB) and Athabasca (ATH). At RSB, we observe polar-cap plasma patches almost always during southward IMF periods. From two-dimensional cross-correlation analyses, we determine velocity vectors of the patches, which indicates the ionospheric convection vector, showing high correlation with the IMF-By and -Bz variations. At ATH, we often observe isolated proton arcs and Stable Auroral Red (SAR) arcs, which are located equatorward of the auroral oval. The appearance of the isolated proton arcs is highly correlated with the Pc I geomagnetic pulsations measured simultaneously at ATH, suggesting interactions between the electromagnetic ion cyclotron (EMIC) waves and protons in the vicinity of the plasmapause and the ring current. Similar interactions without waves are also suggested for the SAR arcs, which appear after the substorm expansion phase even without geomagnetic storms. These observations show promising capability to monitor magnetospheric processes from the ground stations, which would contribute to the future satellite projects, such as THEMIS, ERG, and Scope/Xscale.

Web of Science

Zonal asymmetry of daytime 150-km echoes observed by Equatorial Atmosphere Radar in Indonesia Reviewed

T. Yokoyama, D. L. Hysell, A. K. Patra, Y. Otsuka, M. Yamamoto

ANNALES GEOPHYSICAE   Vol. 27 ( 3 ) page： 967 - 974   2009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS GESELLSCHAFT MBH  

Multi-beam observations of the daytime ionospheric E-region irregularities and the so-called 150-km echoes with the 47-MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (0.20 degrees S, 100.32 degrees E, 10.36 degrees S dip latitude) are presented. 150-km echoes have been frequently observed by the EAR, and their characteristics are basically the same as the equatorial ones, except for an intriguing zonal asymmetry; stronger echoes in lower altitudes in the east directions, and weaker echoes in higher altitudes in the west. The highest occurrence is seen at 5.7 degrees east with respect to the magnetic meridian, and the altitude gradually increases as viewing from the east to west. Arc structures which return backscatter echoes are proposed to explain the asymmetry. While the strength of radar echoes below 105 km is uniform within the wide coverage of azimuthal directions, the upper E-region (105-120 km) echoes also show a different type of zonal asymmetry, which should be generated by an essentially different mechanism from the lower E-region and 150-km echoes.

DOI： 10.5194/angeo-27-967-2009

Web of Science

VHF radar observations of nighttime F-region field-aligned irregularities over Kototabang, Indonesia Reviewed

Y. Otsuka, T. Ogawa, Effendy

EARTH PLANETS AND SPACE   Vol. 61 ( 4 ) page： 431 - 437   2009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

We report, for the first time, continuous observations of the nighttime F-region field-aligned irregularities (FAIs) over Indonesia. A VHF radar with operating frequency of 30.8 MHz and peak power of 20 kW has been operated at Kolotabang (0.20 degrees S, 100.32 degrees E; dip latitude 10.4 degrees S), Indonesia since February 2006. Five beams were allocated between +/- 54 degrees in azimuth around geographic South (126 degrees-234 degrees). From the Continuous observation from February 2006 to November 2007, we found that FAIs appeared frequently at pre-midnight between March and May and Lit post-midnight between May and August. The pre-midnight FAIs coincided well with GPS scintillation observed at the same site. Seasonal and local time variations of the pre-midnight FAI Occurrence are consistent with those of equatorial plasma bubbles reported in previous studies (e.g., Maruyama and Matuura, 1984). These results indicate that the pre-midnight FAIs could be associated with the equatorial plasma bubbles. On the other hand, seasonal and local time variations of the post-midnight FAIs were inconsistent with those of the plasma bubbles. The features of the post-midnight FAIs can be summarized as follows: (1) The post-midnight FAIs are not accompanied by GPS scintillations. (2) Most of the post-midnight FAI regions do not show propagation, but some of them propagate westward. (3) Echo intensity of the post-midnight FAIs was weaker than that of the pre-midnight FAIs. These features are similar to those of the FAI echoes that have been observed at mid-latitude (e.g., Fukao et, al., 1991). At Kototabang, Fukao et al. (2004) have firstly observed FAIs that resemble those Lit mid-latitude. The present paper reports statistical characteristics of the mid-latitude-type FAIs observed at Kototabang.

DOI： 10.1186/BF03353159

Web of Science

New observational deployments for SEALION-Airglow measurements using all-sky imagers

Minoru Kubota, Mamoru Ishii, Takuya Tsugawa, Jyunpei Uemoto, Hidekatsu Jin, Yuichi Otsuka, Kazuo Shiokawa

Journal of the National Institute of Information and Communications Technology   Vol. 56 ( 1-4 ) page： 299 - 307   2009

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Language：English   Publishing type：Research paper (scientific journal)  

An airglow all-sky imager (ASI)-a new monitoring instrument-is scheduled to be set up in February 2010 at Chiang Mai, Thailand, as part of the framework of the Southeast Asia Low-latitude Ionospheric Network (SEALION) being developed in the Southeast Asian region by the National Institute of Information and Communications Technology (NICT). The ASI images airglow having an emission layer near the ionospheric F-layer in order to observe the two-dimensional structure and time evolution of such ionospheric disturbances as plasma bubbles. In Southeast Asia, the Solar-Terrestrial Environment Laboratory (STE) of Nagoya University has been conducting ASI-based observations at Kototabang, Indonesia, ever since 2002. This paper provides detailed insight into the formation, growth and propagation of plasma bubbles as derived from observational data recorded using ASI at Kototabang and using SEALION ionoson-des and GPS receivers. The functional requirements for another ASI to be installed and ASI-based observation schemes are then discussed based on that insight.

Scopus

Nighttime medium-scale traveling ionospheric disturbances detected by network GPS receivers in Taiwan Reviewed

C. C. Lee, Y. A. Liou, Y. Otsuka, F. D. Chu, T. K. Yeh, K. Hoshinoo, K. Matunaga

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 113 ( A12 )   2008.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] The nighttime medium-scale traveling ionospheric disturbances (MSTIDs) are detected by the network GPS receivers in Taiwan for the first time. The two-dimensional mapping technique (a spatial resolution of 0.05 degrees x 0.05 degrees in latitude and longitude with 5 x 5 pixels smoothing) is applied on the data of TEC perturbations (TECp) and GPS phase fluctuations (Delta TEC/min). The results show that the MSTIDs move southwestward to latitude of 20.5 degrees N with a horizontal velocity between 100 and 160 m/s and a wavelength of about 500 km. The wavefronts of the MSTIDs are aligned along the northwest-southeast direction. Furthermore, the greater vertical bar Delta TEC/min vertical bar values are corresponding to the wave peak and trough of TECp. This coincidence indicates that the MSTIDs play an important role for the generation of F region irregularities in the low-latitude ionosphere.

DOI： 10.1029/2008JA013250

Web of Science

Northeastward motion of nighttime medium-scale traveling ionospheric disturbances at middle latitudes observed by an airglow imager

K. Shiokawa, Y. Otsuka, N. Nishitani, T. Ogawa, T. Tsugawa, T. Maruyama, S. E. Smirnov, V. V. Bychkov, B. M. Shevtso

Journal of Geophysical Research: Space Physics   Vol. 113 ( A12 )   2008.12

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Nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed in 630-nm airglow images at middle latitudes are known to have a predominantly northwest-southeast phase surface and to move southwestward in the Northern Hemisphere of Earth. However, the mechanisms of MSTID generation and their systematic southwestward motion have not been clarified. In this paper, we report the &quot;northeastward&quot; motion of the MSTIDs observed at Paratunka, Far East Russia (52.97°N, 158.25°E), using an all-sky 630-nm airglow imager at 2000-2300 LT on 19 August 2007. The MSTIDs moved first southwestward but then back northeastward in the northern part of the images. The northeastward motion of the MSTIDs took place coincident with a F layer height decrease observed by an ionosonde at Paratunka. The F layer height decrease was also confirmed by an enhancement of the 630-nm airglow intensity, which seemed to propagate from northeast to southwest. This fact suggests that the F layer height decrease was caused by poleward wind enhancement rather than westward electric field. These observations imply that the F layer height decrease or the poleward thermospheric wind has some role in the northeastward turning of the MSTID propagation direction. Copyright 2008 by the American Geophysical Union.

DOI： 10.1029/2008JA013417

Web of Science

Scopus

Statistical study of relationship between medium-scale traveling ionospheric disturbance and sporadic E layer activities in summer night over Japan Reviewed

Otsuka, Y., T. Tani, T. Tsugawa, and T. Ogawa

J. Atmos. Solar-Terr. Phys.   Vol. 70   page： 2196-2202   2008.12

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Statistical study of relationship between medium-scale traveling ionospheric disturbance and sporadic E layer activities in summer night over Japan Reviewed

Y. Otsuka, T. Tani, T. Tsugawa, T. Ogawa, A. Saito

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 70 ( 17 ) page： 2196 - 2202   2008.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We investigate the relationship between medium-scale traveling ionospheric disturbance (MSTID) and sporadic E (E-s) layer activities in summer nights by analyzing total electron content (TEC) data obtained from a global positioning system (GPS) network in Japan and ionosonde data obtained at Kokubunji, Japan during May-August in 2001-2005. MSTID activity is defined as delta I/($) over bar, where delta I is standard deviation of the TEC perturbations over Kokubunji within 1 h, and 7 is the background TEC. By analyzing nighttime-averaged (19-02 LT) values of MSTID activity and E, layer parameters, we find that the MSTID activity is closely correlated with f(0)E(s) and f(0)E(s) -f(b)E(s). This result suggests that MSTID and the spatial structures of E, layer could be generated by an electro-dynamical coupling process between the Es layer and F region through polarization electric fields. Furthermore, we suggest that the appearance of the E, layer in the summer hemisphere could play an important role in generating MSTIDs in both hemispheres. (C) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2008.07.008

Web of Science

Decay of 3-m-scale ionospheric irregularities associated with a plasma bubble observed with the Equatorial Atmosphere Radar Reviewed

Susumu Saito, Shoichiro Fukao, Mamoru Yamamoto, Yuichi Otsuka, Takashi Maruyama

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 113 ( A11 )   2008.11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We investigated the decay processes of 3-m-scale ionospheric irregularities associated with plasma bubbles by multi-instrument observations. The observations were made using the Equatorial Atmosphere Radar (EAR), a 630-nm airglow imager, and ionosondes. The EAR&apos;s electronic beam steering capability allowed us to see the temporal evolution of 3-m-scale irregularities associated with plasma bubbles in two-dimensional views. Around midnight on 28 and 31 March 2006, the EAR detected plasma bubbles that drifted into its field of view. When their eastward drift motion slowed and they finally stopped moving eastward, the 3-m-scale irregularities decayed. These results suggest that instability driven by the eastward neutral wind kept the 3-m-scale irregularities in matured plasma bubbles, which are in the well-developed stage, active for several hours after sunset. The stagnated plasma bubbles drifted downward with the ambient westward electric field. This drift resulted in the backscatter echo patterns seen in the range-time-intensity (RTI) plots which look like those associated with mid-latitude plumes.

DOI： 10.1029/2008JA013118

Web of Science

F-3 layer during penetration electric field Reviewed

Balan, N, Thampi, SV, Lynn, K, Otsuka, Y, Alleyne, H, Watanabe, S, Abdu, MA, Fejer, BG

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 113   2008.9

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DOI： 10.1029/2008JA013206

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Simultaneous appearance of isolated auroral arcs and Pc 1 geomagnetic pulsations at subauroral latitudes Reviewed

K. Sakaguchi, K. Shiokawa, Y. Miyoshi, Y. Otsuka, T. Ogawa, K. Asamura, M. Connors

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 113 ( A5 )   2008.5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We have been conducting observations of aurora and geomagnetic pulsations at Athabasca, Canada, located at a subauroral latitude ( magnetic latitude: 62 degrees, L similar to 4.6), using an all-sky imager and an induction magnetometer. Isolated auroral arcs at wavelengths of 557.7 nm, 630.0 nm, and 486.1 nm ( H-beta) were often observed at latitudes separated equatorward from the main auroral oval. From a 1-year observation ( 4 September 2005 to 3 September 2006), we found 13 isolated arc events. All these isolated arcs occurred coincidentally with Pc 1 geomagnetic pulsations, although there were nine other Pc 1 events without isolated arcs in the field of view of the imager. The arcs were observed in both pre- and post-midnight sectors and tended to appear during the late recovery phase of geomagnetic storms. The isolated arcs had limited latitudinal and longitudinal widths of less than 230 km and 250 - 800 km, respectively. We found that as isolated arcs moved equatorward ( poleward), the frequencies of the simultaneous Pc 1 pulsations increased ( decreased). Using the Tsyganenko-02 magnetic field model, the observed Pc 1 frequencies were almost the same as the frequencies of He+ electromagnetic ion cyclotron ( EMIC) waves at the equatorial plane connected to observed isolated arcs. These results indicate that interactions of spatially localized EMIC waves with ring current ions cause high-energy ion precipitation and associated isolated auroras at subauroral latitudes. These results also imply that the dynamics and instabilities in the inner magnetosphere can be monitored as low-latitude auroral emissions away from the ordinary auroral oval.

DOI： 10.1029/2007JA012888

Web of Science

Daytime 150-km echoes observed with the Equatorial Atmosphere Radar in Indonesia: First results Reviewed

A. K. Patra, T. Yokoyama, Y. Otsuka, M. Yamamoto

GEOPHYSICAL RESEARCH LETTERS   Vol. 35 ( 6 )   2008.3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Results of the daytime 150-km echoes from 10.36 degrees S magnetic latitude observed using the Equatorial Atmosphere Radar (EAR), which are first of its kind from magnetic southern hemisphere, are presented. The echoing region of 145-165 km shows forenoon descent and afternoon ascent with signal intensity modulation with time resembling a necklace and the Doppler spectra are narrow, which are quite similar to those reported earlier from other locations. Westward and upward/southward irregularity drifts are found to be consistent with daytime background electric fields, providing a tool to investigate low latitude daytime electrodynamics. Detectability of these echoes suggests the absence of strong latitudinal dependence in contrast to the earlier belief, suggesting a possibility of their detection at mid-latitudes. Also the echoes show zonal anisotropy that is opposite to that observed over Pohnpei. The implications of these observations are discussed in the light of present understanding of 150-km echoing phenomenon.

DOI： 10.1029/2007GL033130

Web of Science

First three-dimensional simulation of the Perkins instability in the nighttime midlatitude ionosphere Reviewed

Tatsuhiro Yokoyama, Yuichi Otsuka, Tadahiko Ogawa, Mamoru Yamamoto, David L. Hysell

GEOPHYSICAL RESEARCH LETTERS   Vol. 35 ( 3 )   2008.2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

A three-dimensional numerical simulation in the nighttime midlatitude ionosphere is developed for the first time and applied to the Perkins instability in the midlatitude F region. Growth of the Perkins instability is successfully reproduced under nighttime condition, and the numerical results basically agree with a linear theory and previous two-dimensional numerical studies. Northwest-southeast (NW-SE) alignment of density perturbations is generated from random seeding by applying a southeastward neutral wind. The perturbations are dominant at altitudes of 200 300 km where a steep density gradient exists, which is consistent with the altitude of 630-nm airglow emission that often shows NW-SE alignment. Further simulation in terms of the E-F coupling can be done in the near future.

DOI： 10.1029/2007GL032496

Web of Science

Optical and radio observations and AMIE/TIEGCM modeling of nighttime traveling ionospheric disturbances at midlatitudes during geomagnetic storms

K. Shiokawa, T. Tsugawa, T. Tsugawa, Y. Otsuka, T. Ogawa, G. Lu, A. Saito, M. Yamamoto

Geophysical Monograph Series   Vol. 181   page： 271 - 281   2008

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© 2008 by the American Geophysical Union. This paper summarizes our recent observations of storm-time traveling ionospheric disturbances (TIDs) over Japan. The storm-time TIDs, which are often referred as large-scale TIDs or traveling atmospheric disturbances, are generated at high-latitude auroral zone and propagate predominantly equatorward at midlatitudes as atmospheric waves in the thermosphere. The imaging measurements by airglow imagers and GPS networks give reliable wave parameters (wavelength, phase velocity, and amplitude) of TIDs. Comparison of imaging observations of TIDs between the northern and southern hemispheres revealed their nonconjugacy. Combinations of these imaging observations with vertical sounding measurements by ionosondes and radars provide a comprehensive description of TID structures at midlatitudes. Comparison between these observations and the assimilative mapping of the ionospheric electrodynamics technique with the thermosphere-ionosphere electrodynamics general circulation model results in several insights into the generation and propagation of TIDs at the high-latitude source region as well as limitations of the current global model.

DOI： 10.1029/181GM24

Web of Science

Scopus

Discovery of anoumoulous stripes over the amazon by the PALSAR onboard ALOS satellite Reviewed

Masanobu Shimada, Yasushi Muraki, Yuichi Otsuka

International Geoscience and Remote Sensing Symposium (IGARSS)   Vol. 2 ( 1 ) page： II387 - II390   2008

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Language：English   Publishing type：Research paper (international conference proceedings)  

We discovered anomalous stripes over the large area of Amazon when the region was scanned by the radar (PALSAR) onboard ALOS satellite. The stripes appeared on the local midnight along the geomagnetic field line with a typical characteristic width of 600m. ©2008 IEEE.

DOI： 10.1109/IGARSS.2008.4779009

Scopus

Medium-scale traveling ionospheric disturbances detected with dense and wide TEC maps over North America Reviewed

T. Tsugawa, Y. Otsuka, A. J. Coster, A. Saito

GEOPHYSICAL RESEARCH LETTERS   Vol. 34 ( 22 )   2007.11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Nighttime and daytime medium- scale traveling ionospheric disturbances ( MSTIDs) are detected with dense and wide detrended total electron content ( TEC) maps over North America using multiple GPS receiver networks. The TEC maps cover a wide region of 60-130 degrees W and 24 - 54 degrees N (30-65 degrees N in geomagnetic latitude), and have a spatial resolution of 1.05 degrees x1.05 degrees in latitude and longitude (0.15 degrees x0.15 degrees with 7x7 pixel smoothing) and a temporal resolution of 30 seconds. The TEC maps reveal, for the first time, that the nighttime MSTIDs propagate southwestward with 200 - 500 km wavelengths over North America and have wavefronts longer than similar to 2,000 km. We also observe that daytime MSTIDs with 300 - 1,000 km wavelengths propagate southeastward until mid-afternoon and southwestward in the late afternoon. In the mid-to-late afternoon, these MSTIDs propagating in the different directions are superimposed. The TEC maps can be a new powerful tool to investigate the MSTIDs.

DOI： 10.1029/2007GL031663

Web of Science

Summer-winter hemispheric asymmetry of the sudden increase in ionospheric total electron content and of the O/N2 ratio: Solar activity dependence Reviewed

T. Tsugawa, S. R. Zhang, A. J. Coster, Y. Otsuka, J. Sato, A. Saito, Y. Zhang, L. J. Paxton

Journal of Geophysical Research: Space Physics   Vol. 112 ( A8 )   2007.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

The solar activity dependence of the summer-winter hemispheric asymmetry (SWHA) of the sudden increase in total electron content (SITEC) due to solar flares and of the O/N2 ratio is statistically analyzed using global GPS-total electron content data and TIMED Global Ultraviolet Imager column O/N2 ratio data. We focus on observations with nonnegligible residuals of the solar zenith angle (SZA) dependency of SITEC. We examined 109 SITEC events associated with flares larger than M5 X-ray class flare from 2000 through 2006 and compared the residual SITEC (δ) to the O/N2 ratio. We observed that the latitude gradient of δ has not only an annual variation but also a year-to-year variation that is similar to those of the O/N2 ratio. The SWHA magnitude (defined as the annual maximum of latitude gradient) of both δ and O/N2 decreases as the solar activity declines toward its minimum. The correlation coefficient between the annual SWHA magnitudes of δ and those of O/N2 is 0.92, indicating strongly that the SWHA of O/N2 is responsible for that of SITEC in both the annual and year-to-year variations. The X-ray classes of the solar flares have no clear correlation with the solar activity, F10.7 index. We observe that the SWHA magnitude of δ does not depend on the magnitude of solar flare but rather on the background solar activity through the SWHA magnitude of the O/N2. Copyright 2007 by the American Geophysical Union.

DOI： 10.1029/2007JA012415

Web of Science

Scopus

Low-latitude total electron content enhancement at low geomagnetic activity observed over Japan Reviewed

Ivan Kutiev, Yuichi Otsuka, Akinori Saito, Takuya Tsugawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 112 ( A7 )   2007.7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] Numerous total electron content (TEC) values derived from GPS signals are averaged within 1.5 degrees x 1.5 degrees cells in a 1-hour time frame, and the relative deviations of these average values from corresponding monthly medians are used to produce latitude-time plots over Japan. The paper analyzes the appearance and development of enhancements of TEC of equatorial origin (ETEs), occurring outside initial and main phases of geomagnetic storms. ETE structures appear mainly as single-crest structures in the evening hours local time, with TEC peak around 1900 LT. TEC usually decreases with latitude, and the structures disappear below 40 degrees N. In some cases the TEC peak is found above the plot boundary of 24 degrees N, as depletions toward the equator are also observed. The observed enhanced structures are linked to the well-known evening prereversal enhancement of ion drift in the equatorial F region. Double-crest ETEs are also observed, with the second peak occurring in early morning hours. Most of the ETE events appear in periods of low geomagnetic activity, 1-3 days after the main phase of the storms. In some cases the time of rising of ETE structures coincides with the increase of interplanetary electric field (IEF), a fact showing the importance of directly penetrating electric field in formation of ETEs. Often, ETEs appear repeatedly in 2 or 3 consecutive days. It is supposed that planetary atmospheric waves are responsible for this phenomenon. Most of the observed features of ETEs can be explained by the published results of simulations of the coupled thermosphere-ionosphere-plasmasphere (CTIP) model and the thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). It is suggested that ETE structures are produced mainly by a disturbance winds dynamo electric field, built up after the main phase of the storms. Some ETE events, appearing at the end of a prolonged period of low geomagnetic activity, can be linked to directly penetrating IEF in equatorial ionosphere. The intensity of these electric fields is much smaller than is needed to explain the observations.

DOI： 10.1029/2007JA012385

Web of Science

Simultaneous observations of nighttime medium-scale traveling ionospheric disturbances and E region field-aligned irregularities at midlatitude Reviewed

Y. Otsuka, F. Onoma, K. Shiokawa, T. Ogawa, M. Yamamoto, S. Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 112 ( A6 )   2007.6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] We report on simultaneous observations of very high frequency ( VHF) radar backscatter from field-aligned irregularities (FAI) in the E region and medium-scale traveling ionospheric disturbances (MSTID) in 630-nm airglow images. On the night of 6 August 2002, the powerful 46.5-MHz middle and upper atmosphere ( MU) radar with five beam directions at Shigaraki, Japan, observed typical quasiperiodic (QP) echoes from FAI in the E region. During this QP echo event, an all-sky charge-coupled device imager at the MU radar site detected MSTID in 630-nm airglow images. The MSTID also propagated southwestward at 80 m/s with fluctuation amplitudes of 50% from the background intensity and a wavelength of 380 km. Period of the MSTID was almost same as that of the Doppler velocity in the QP echoes. The airglow enhancement ( depletion) caused by the MSTID coincided with southeastward ( northwestward) velocity in the QP echo. These results indicate that the electric fields associated with the F region MSTID could be closely coupled to those associated with QP echoes in the E region.

DOI： 10.1029/2005JA011548

Web of Science

Ground observation and AMIE-TIEGCM modeling of a storm-time traveling ionospheric disturbance Reviewed

Shiokawa, K, Lu, G, Otsuka, Y, Ogawa, T, Yamamoto, M, Nishitani, N, Sato, N

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 112 ( A5 )   2007.5

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DOI： 10.1029/2006JA011772

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Simultaneous ground and satellite observations of an isolated proton arc at subauroral latitudes Reviewed

K. Sakaguchi, K. Shiokawa, A. Ieda, Y. Miyoshi, Y. Otsuka, T. Ogawa, M. Connors, E. F. Donovan, F. J. Rich

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 112 ( A4 )   2007.4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] We observed an isolated proton arc at the Athabasca station (MLAT: 62 degrees N) in Canada on 5 September 2005, using a ground-based all-sky imager at wavelengths of 557.7 nm, 630.0 nm, and 486.1 nm (H beta). This arc is similar to the detached proton arc recently observed by the IMAGE satellite [Immel et al., 2002]. The arc appeared at 0500-0640 UT (2100-2240 MLT), coincident with strong Pc 1 geomagnetic pulsations in the frequency range of the electromagnetic ion cyclotron (EMIC) wave. The isolated arc did not change its structure and intensity during the late growth and expansive phases of a small substorm that occurred at 0550 UT. From particle data obtained by the NOAA 17 satellite, we found that the isolated arc was associated with the localized enhancement of ion precipitation fluxes at an energy range of 30-80 keV at L similar to 4. Trapped ion flux enhancements ( ring current ions) were also observed at two latitudinally separated regions. The localized ion precipitation was located at the outer boundary of the inner ring current ions. The DMSP F13 satellite observed signatures of an ionospheric plasma trough near the conjugate point of the arc in the Southern Hemisphere. The trough is considered to be connected to the plasmapause. These results indicate that the source region of the isolated arc was located near the plasmapause and in the ring current. We conclude that the observed isolated proton arc at subauroral latitudes was caused by the EMIC waves, which were generated near the plasmapause and resonantly scattered the ring current protons into the loss cone.

DOI： 10.1029/2006JA012135

Web of Science

An intense gravity wave near the mesopause region observed by a Fabry-Perot interferometer and an airglow imager Reviewed

K. Shiokawa, S. Suzuki, Y. Otsuka, T. Ogawa, T. Nakamura, T. Horinouchi

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES   Vol. 112 ( D7 )   2007.4

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[1] We report an intense gravity wave event observed at Shigaraki, Japan, at 1500 - 2000 UT (0000 - 0500 LT) on 22 December 2001 using a Fabry-Perot interferometer, which measures the Doppler shift of nocturnal airglow emission at a wavelength of 557.7 nm (emission altitude: 90 - 100 km). The wave had a period of 1.5 hours and a peak-to-peak amplitude of 68 m/s in the horizontal wind velocity. The hodograph of the east-west and north-south wind oscillations showed a polarization direction of the wave of ENE-WSW. An all-sky airglow imager at Shigaraki also recorded a similar southwestward moving wave with a period of similar to 1.5 hours and a velocity of 146 m/s in the 557.7-nm airglow images. The horizontal wavelength was estimated to be 790 km from the images. These facts indicate that the wave was generated in the northeast of Japan, where a well-developed low-pressure cell (968 hPa) and a distortion of the jet stream were observed in the troposphere. However, the large horizontal wavelength, the fast phase velocity, and the possible source location apart from the exit of the jet streak are different from those predicted from previous numerical simulations.

DOI： 10.1029/2006JD007385

Web of Science

Medium-scale traveling ionospheric disturbances observed by GPS receiver network in Japan: a short review Reviewed

Takuya Tsugawa, Nobuki Kotake, Yuichi Otsuka, Akinori Saito

GPS SOLUTIONS   Vol. 11 ( 2 ) page： 139 - 144   2007.3

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Medium-scale traveling ionospheric disturbances (MSTID) are wave-like perturbations of the ionospheric plasma with wavelengths of several hundred kilometres and velocities of several hundred metres per second. MSTID is one of the most common ionospheric phenomena that generally induce the perturbations of ionospheric total electron content (TEC) by similar to 10(16) electron/m(2), which corresponds to -54 ns (16.2 cm) delay at GPS L1 signal. In the past decade, several new characteristics on MSTIDs have been revealed by the TEC observations using the dense GPS receiver network in Japan. In this paper, we provide a short review of these new observations and summarize the morphological characteristics of MSTIDs in Japan.

DOI： 10.1007/s10291-006-0045-5

Web of Science

A concentric gravity wave structure in the mesospheric airglow images Reviewed

S. Suzuki, K. Shiokawa, Y. Otsuka, T. Ogawa, K. Nakamura, T. Nakamura

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES   Vol. 112 ( D2 )   2007.1

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A concentric gravity wave pattern was observed by an all-sky airglow imager located at Shigaraki, Japan (34.9 degrees N, 136.1 degrees E) on 3 October 2002. The wave pattern was identified over 2.5 hours (1330-1600 UT or 2230-0100 LT) both in the OI (557.7 nm) and OH (720-910 nm) band images. It had a wavelength of about 35 km and propagated concentrically northeastward with a horizontal phase velocity of about 80 m/s. The wave had a short period of about 7 min and a momentum flux of 2.2 m(2)/s(2). We identified the source of the waves as a cumulonimbus in the satellite cloud images and in radar precipitation images near the center of the concentric pattern. We conclude that this rare wave pattern was caused by a weak wind system that extended from the stratosphere to the mesopause region in an autumn transition.

DOI： 10.1029/2005JD006558

Web of Science

Simultaneous observations of nighttime medium-scale traveling ionospheric disturbances and E-region field-aligned irregularities at midlatitude Reviewed

Otsuka, Y., F. Onoma, K. Shiokawa, T. Ogawa, M. Yamamoto, and S. Fukao

J. Geophys. Res     page： doi:10.1029/2005JA011548   2007

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Statistical study of medium-scale traveling ionospheric disturbances observed with the GPS networks in Southern California Reviewed

Nobuki Kotake, Yuichi Otsuka, Tadahiko Ogawa, Takuya Tsugawa, Akinori Saito

EARTH PLANETS AND SPACE   Vol. 59 ( 2 ) page： 95 - 102   2007

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Using global positioning system (GPS) data taken from 350 dual-frequency GPS receivers in Southern California in 2002, we investigated two-dimensional maps of total electron content (TEC) perturbations with a time resolution of 30 s and a spatial resolution of 0.15 degrees x0.15 degrees in longitude and latitude to reveal statistical characteristics of medium-scale traveling ionospheric disturbances (MSTIDs). We found that MSTIDs can be categorized into three types. One type is daytime MSTIDs, which frequently occur in winter and equinoxes. Since most of the daytime MSTIDs propagated southeastward, we speculate that the daytime MSTIDs could be caused by atmospheric gravity waves in the thermosphere. A second type is nighttime MSTIDs, which frequently occur in summer. Nighttime MSTIDs propagate southwestward. This propagation direction is consistent with the idea that polarization electric fields could play an important role in generating nighttime MSTIDs. The third is dusk MSTIDs, which frequently occur in summer and propagate northwestward. Dusk MSTIDs could be caused by gravity waves originating from the sunset terminator because they have wavefronts almost parallel to the sunset terminator.

DOI： 10.1186/BF03352681

Web of Science

The ionospheric response in the Brazilian sector during the super geomagnetic storm on 20 November 2003 Reviewed

F. Becker-Guedes, Y. Sahai, P. R. Fagundes, E. S. Espinoza, V. G. Pillat, W. L. C. Lima, Su. Basu, Sa. Basu, Y. Otsuka, K. Shiokawa, E. M. MacKenzie, X. Pi, J. A. Bittencourt

ANNALES GEOPHYSICAE   Vol. 25 ( 4 ) page： 863 - 873   2007

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS GESELLSCHAFT MBH  

A very intense geomagnetic storm (superstorm) began with storm sudden commencement (SSC) at 08:03 UT on 20 November 2003, as a result of the coronal mass ejection (CME) by sunspot 484 hurled into space on 18 November 2003. The geomagnetic storm attained vertical bar D-st vertical bar(max)=472 nT at 20:00 UT (20 November). In this paper we present the simultaneous ionospheric sounding observations, using the Canadian Advanced Digital Ionosondes (CADIs), carried out from Palmas (PAL; 10.2 degrees S, 48.2 degrees W; dip latitude 5.5 degrees S; a near equatorial station) and Sao Jose dos Campos (SJC; 23.2 degrees S. 45.9 degrees W; dip latitude 17.6 degrees S; station located under the crest of equatorial ionospheric anomaly), Brazil. In addition, total electron content (TEC) measurements from several GPS receiving stations in the Brazilian sector during this storm are presented. The simultaneous ionospheric sounding observations carried out at SJC and PAL, and TEC observations on 3 consecutive days viz., 19 November (quiet), 20 November (disturbed) and 21 November (recovery phase) are presented. Salient features from the ionospheric observations in the Brazilian sector during the superstorm are discussed. The difference in the observed ionospheric response at the two stations (PAL and SJC) is considerable. This is not surprising given that PAL is close to the magnetic equator and SJC is near the crest of the equatorial ionospheric anomaly (EIA). It should be pointed out that soon after the SSC (about 4 h later), the F-region critical frequency (foF2), the F-region peak height (hpF2), and variations of virtual heights at different frequencies (iso-frequency plots) all show wavelike oscillations of the F-region during daytime at both the ionospheric sounding stations. Unusual rapid uplifting of F-region at PAL was observed during both the main and recovery phases of the storm.

DOI： 10.5194/angeo-25-863-2007

Web of Science

Response of nighttime equatorial and low latitude F-region to the geomagnetic storm of August 18, 2003, in the Brazilian sector Reviewed

Y. Sahai, F. Becker-Guedes, P. R. Fagundes, W. L. C. Lima, Y. Otsuka, C. -S. Huang, E. S. Espinoza, X. Pi, A. J. de Abreu, M. J. A. Bolzan, V. G. Pillat, J. R. Abalde, A. A. Pimenta, J. A. Bittencourt

ADVANCES IN SPACE RESEARCH   Vol. 39 ( 8 ) page： 1325 - 1334   2007

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This paper presents an investigation of geomagnetic storm effects in the equatorial and low latitude F-region in the Brazilian sector during the intense geomagnetic storm on 18 August, 2003 (SSC 14:21 UT on 17/08; Sigma Kp = 52+; Ap = 108; vertical bar Dst vertical bar(max) = 168 at 1600 UT on 18/08). Simultaneous ionospheric sounding measurements from two stations, viz., Palmas (10.2 degrees S, 48.2 degrees W; dip latitude 5.7 degrees S) and Sao Jose dos Campos (23.2 degrees S, 45.9 degrees W; dip latitude 17.6 degrees S), Brazil, are presented for the nights of 16-17, 17-18 and 18-19 August, 2003 (quiet, disturbed and recovery phases). Both stations are equipped with the Canadian Advanced Digital Ionosonde (CADI). Quiet and disturbed conditions of the F-region ionosphere are compared using data collected from the two stations. The relationship between magnetospheric disturbance and low-latitude ionospheric dynamics, and generation of ionospheric irregularities are discussed. On the disturbed nights (17-18 and 18-19 August), the low latitude station S. J. Campos showed strong enhancements in the F-region critical frequency (foF2), whereas the near equatorial station Palmas showed strong uplifting of the F-layer about 1 h earlier. Normally during the June solstice months (May-August) in the Brazilian sector, large-scale ionospheric irregularities in form of plasma bubbles are rarely observed. On the night of 17-18 August, ionsospheric sounding observations at Palmas showed the presence of bottomside spread-F, whereas on the night of 18-19 August, the observations at Palmas and S. J. Campos showed the presence of plasma bubbles when the storm recovery phase had just started. The complementary GPS data available from several stations in the "Rede Brasileira de MonitoramentoContinuo de GPS (Brazilian Network for Continuous GPSMonitoring)" are used to obtain the vertical total electron content (VTEC) and the rate of change of TEC per minute on UT days 18 and 19 August, 2003 and presented. Also, several global ionospheric TEC maps from the worldwide network of GPS receivers are presented, showing widespread latitudinal and longitudinal TEC changes during the different phases of the storm. All the observations (local ionospheric sounding and GPS network measurements, and global GPS measurements) presented in this investigation related to the geomagnetic storm on 18 August indicate that the equatorial and low latitude region in the Brazilian sector had much stronger effect during the recovery phase compared with the main phase. A comparison of the observed disturbance drifts with the Fejer-Scherliess storm-time model drifts indicate that the modeled drifts are not consistent with the present observations. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2007.02.064

Web of Science

Development of airglow temperature photometers with cooled-CCD detectors Reviewed

K. Shiokawa, Y. Otsuka, S. Suzuki, T. Katoh, Y. Katohl, M. Satoh, T. Ogawa, H. Takahashi, D. Gobbli, T. Nakamura, B. P. Williams, C.-Y. She, M. Taguchi, T. Shimomai

EARTH PLANETS AND SPACE   Vol. 59 ( 6 ) page： 585 - 599   2007

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We have developed three airglow temperature photometers with cooled-CCD detectors. The photometers measure rotational temperatures using the airglow emissions of OH and 02 near the mesopause region (altitude: 80-100 km). The photometers also measure six other airglow and auroral lines at wavelengths of 55.7, 630.0, 777.4, 589.3, 427.8, and 486.1 nm. The CCD detectors are used to distinguish the emission lines in these airglow bands, similarly to those used by the Spectral Airglow Temperature Imagers (SATI). In this paper, we describe the configuration of the photometers, their calibration, the data processing to extract rotational temperatures and emission intensities from the measured airglow spectra, as well as the initial deployment at Platteville, Colorado (40.2 degrees N, 255 degrees E), when their observations were compared with the concurrent and nearly collocated observations by a sodium lidar. We obtain a good correlation and some systematic difference of temperatures from the photometers and the lidar, and discuss possible causes of the temperature difference.

DOI： 10.1186/BF03352721

Web of Science

Gravity wave momentum flux in the upper mesosphere derived from OH airglow imaging measurements Reviewed

Shin Suzuki, Kazuo Shiokawa, Yuichi Otsuka, Tadahiko Ogawa, Minoru Kubota, Masaki Tsutsumi, Takuji Nakamura, David C. Fritts

EARTH PLANETS AND SPACE   Vol. 59 ( 5 ) page： 421 - 428   2007

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We report procedures to identify small-scale (20-100 km) atmospheric gravity waves from OH airglow images to estimate momentum fluxes carried by the waves. We also deduce contamination of background continuum emission in OH image, by comparing a simultaneous observation of OH lines measured by the Spectral Airglow Temperature Imager (SATI). We applied the procedures to a one-night dataset obtained at Shigaraki, Japan (34.9 degrees N, 136.1 degrees E) on November 19, 1999. The background wind, which is essential for deriving the intrinsic parameters of gravity waves, was measured by the Middle and Upper Atmosphere (MU) radar. Contamination of background continuum emission with the OH filter was deduced to be 30%. From these procedures, we found that the gravity waves identified in the OH images were mainly propagating southward or southeastward with horizontal wavelengths of 60-90 km and apparent phase speeds of 40-80 m/s. The estimated momentum fluxes on this night was 1-15 m(2) s(-2), with an average of 4.9 m(2) s(-2).

DOI： 10.1186/BF03352703

Web of Science

Summer-winter hemispheric asymmetry of sudden increase in ionospheric total electron content induced by solar flares: A role of O/N₂ ratio Reviewed

T. Tsugawa, T. Sadakane, J. Sato, Y. Otsuka, T. Ogawa, K. Shiokawa, A. Saito

Journal of Geophysical Research A: Space Physics   Vol. 111 ( A11 )   2006.11

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DOI： 10.1029/2006JA011951

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A fast-propagating, large-scale atmospheric gravity wave observed in the WAVE2004 campaign Reviewed

Minoru Kubota, Seiji Kawamura, Makoto Abo, Yoshiko Koizumi, Yasuhiro Murayama, Miho Yamamori, Kazuo Shiokawa, Yuichi Otsuka, Michihiro Uchiumi, Kiyoshi Igarashi, Takumi Abe, Koh-Ichiro Oyama, Naomoto Iwagami

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES   Vol. 111 ( D21 )   2006.11

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The Waves in Airglow Campaign in 2004 (WAVE2004), which aimed to elucidate the formation process of waves in airglow structures from both dynamical and chemical perspectives, was conducted using rocket-borne and ground-based instruments in Japan on 17 January 2004. In this experiment, we observed a large-scale atmospheric gravity wave (AGW), which appeared in both the vertical profiles of sodium density obtained by a Na lidar and the horizontal distributions of airglow emission obtained by an all-sky imager (ASI). Vertical propagation of the AGW accompanied by a shortening of its vertical wavelength was clearly visualized using the Na lidar data. The horizontal wavelength, horizontal phase velocity, period, and propagation direction of the AGW were estimated from the ASI data as 673-774 km, 107-122 m/s, similar to 1.75 hours, and north-northeastward, respectively. Using these parameters and the MF radar wind, vertical wavelengths of the wave were calculated from the dispersion relation of gravity waves. The calculated vertical wavelengths were comparable at altitudes of 85.5 km and 93.25 km to those estimated from the variation of the sodium density. Using a simple ray tracing technique, the AGW was traced back to the southern edge of the distorted jet stream near tropopause. This result strongly suggests that an unstable baroclinic wave associated with ageostrophic motions in the jet stream was the wave source of the large-scale AGW observed in the WAVE2004.

DOI： 10.1029/2005JD006788

Web of Science

Detection of ruptures of Andaman fault segments in the 2004 great Sumatra earthquake with coseismic ionospheric disturbances Reviewed

Kosuke Heki, Yuichi Otsuka, Nithiwatthn Choosakul, Narong Hemmakorn, Tharadol Komolmis, Takashi Maruyama

Journal of Geophysical Research: Solid Earth   Vol. 111 ( B9 )   2006.9

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We Near-field coseismic perturbations of ionospheric total electron content (TEC), caused by direct acoustic waves from focal regions, can be observed with Global Positioning System (GPS). They appear 10-15 min after the earthquake with typical periods of ∼4-5 min and propagate as fast as ∼1 km/s toward directions allowed by ambient geomagnetic fields. Ionospheric disturbance, associated with the 2004 December 26 great Sumatra-Andaman earthquake, was recorded with nine continuous GPS receiving stations in Indonesia and Thailand. Here we explore the possibility to constrain the rupture process of the earthquake with the observed ionospheric disturbances. We assumed linearly distributed point sources along the zone of coseismic uplift extending ∼1300 km from Sumatra to the Andaman Islands that excited acoustic waves sequentially as the rupture propagate northward by 2.5 km/s. TEC variations for several satellite-receiver pairs were synthesized by simulating the propagation of acoustic waves from the ground to the ionosphere and by integrating the TEC perturbations at intersections of line of sights and the ray paths. The TEC perturbations from individual point sources were combined using realistic ratios, and the total disturbances were compared with the observed signals. Prescribed ratios based on geodeticatly inferred coseismic uplifts reproduced the observed signals fairly well. Similar calculation using a rupture propagation speed of 1.7 km/s degraded the fit. Suppression of acoustic waves from the segments north of the Nicobar Islands also resulted in a poor fit, which suggests that ruptures in the northern half of the fault were slow enough to be overlooked in short-period seismograms but fast enough to excite atmospheric acoustic waves. Coseismic ionospheric disturbance could serve as a new indicator of faulting sensitive to ruptures with timescale up to 4-5 min. Copyright 2006 by the American Geophysical Union.

DOI： 10.1029/2005JB004202

Web of Science

Scopus

Estimating drift velocity of polar cap patches with all-sky airglow imager at Resolute Bay, Canada Reviewed

K. Hosokawa, K. Shiokawa, Y. Otsuka, A. Nakajima, T. Ogawa, J. D. Kelly

GEOPHYSICAL RESEARCH LETTERS   Vol. 33 ( 15 )   2006.8

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Highly sensitive all-sky airglow imager has been operative at Resolute Bay, Canada (74.73 degrees N, 265.07 degrees E; AACGM latitude 82.9 degrees) since January 2005. We present, as a first result from the imager, an event of polar cap patches drifting anti-sunward during the southward IMF conditions. Magnitude and direction of patch drift velocities are computed with a temporal resolution of 2 min by using the newly developed patch-tracking algorithm based on 2D cross correlation analysis. It is well visualized that the patches change their moving speed and direction drastically in a short time scale ( a few minutes). Speed of the patch is primally controlled by the IMF Bz. Dawn-dusk component of the patch drift velocities is well correlated with the IMF By in agreement with published By dependence of the nightside polar cap convection. However, response of the patch drift direction to the IMF By is found to be much slower ( approximate to 20 min) than that of the drift speed to the IMF Bz ( almost instantaneous).

DOI： 10.1029/2006GL026916

Web of Science

A multi-instrument measurement of a mesospheric front-like structure at the equator Reviewed

Kazuo Shiokawa, Shin Suzuki, Yuichi Otsuka, Tadahiko Ogawa, Takuji Nakamura, Martin G. Mlynczak, James M. Russell

JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN   Vol. 84A   page： 305 - 316   2006.7

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We have made a comprehensive measurement of a front-like structure in the mesosphere at the equator at Kototabang, Indonesia (0.2 degrees S, 100.3 degrees E), using an airglow imager, an airglow temperature photometer, a meteor radar, and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. The event was detected in airglow images of both OH-band (peak emission altitude: 87 km) and 557.7-nm (96-km) emissions, as an east-west front-like structure propagating northward with a velocity of 52-58 m/s. Wave trains with a horizontal wavelength of 30-70 km were observed after the passage of the front. The airglow intensity decreased for all the mesospheric emissions of OI (557.7 nm), OH-band, O-2-band (altitude: 94 km), and Na (589.3 nm) (90 km) after the front passage. The rotational temperatures of both the OH-band and the O-2-band also decreased similar to 10 K. An intense shear in northward wind velocity of 80 m/s/6 km was observed at altitudes of 84-90 km by the meteor radar. The kinetic temperature profile at altitudes of 20-120 km was observed near Kototabang by TIMED/SABER. The front-like structure and trailing waves were similar to those of the mesospheric tidal bore. However, we found that the ducting condition, which is necessary to form a bore, was not satisfied for the observed wave parameters. We speculate that the intense wind shear may play some role for the generation of the front-like structure.

DOI： 10.2151/jmsj.84A.305

Web of Science

Equatorial ionospheric scintillations and zonal irregularity drifts observed with closely-spaced GPS receivers in Indonesia Reviewed

Yuichi Otsuka, Kazuo Shiokawa, Tadahiko Ogawa

JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN   Vol. 84A   page： 343 - 351   2006.7

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To investigate drift velocities of a few hundred meter-scale irregularities associated with equatorial plasma bubbles, we used three single-frequency GPS receivers at the Equatorial Atmosphere Radar (EAR) site at Kototabang, Indonesia (0.20 degrees S, 100.32 degrees E; geomagnetic latitude 10.6 degrees S), since January 2003. The GPS receivers sampled GPS signal intensity at a rate of 20 Hz. Distances between the receivers were 116, 127, and 152 m. An analysis of scintillation index (S-4) in two years (2003-2004) revealed that the scintillations often occurred between 2000-0100 LT at equinoxes and that their occurrence rate was higher during March-April than during September-October. Drift velocities of irregularities were measured using cross-correlation analysis with the time series of the GPS signal intensity obtained from the three receivers. From a statistical analysis of the drift velocities, the eastward component of drift velocity just after sunset is found to be greater during March-April than during September-October. Based on these results, for the first time, we suggest that the east-west component of plasma drift velocity (or vertical electric field) may be related to the evolution of plasma irregularities causing scintillations throughout the mechanism causing the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocity could be attributed to the equinoctial asymmetry of neutral winds in the thermosphere.

DOI： 10.2151/jmsj.84A.343

Web of Science

Ionospheric disturbances over Indonesia and their possible association with atmospheric gravity waves from the troposphere Reviewed

Tadahiko Ogawa, Yuichi Otsuka, Kazuo Shiokawa, Akinori Saito, Michi Nishioka

JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN   Vol. 84A   page： 327 - 342   2006.7

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As part of the CPEA (Coupling Processes in the Equatorial Atmosphere) project, we have been conducting ground-based optical and radio observations of the ionosphere and thermosphere at Kototabang in Indonesia, Japan, and Australia. First, this paper gives a brief overview of some results, paying special attention to 100-1000 km scale plasma disturbances in the F region ionosphere, i.e., medium-scale traveling ionospheric disturbances (MSTIDs) and plasma bubbles. MSTIDs over the equator are observed within and in the south of the F region equatorial anomaly crest and have, on the average, a phase velocity of 300 m s(-1) toward the south, a period of 40 min, and a wavelength of 700 km. Plasma bubbles move to the east at similar to 100 m s(-1), have a scale of about 100 km with spacings of 200-250 km, and are embedded within plasma structures with a scale of about 1000 km. Interestingly, giant plasma bubbles can be simultaneously detected at lower midlatitudes in southern and northern hemispheres that are connected by the geomagnetic field line, and are very identical in appearance in the both hemispheres. Then, we present newly-obtained characteristics of equatorial ionospheric scintillations of 1.6-GHz GPS radio waves associated with plasma bubbles. Continuous scintillation observations for two years at Kototabang indicate that the scintillations appear predominantly from sunset to midnight in equinoctial months. Such a seasonal variation is also recognized from a statistical study of bubble occurrences over the Philippines, Singapore, and Indonesia. To investigate possible dynamical coupling between the ionosphere/thermosphere and troposphere over the equator, we compare the scintillation (bubble) activity and Earth's brightness temperature variation over the Indian Ocean measured by meteorological satellites. The results indicate that there can exist meaningful correlations between the scintillation occurrence and tropospheric disturbance at 80 degrees-95 degrees E longitudes, i.e. 5 degrees-20 degrees west of Kototabang. Possible processes to seed plasma bubbles are discussed.

DOI： 10.2151/jmsj.84A.327

Web of Science

Quasiperiodic southward moving waves in 630-nm airglow images in the equatorial thermosphere Reviewed

K. Shiokawa, Y. Otsuka, T. Ogawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 111 ( A6 )   2006.6

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We report quasiperiodic southward moving waves, which are commonly observed in the OI 630-nm airglow images (emission altitudes of 200-300 km) near the equator, in 2-year airglow observations at Kototabang, Indonesia (0.2 degrees S, 100.3 degrees E, geomagnetic latitude of -10.4 degrees). The waves have predominantly east-west phase fronts and repeatedly propagate southward with a velocity of 310 +/- 110 m/s and a period of 40 +/- 15 min. They are frequently observed in May-July with an occurrence rate of 53% and are also observed in other seasons with occurrences of similar to 20%. The waves are observed in and to the south (geomagnetically poleward) of the equatorial ionospheric anomaly, which is identified as an airglow enhancement region moving gradually to lower geomagnetic latitudes at the premidnight local times. We suggest that gravity waves in the lower thermosphere below similar to 300 km are a plausible cause of the observed quasiperiodic waves in the airglow images.

DOI： 10.1029/2005JA011406

Web of Science

Characteristics and implications of Doppler spectra of E region quasi-periodic echoes observed by the multibeam middle and upper atmosphere radar Reviewed

Ogawa, T, Otsuka, Y, Yamamoto, M

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 111 ( A5 )   2006.5

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Language：Japanese  

DOI： 10.1029/2005JA011335

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Climatological study of GPS total electron content variations caused by medium-scale traveling ionospheric disturbances Reviewed

N Kotake, Y Otsuka, T Tsugawa, T Ogawa, A Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 111 ( A4 )   2006.4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Using global positioning system (GPS) data taken from the International GNSS Service (IGS), we investigated total electron content (TEC) perturbations associated with medium-scale traveling ionospheric disturbances (MSTIDs). We analyzed TEC data taken from four or five GPS receivers in each of six regions ( Japan, Europe, eastern United States, western United States, Australia, and South America) in 3 years ( 1998, 2000, and 2001). To derive perturbation components of TEC (I&apos;), we subtracted the 1-hour running average form the time sequence of TEC for each satellite-receiver pair. Standard deviation of I&apos; within 1 hour, delta I, was calculated every hour, and MSTIDs activity were defined as delta I/(I) over bar, where (I) over bar is the 1-hour average of absolute vertical TEC. We found that MSTIDs activity during daytime is different from that during nighttime with respect to seasonal, solar activity, longitudinal, and latitudinal dependences. Daytime MSTIDs activity are high in winter in all six regions. On the other hand, seasonal variation of nighttime MSTIDs activity is coupled with its longitudinal variation. In the Japanese and Australian longitudinal sector, nighttime MSTIDs are most active near the June solstice, whereas it is most active near the December solstice in the European longitudinal sector. Nighttime MSTIDs activity at the Japanese and Australian longitudinal sector shows negative correlation with solar activity, whereas solar activity dependence is not seen in daytime MSTIDs activity. These results suggest that mechanisms causing MSTIDs could be different between daytime and nighttime.

DOI： 10.1029/2005JA011418

Web of Science

Geomagnetic conjugate observations of large-scale traveling ionospheric disturbances using GPS networks in Japan and Australia Reviewed

T Tsugawa, K Shiokawa, Y Otsuka, T Ogawa, A Saito, M Nishioka

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 111 ( A2 )   2006.2

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The geomagnetic conjugacy of large-scale traveling ionospheric disturbances (LSTIDs) was studied using total electron content (TEC) data derived from GPS networks in Japan and Australia. The number of simultaneous occurrences of LSTIDs within +/- 1 hour in both hemispheres was 5 out of 20 (21) events in Japan (Australia) when Kp &gt;= 5- and 0 out of 15 (10) when Kp &lt;= 4+. As for the LSTIDs observed simultaneously in both hemispheres, the propagation velocities of equatorward LSTID were comparable between the two hemispheres, with differences of 10-40%. The crossing times at 30 degrees geomagnetic latitude of the simultaneous LSTIDs over Japan and Australia were also different by several tens of minutes for all five events. These observational results indicate that the LSTIDs observed almost simultaneously in both hemispheres are not connected electromagnetically through the geomagnetic field but are generated by atmospheric gravity waves propagating to the equator independently in the two hemispheres.

DOI： 10.1029/2005JA011300

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Equatorial ionospheric scintillations and zonal irregularity drifts observed with closely-spaced GPS receivers in Indonesia Reviewed

Otsuka, Y., K. Shiokawa, and T. Ogawa

J. Meteor. Soc. Japan   Vol. 84A   page： 343-351   2006

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GPS detection of total electron content variations over Indonesia and Thailand following the 26 December 2004 earthquake Reviewed

Otsuka, Y., N. Kotake, T. Tsugawa, K. Shiokawa, T. Ogawa, Effendy, S. Saito, M. Kawamura, T. Maruyama, N. Hemmakorn, and T. Komolmis

Earth, Planets and Space   Vol. 58   page： 159-165   2006

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A Statistical Study of Ionospheric Irregularities Observed with a GPS Network in Japan

Y. Otsuka, T. Aramaki, T. Ogawa, A. Saito

Recurrent Magnetic Storms: Corotating Solar Wind Streams   Vol. 167   page： 271 - +   2006

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© 2006 by the American Geophysical Union. All rights reserved. The Geographical Survey Institute of Japan has installed a network of about 1000 dual-frequency GPS receivers in Japan with their mutual distance of about 25 km. Phases and pseudo ranges of dual-frequency GPS signals are recorded every 30 s. The dense distribution of the GPS receivers allows us to reveal twodimensional structures of the ionospheric plasma density irregularities with scale sizes of the order of several kilometers. We analyzed TEC data obtained from the GPS network of Japan in 2000. It was found that the irregularity characteristics over Japan depended on latitude. The results are as follows: (1) at the northern part, the irregularities appeared only during geomagnetic storms. (2) At the middle part, they had the most frequent occurrence in summer nighttime and were usually accompanied by Medium-Scale Traveling Ionospheric Disturbances (MSTID). (3) At the southern part, they were associated with the equatorial plasma bubbles and their occurrences were highest in the equinoctial nighttime.

DOI： 10.1029/167GM21

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Scopus

GPS observations of post-storm TEC enhancements at low latitudes Reviewed

Ivan Kutiev, Yuichi Otsuka, Akinori Saito, Shigeto Watanabe

EARTH PLANETS AND SPACE   Vol. 58 ( 11 ) page： 1479 - 1486   2006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

In a previous work (J. Geophys. Res., 110(A01308), 1-11, 2005), the authors developed an original approach to the processing of total electron content (TEC) data obtained by GPS signals from the Japan receiver network. This approach includes removing the diurnal and seasonal variation carried by 27-day medians and the solar rotation periodicity. The relative deviations of TEC from the median-from all measured locations at a given hour-were then approximated by a regression line along the main prolongation of the Japan islands, between latitudes 24 degrees and 45 degrees N. The two variables of the regression line, the average value at the center and the slope were obtained as a time series, and their behavior during geomagnetic storms in the period 2000-2002 were analyzed. One interesting result was the observed enhancement of TEC at the end of the recovery phase of the storms. The slope variations clearly showed that this enhancement started from the south and was interpreted as a poleward expansion of equatorial crest. In the present paper we further analyze this post-storm phenomenon, adding foF2 data from Japanese Kokubunji and Okinawa ionosondes. We also show the latitude extension of the poleward expansion by using lat/UT contour plots. The results confirm that most of the post-storm TEC enhancements are part of the equatorial crest region which extends poleward during nighttime. In some cases, the enhanced TEC structures develop by separating from the crest region. Daytime TEC enhancements were also observed. Their structures are not confined to the equatorial crests region, but occupy the whole latitude range considered in this study. TEC post-storm enhancements were generally found to be in agreement with foF2 variations.

DOI： 10.1186/BF03352647

Web of Science

GPS detection of total electron content variations over Indonesia and Thailand following the 26 December 2004 earthquake Reviewed

Yuichi Otsuka, N. Kotake, Takuya Tsugawa, Kazuo Shiokawa, Tadahiko Ogawa, E. Effendy, Susumu Saito, Masabumi Kawamura, Takashi Maruyama, N. Hemmakorn, T. Komolmis

Earth, Planets and Space   Vol. 58 ( 2 ) page： 159 - 165   2006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan , The Geodetic Society of Japan , The Japanese Society for Planetary Sciences  

We report the response of the ionosphere to the large earthquake that occurred in West Sumatra, Indonesia, at 0058 UT on December 26, 2004. We have analyzed Global Positioning System (GPS) data obtained at two sites in Sumatra and at three sites in Thailand to investigate total electron content (TEC) variations. Between 14 and 40 min after the earthquake, TEC enhancements of 1.6-6.9 TEC units (TECU) were observed at subionospheric points located 360-2000 km north of the epicenter. From the time delays of the observed TEC enhancements, we find that the TEC enhancements propagated northward from the epicenter. The time delays between the earthquake and rapid increases in TEC, which occurred near the epicenter, are consistent with the idea that acoustic waves generated by the earthquake propagated into the ionosphere at the speed of sound to cause the TEC variations. A small TEC enhancement of 0.6 TECU was observed south of the epicenter, while no TEC enhancements were seen east of the epicenter. From a model calculation, we find that this directivity of the TEC variations with respect to the azimuth from the epicenter could be caused partially by the directivity in the response of the electron density variation to the acoustic waves in the neutral atmosphere. © 2006, The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences. All rights reserved.

DOI： 10.1186/BF03353373

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Scopus

Ionospheric variations at midlatitude detected by a dense GPS receiver array in Japan Reviewed

A. Saito, Y. Otsuka, T. Tsugawa, K. Matsunaga, K. Hoshinoo

Proceedings of the Annual Meeting - Institute of Navigation     page： 425 - 428   2005.12

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Ionospheric variations over Japan were investigated with high spatial and temporal resolution data of GEONET, which consists of about 1,200 GPS receivers. Total Electron Content (TEC) data derived from GEONET clarified that various ionospheric phenomena cause the variation of the ionosphere in the region around 30 degrees of the geomagnetic latitude, and affect on the navigation system. The large day-to-day variation of TEC gradient over Japan was found to be attributed to the day-to-day variation of the Equatorial Ionization Anomaly (EIA) because Japan is located on the northern slope of EIA. The amplitude and location of EIA have large variability because they are controlled by a complex system among the neutral and the ionized atmospheres, lower and upper atmospheres, and low-latitude and high-latitude ionospheres. As a result, the variation of EIA is difficult to be predicted. Meso-scale ionospheric structures, such as medium-scale traveling ionospheric disturbances (MSTID) and plasma bubbles, drift in about 100m/s of velocity and cause variations of short temporal scale. They were often observed to be accompanied by smaller scale structures that generate phase and amplitude scintillations of the GPS radio waves. The activity of MSTID has maxima in summer and winter. During these maximum periods, they were detected by GEONET almost everyday. Plasma bubbles were detected in the lowest latitude area of Japan, and found to reach the central part of Japan in a few days in a year. Most of them were in the geomagnetically disturbed periods. The geomagnetic disturbances also generate large-scale traveling ionospheric disturbances and reduction of TEC.

Scopus

Efficacy of STI571 for a patient with metastatic gastrointestinal stromal tumor Reviewed

Sasaki, Y, Niwa, Y, Ando, N, Otsuka, Y, Ohmiya, N, Hirooka, Y, Itoh, A, Furuta, S, Goto, H

HEPATO-GASTROENTEROLOGY   Vol. 52 ( 66 ) page： 1764 - 1767   2005.11

Observations of equatorial plasma bubbles using broadcast VHF radio waves Reviewed

H Nakata, Nagashima, I, K Sakata, Y Otsuka, Y Akaike, T Takano, S Shimakura, K Shiokawa, T Ogawa

GEOPHYSICAL RESEARCH LETTERS   Vol. 32 ( 17 )   2005.9

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The propagation of VHF radio waves affected by field-aligned irregularities within equatorial plasma bubbles is examined. Continuous observation of VHF radio waves at Tateyama, Japan, shows that broadcast radio waves transmitted from Southeast Asia propagate to Japan. Using a ray-tracing calculation combined with a model of scattering by field-aligned irregularities, we determined scatter points suitable for the reception of these radio waves. The results of the calculation show that radio waves transmitted from the Philippines can propagate to Japan owing to field-aligned irregularities within equatorial plasma bubbles located west of Japan. Comparing with all-sky imager data observed at Sata, Japan, we found that the radio waves are received at Tateyama when the plasma bubbles are observed at Sata. The results of this study show that the reception of these radio waves is closely correlated with the equatorial plasma bubbles.

DOI： 10.1029/2005GL023243

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Geomagnetic conjugate observation of nighttime medium-scale and large-scale traveling ionospheric disturbances: FRONT3 campaign

Shiokawa, K; Otsuka, Y; Tsugawa, T; Ogawa, T; Saito, A; Ohshima, K; Kubota, M; Maruyama, T; Nakamura, T; Yamamoto, M; Wilkinson, P

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 110 ( A5 ) page： doi:10.1029/2004JA010845   2005.5

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DOI： 10.1029/2004JA010845

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Total electron content behavior over Japan during geomagnetic storms Reviewed

Kutiev, I, S Watanabe, Y Otsuka, A Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 110 ( A1 )   2005.1

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[ 1] The total electron content (TEC) obtained from GPS signals is used to study ionospheric dynamics over Japan during geomagnetically disturbed conditions. The numerous TEC measurements are averaged in cells with a size 1.5degrees x 1.5degrees geographic scale and formatted as time series within the years 2000 - 2002. To extract the storm time changes of TEC, the diurnal and 27-day periodicities are subsequently removed. Diurnal variations are removed by replacing absolute TEC values in each cell with their relative deviations (RTEC) from medians. The hourly RTEC values from all cells within the central 4degrees-wide band over Japan area are then approximated by a plane surface. This surface is represented by two parameters: its value at the center (rt) and the slope (b) along the main axis, taken as constants of the linear regression. The 27-day periodicity was approximated by Fourier waves with main period of 640 hours and two harmonics separately for rt and b and subtracted from them. The analysis of rt and b behavior during a number of geomagnetic storms allowed us to reveal several repeatable features of average TEC behavior. It was found that TEC behavior during the storms is similar to that of f(o)F(2) at the F region and was local time - dependent. A marked poleward expansion of the equatorial ionosphere ( crest region) at the end of recovery phase is persistently observed feature, produced probably by intensified eastward zonal winds. Such an expansion of equatorial ionosphere is observed also during isolated substorms, outside main geomagnetic storms. An oscillation-like change of positive and negative disturbances with period of 24 hours is observed during a 4-day period, following a moderate storm. In the absence of geomagnetic activity driver that effect is probably caused by the alternative expansion and contraction of equatorial ionosphere.

DOI： 10.1029/2004JA010586

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Relationship between propagation direction of gravity waves in OH and OI airglow images and VHF radar echo occurrence during the SEEK-2 campaign Reviewed

F Onoma, Y Otsuka, K Shiokawa, T Ogawa, M Yamamoto, S Fukao, S Saito

ANNALES GEOPHYSICAE   Vol. 23 ( 7 ) page： 2385 - 2390   2005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EUROPEAN GEOSCIENCES UNION  

We report simultaneous observations of atmospheric gravity waves (AGW) in OI (557.7 nm) and OH air-glow images and VHF radar backscatter from field-aligned irregularities (FAI) in the E-region during the SEEK-2 (Sporadic-E Experiment over Kyushu 2) campaign period from 29 July to 9 August 2002. An all-sky imager was operated at Nishino-Omote (30.5 degrees N, 130.1 degrees E), Japan. On 14 nights. 17 AGW events were detected in OI and OH air-glow images. AGW propagated mostly toward the northeast or southeast. From comparison with the E-region FAI occurrence, which is detected by a nearby VHF radar (31.57 MHz), we found that AGW tended to propagate southeastward during FAI events. This result suggests that the interaction between AGW and E-region plasma plays an important role in generating FAI. Furthermore, polarization electric fields generated directly by AGW may contribute to the FAI generation.

DOI： 10.5194/angeo-23-2385-2005

Web of Science

Simultaneous ground- and satellite-based airglow observations of geomagnetic conjugate plasma bubbles in the equatorial anomaly Reviewed

T Ogawa, E Sagawa, Y Otsuka, K Shiokawa, TI Immel, SB Mende, P Wilkinson

EARTH PLANETS AND SPACE   Vol. 57 ( 5 ) page： 385 - 392   2005

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We compare, for the first time, geomagnetically-conjugate plasma bubbles observed by ground-based 01 630.0-nm all-sky imagers at Shigaraki, Japan (34.8 degrees N, 136.1 degrees E; magnetic latitude 25.4 degrees N) and Darwin, Australia (12.4 degrees S, 131.0 degrees E; magnetic latitude 22.0 degrees S), with global-scale plasma structures (similar to 10,000 km in longitude) in the equatorial anomaly simultaneously detected with an OI 135.6-nm imager on the IMAGE satellite at similar to 7 earth radii. As found previously, global-scale plasma structures in both hemisphere imaged by IMAGE consist of an array of geomagnetically-conjugate small- to medium-scale (a few hundreds to 1000 km in longitude) wavy structures that move to the east at similar to 100 in s(-1). We find the following: 1) plasma bubbles detected with the all-sky imagers reach an apex altitude of 1800 km over the geomagnetic equator while moving to the east at - 100 m s(-1) with spacings of 200-250 km. 2) Bubbles observed with the all-sky imagers and IMAGE are embedded within the small- to medium-scale wavy structures, and some of them are located near the crest of ail enhanced electron density region associated with the wavy structures. 3) The bubbles and wavy structures that are generated near sunset slant to the west with increasing latitude in both hemispheres, and tilts do not change with longitude (i.e., local time). The results suggest that the generation and evolution of plasma bubbles are closely related to those of the small- to medium-scale plasma structures.

DOI： 10.1186/BF03351822

Web of Science

The first coordinated observations of mid-latitude E-region quasi-periodic radar echoes and lower thermospheric 557.7-nm airglow Reviewed

T Ogawa, Y Otsuka, F Onoma, K Shiokawa, M Yamamoto

ANNALES GEOPHYSICAE   Vol. 23 ( 7 ) page： 2391 - 2399   2005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EUROPEAN GEOSCIENCES UNION  

We present the first coordinated observations of quasi-periodic (QP) radar echoes from sporadic-E (E-S) field-aligned irregularities (FAIs), OI 557.7-nm airglow, and neutral winds in a common volume over Shigaraki, Japan (34.9 degrees N. 136.1 degrees E) on the night of 5 August 2002 during the SEEK-2 campaign. QP echo altitudes of 90-110 km were lower than usual by 10 km, enabling us to make a detailed comparison among QP echoes, airglow intensity, and neutral wind at around 96 km altitude. Eastward movement of the QP echo regions is consistent with the motions of neutral winds, airglow structures, and FAIs, suggesting that the electrodynamics of E-S-layers is fundamentally controlled by the neutral atmospheric dynamics. During the QP echo event, the echo altitudes clearly went up (down) in harmony with an airglow enhancement (subsidence) that also moved to the east. This fact suggests that the eastward-moving enhanced airglow region included an upward (downward) component of neutral winds to raise (lower) the altitude of the wind-shear node responsible for the E-S formation. The airglow intensity, echo intensity, and Doppler velocity of FAIs at around 96km altitude fluctuated with periods from 10min to 1 h, indicating that these parameters were modulated with short-period atmospheric disturbances. Some QP echo regions below 100 km altitude contained small-scale QP structures in which very strong neutral winds exceeding 100 m/s existed. The results are compared with recent observations, theories, and simulations of QP echoes.

DOI： 10.5194/angeo-23-2391-2005

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Transition region of TEC enhancement phenomena during geomagnetically disturbed periods at mid-latitudes Reviewed

K Unnikrishnan, A Saito, Y Otsuka, M Yamamoto, S Fukao

ANNALES GEOPHYSICAE   Vol. 23 ( 11 ) page： 3439 - 3450   2005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EUROPEAN GEOSCIENCES UNION  

Large-scale TEC perturbations/enhancements observed during the day sectors of major storm periods, 1213 February 2000, 23 September 1999, 29 October 2003, and 21 November 2003, were studied using a high resolution GPS network over Japan. TEC enhancements described in the present study have large magnitudes (&gt;= 25 x 10(16) electrons/m(2)) compared to the quiet-time values and long periods (&gt;= 120 min). The sequential manner of development and the propagation of these perturbations show that they are initiated at the northern region and propagate towards the southern region of Japan, with velocities &gt; 350 m/s. On 12 February 2000, remarkably high values of TEC and background content are observed at the southern region, compared to the north, because of the poleward expansion of the equatorial anomaly crest, which is characterized by strong latitudinal gradients near 35 degrees N (26 degrees N geomagnetically). When the TEC enhancements, initiating at the north, propagate through the region 39-34 degrees N (30-25 degrees N geomagnetically), they undergo transitions characterized by a severe decrease in amplitude of TEC enhancements. This may be due to their interaction with the higher background content of the expanded anomaly crest. However, at the low-latitude region, below 34 degrees N, an increase in TEC is manifested as an enhanced ionization pattern (EIP). This could be due to the prompt penetration of the eastward electric field, which is evident from high values of the southward Interplanetary Magnetic Field component (IMF B-z) and AE index. The TEC perturbations observed on the other storm days also exhibit similar transitions, characterized by a decreasing magnitude of the perturbation component, at the region around 39-34 degrees N. In addition to this, on the other storm days, at the low-latitude region, below 34 degrees N, an increase in TEC (EIP feature) also indicates the repeatability of the above scenario. It is found that, the latitude and time at which the decrease in magnitude of the perturbation component/amplitude of the TEC enhancement are matching with the latitude and time of the appearance of the high background content. In the present study, on 12 February 2000, the F-layer height increases at Wakkanai and Kokubunji, by exhibiting a typical dispersion feature of LSTID, or passage of an equator-ward surge, which is matching with the time of occurrence of the propagating TEC perturbation component. Similarly, on 29 October 2003, the increase in F-layer heights by more than 150 km at Wakkanai and 90 km at Kokubunji around 18:00 JST, indicates the role of the equatorward neutral wind. On that day, TEC perturbation observed at the northern region, after 18:30 JST, which propagates towards south, could be caused mainly by the equatorward neutral wind, leading to an F-layer height increase. These observations imply the role of the equatorward neutral wind, which increases the F-layer height, by lifting the ionization to the regions of lower loss during daytime, when production is still taking place, which, in turn, increases the TEC values.
Large-scale traveling ionospheric disturbances (LSTIDs) are considered as ionospheric manifestations of the passage of Atmospheric Gravity Waves (AGWs) that are generated at the high latitude by energy input from the magnetosphere to the low-latitude ionosphere. This study shows that large-scale TEC perturbations observed here are produced at the northern region due to the combined effects of the equatorward neutral wind, the subsequent F-layer height increase, and LSTIDs. When these perturbation components propagate through the region, 39-34 degrees N, they undergo transitions characterised by a decrease in magnitude. Also, at the low-latitude region, below 34 degrees N, an increase in the TEC exhibits EIP feature, due to the prompt penetration of the eastward electric field.

DOI： 10.5194/angeo-23-3439-2005

Web of Science

Spatial relationship of equatorial plasma bubbles and field-aligned irregularities observed with an all-sky airglow imager and the Equatorial Atmosphere Radar Reviewed

Y Otsuka, K Shiokawa, T Ogawa, T Yokoyama, M Yamamoto, S Fukao

GEOPHYSICAL RESEARCH LETTERS   Vol. 31 ( 20 )   2004.10

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report, for the first time, simultaneous two-dimensional observations of 630-nm airglow depletions and radar backscatter from field-aligned irregularities (FAI) associated with equatorial plasma bubbles. Spatial distributions of backscatter were obtained by performing east-west scans with the 47-MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia. A 630-nm airglow depletion, caused by a plasma bubble, was simultaneously observed with an all-sky airglow imager. Both the airglow depletion and backscatter region appeared as band-like structure elongated in the meridional direction with a zonal width of about 100 km. To compare the spatial structures of backscatter with that of airglow depletion, the backscatter was projected onto a horizontal plane at 250-km altitude. Backscatter was found to occur within the entire airglow-depleted region.

DOI： 10.1029/2004GL020869

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Statistical characteristics of gravity waves observed by an all-sky imager at Darwin, Australia Reviewed

Shin Suzuki, K. Shiokawa, Y. Otsuka, T. Ogawa, P. Wilkinson

Journal of Geophysical Research D: Atmospheres   Vol. 109 ( D20 ) page： D20 - S07   2004.8

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An all-sky airglow imager with a cooled charge-coupled device camera in place at Darwin (12.4°S, 131.0°E), Australia, since October 2001 has been used to obtain two-dimensional gravity wave images in the mesopause region. Using airglow images of the OI (557.7 nm, emission altitude ∼96 km) and OH band (720-910 nm, ∼86 km) emissions obtained for October 2001 to August 2002, we investigated the wavelengths, phase velocities, and propagation directions of gravity waves. Wave occurrence in OH images (60-90%) is higher than in OI images (30-70%) for all seasons. The waves have wavelengths of less than 90 km (peak: 30-50 km) and phase velocities ofless than 90 m/s (peak: 30-60 m /s). Most of the waves propagate in the meridional direction, and the directionality strongly depends on the season. In winter, waves propagate both poleward and equatorward, while in summer almost all waves propagate poleward. An examination of airglow-imaging statistics at Adelaide (35°S, 138°E), Australia, obtained by Walterscheid et al. [1999], leads us to conclude that this clear directionality is caused by the location of the wave sources and by the wave ducting processes
that is, poleward waves in summer come from an equatorial convective source through a thermal duct structure. The effect of wind filtering on the waves is also discussed for zonal wave propagation. Copyright 2004 by the American Geophysical Union.

DOI： 10.1029/2003JD004336

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Scopus

Geomagnetic conjugate observations of medium-scale traveling ionospheric disturbances at midlatitude using all-sky airglow imagers Reviewed

Y Otsuka, K Shiokawa, T Ogawa, P Wilkinson

GEOPHYSICAL RESEARCH LETTERS   Vol. 31 ( 15 )   2004.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report for the first time simultaneous observations of medium-scale traveling ionospheric disturbances (MSTIDs) at geomagnetic conjugate points in both hemispheres, using two all-sky airglow imagers at midlatitudes. A 630-nm all-sky CCD imager at Sata, Japan, detected MSTIDs with a wavefront elongated from NW to SE on the night of August 9, 2002. During this event, MSTIDs with a wavefront elongated from SW to NE were observed at the geomagnetic conjugate point, Darwin, Australia. To investigate geomagnetic conjugacy of the MSTID structures, the Darwin images were mapped The MSTID structures mapped from Darwin to its magnetic conjugate points along the geomagnetic field lines ( B) coincide closely with those in the Sata images. This result suggests that polarization electric field (E-p) plays an important role in the generation of MSTIDs. E-p maps along B and moves the F region plasma upward or downward by E x B drifts, causing plasma density perturbations with structures mirrored in the northern and southern hemispheres.

DOI： 10.1029/2004GL020262

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Comparison of OH rotational temperatures measured by the spectral airglow temperature imager (SATI) and by a tilting-filter photometer Reviewed

K Shiokawa, Y Otsuka, T Ogawa, H Takahashi, T Nakamura, T Shimomai

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 66 ( 11 ) page： 891 - 897   2004.7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Measurements of OH rotational temperatures have been carried out at Shigaraki, Japan(34.8degreesN, 136.1degreesE), for 8 nights in September-December:2000, using the Spectral Airglow Temperature Imager (SATI) and a tilting-filter zenith photometer (MC4), simultaneously. Clear positive correlation was obtained between the two temperatures. However, the absolute temperatures obtained by SATI were unusually low (similar to130-180 K) with differences of similar to60 K from the MC4 temperatures. We discuss possible causes of the temperature differences, particularly for SATI, which newly uses OH(6-2) Q-branch lines (MC4 uses OH(6-2) P-branch lines) and imaging optics for the temperature measurement. (C) 2004 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2004.03.003

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A statistical study of large-scale traveling ionospheric disturbances using the GPS network in Japan Reviewed

Tsugawa, T, Saito, A, Otsuka, Y

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 109 ( A6 ) page： A06302   2004.6

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Language：Japanese  

DOI： 10.1029/2003JA010302

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Simultaneous mesosphere/lower thermosphere and thermospheric F region observations during geomagnetic storms Reviewed

Balan, N, Kawamura, S, Nakamura, T, Yamamoto, M, Fukao, S, Igarashi, K, Maruyama, T, Shiokawa, K, Otsuka, Y, Ogawa, T, Alleyne, H, Watanabe, S, Murayama, Y

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 109 ( A4 )   2004.4

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DOI： 10.1029/2003JA009982

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Spatial relationship of equatorial plasma bubbles and field-aligned irregularities observed with an all-sky airglow imager and the Equatorial Atmosphere Radar Reviewed

Otsuka, Y., K. Shiokawa, T. Ogawa, T. Yokoyama, M. Yamamoto, and S. Fukao

Geophys. Res. Lett.   Vol. 31 ( L20802 ) page： 10.1029/2004GL020869   2004

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

We report, for the first time, simultaneous two-dimensional observations of 630-nm airglow depletions and radar backscatter from field-aligned irregularities (FAI) associated with equatorial plasma bubbles. Spatial distributions of backscatter were obtained by performing
east-west scans with the 47-MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia. A 630-nm airglow depletion, caused by a plasma bubble, was simultaneously
observed with an all-sky airglow imager.
Both the airglow depletion and backscatter region appeared as band-like structure elongated in the meridional direction with a zonal width of about 100 km. To compare the spatial structures of backscatter with that of airglow depletion,
the backscatter was projected onto a horizontal plane at 250-km altitude. Backscatter was found to occur within the entire airglow-depleted region.

*Geomagnetic conjugate observations of medium-scale traveling ionospheric disturbances at midlatitude using all-sky airglow imagers Reviewed

Otsuka, Y., K. Shiokawa, T. Ogawa, and P. Wilkinson

Geophys. Res. Lett.   Vol. 31 ( L15803 ) page： doi:10.1029/2004GL020262   2004

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We report for the first time simultaneous observations of medium-scale traveling ionospheric disturbances (MSTIDs) at geomagnetic conjugate points in both hemispheres, using two all-sky airglow imagers at midlatitudes. A 630-nm all-sky CCD imager at Sata, Japan, detected MSTIDs with a wavefront elongated from NW to SE on the night of August 9, 2002. During this event, MSTIDs with a wavefront elongated from SW to NE were observed at the geomagnetic conjugate point, Darwin, Australia. The MSTID structures mapped from Darwin to its magnetic conjugate points along the geomagnetic field lines B coincide closely with those in the Sata images. This result suggests that polarization electric field (Ep) plays an important role in the generation of MSTIDs. Ep maps along B and moves the F region plasma upward or downward by ExB drifts, causing plasma density perturbations with structures mirrored in the northern and southern hemispheres.

Generation of large-scale equatorial F-region plasma depletions during low range spread-F season Reviewed

Y Sahai, PR Fagundes, Abalde, JR, AA Pimenta, JA Bittencourt, Y Otsuka, VH Rios

ANNALES GEOPHYSICAE   Vol. 22 ( 1 ) page： 15 - 23   2004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EUROPEAN GEOPHYSICAL SOC  

All-sky imaging observations of the F-region OI 630nm nightglow emission allow us to visualize large-scale equatorial plasma depletions, generally known as trans- equatorial plasma bubbles. Strong range type spread-F is the radio signature of these (magnetically) north-south aligned plasma depletions. An extensive database of the 01 630nm emission all-sky imaging observations has been obtained at Cachoeira Paulista (22.7degreesS, 45.0degreesW; dip latitude similar to16degreesS), Brazil, between the years 1987 and 2000. An analysis of these observations revealed that relatively few large-scale ionospheric plasma depletions occur during the months of May to August (southern winter, June solstice) in the Brazilian sector. Of the few that are observed during, these months, some occur in association with geomagnetic storms and some do not. In this paper, a detailed analysis of the events when large-scale ionospheric plasma depletions were initiated and evolved during the June solstice periods are presented and discussed.

DOI： 10.5194/angeo-22-15-2004

Web of Science

Time evolution of high-altitude plasma bubbles imaged at geomagnetic conjugate points Reviewed

K Shiokawa, Y Otsuka, T Ogawa, P Wilkinson

ANNALES GEOPHYSICAE   Vol. 22 ( 9 ) page： 3137 - 3143   2004

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Temporal and spatial evolution of two high-altitude plasma bubbles (evening and midnight) was observed on 4 April 2002, at geomagnetic conjugate points at Sata, Japan (magnetic latitude 24degrees N), and Darwin, Australia (magnetic latitude 22degrees S), using two 630-nm airglow imagers. The apex height of the bubbles reached similar to1500km. The upward velocity of the evolution was faster in the evening (similar to170m/s at 20:00-21:00 LT) than around midnight (similar to28 m/s at 23:00-00:00 LT). Bifurcating features of the bubbles into a smaller scale size of similar to50 km were clearly seen for both the evening and midnight bubbles, showing fairly good conjugacy between the Northern and Southern Hemispheres.

DOI： 10.5194/angeo-22-3137-2004

Web of Science

Ray-tracing Calculation of VHF Radio Waves Scattered by Field-aligned Irregularities Associated with Equatorial Plasma Bubbles Reviewed

Hiroyuki Nakata, Yoshiaki Akaike, Yuichi Otsuka, Toshiaki Takano, Satoshi Ujigawa, Ikuo Nagashima

IEEJ Transactions on Fundamentals and Materials   Vol. 124 ( 12 ) page： 1253 - 1254   2004

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In order to explain the propagation of VHF radio waves for TV broadcasting transmitted from Southeast Asia associated with equatorial plasma bubbles, we have examined ray paths of the radio waves scattered by field-aligned irregularities in equatorial plasma bubbles. In determining the ray paths of the radio waves, a ray tracing calculation combined with a model of the scattering by field-aligned irregularities is used. It is found that VHF radio waves transmitted from Philippines can propagate to Japan due to scattering by field-aligned irregularities located above the East China Sea. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.124.1253

Scopus

Thermospheric wind during a storm-time large-scale traveling ionospheric disturbance Reviewed

K Shiokawa, Y Otsuka, T Ogawa, S Kawamura, M Yamamoto, S Fukao, T Nakamura, T Tsuda, N Balan, K Igarashi, G Lu, A Saito, K Yumoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 108 ( A12 )   2003.12

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[1] A prominent large-scale traveling ionospheric disturbance (LSTID) was observed in Japan during the major magnetic storm (Dst similar to -358 nT) of 31 March 2001. It was detected as enhancements of the 630-nm airglow and foF2, GPS-TEC variations, and a decrease in F-layer virtual height at 1700 - 1900 UT (0200 - 0400 LT). It moved equatorward with a speed of similar to600 m/s. The decrease in the F-layer height was also detected by the MU radar at Shigaraki. Thermospheric wind variations were observed by the MU radar through ion drift measurement and by a Fabry-Perot interferometer (FPI) through a Doppler shift of the 630-nm airglow line at Shigaraki. The wind data show a turn of the meridional wind from -94 m/s (equatorward) to + 44 m/s (poleward) during the LSTID, indicating that an intense poleward wind in the thermosphere passed over Shigaraki as an atmospheric gravity wave and caused the observed ionospheric features of the LSTID. Intense poleward wind was also detected at mesospheric altitudes (95 - 100 km) by the MU radar ( through meteor echoes) and by the FPI ( through the 558-nm airglow) with a delay of similar to2 hours from the thermospheric wind, indicating downward phase progression of the wave. Generation of the observed poleward wind in the auroral zone was investigated using magnetic field data and auroral energy input estimated by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. We suggest that simple atmospheric heating and/or the Lorentz force in the auroral zone do not explain the observed poleward wind enhancement.

DOI： 10.1029/2003JA010001

Web of Science

Optical and radio measurements of a 630-nm airglow enhancement over Japan on 9 September 1999 Reviewed

Y Otsuka, T Kadota, K Shiokawa, T Ogawa, S Kawamura, S Fukao, Zhang, SR

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 108 ( A6 )   2003.6

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Using a comprehensive data set from optical and radio instruments, we investigate a midnight brightness wave that appeared in 630-nm airglow images over Japan on the night of 9 September 1999. This may be the first such observation of the brightness wave with an all-sky imager in the East-Asian longitudinal sector. The imager at Shigaraki (35.6degreesN, 136.1degreesE) tracked a north-northeastward propagation of the wave with an apparent velocity of 500 m s(-1) after midnight. Ionosonde observations at five stations in Japan showed that rapid descent of the F-2 layer propagated northward beyond 35degreesN with decreasing amplitude. Incoherent scatter observations with the MU radar at Shigaraki also revealed that the F-2 peak altitude decreased from 360 to 280 km during the event. During the F-2 layer descent the altitude profile of the electron density became sharp, enhancing the F-2 peak electron density. After the F-2 layer altitude reached 280 km, electron density in the F-2 layer rapidly decreased because of increased neutral density at low altitude. A Fabry-Perot interferometer (FPI) at Shigaraki observed northward neutral winds of 10-70 m s(-1) during the event. A model calculation demonstrates that the meridional winds estimated from the MU radar electron density profiles are fairly consistent with those observed with the FPI. From these results we conclude that the observed northward wind enhancements, probably caused by the midnight temperature maximum, pushed down the plasma in the F-2 layer to lower altitudes along the geomagnetic field to cause the 630-nm airglow intensity enhancement.

DOI： 10.1029/2002JA009594

Web of Science

Ground and satellite observations of nighttime medium-scale traveling ionospheric disturbance at midlatitude Reviewed

K Shiokawa, Y Otsuka, C Ihara, T Ogawa, FJ Rich

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 108 ( A4 )   2003.4

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[1] We have investigated a nighttime medium-scale traveling ionospheric disturbance (MSTID) observed by an airglow imager at Shigaraki (34.9degreesN, 25.4degreesMLAT), Japan, on 17 May 2001. The structure was identified in the airglow images of OI (630.0 nm and 777.4 nm) as NW-SE band structures (horizontal wavelength: 230 km) moving southwestward with a velocity of 50 m/s. Neutral wind velocity was measured simultaneously from the Doppler shift of the 630.0-nm emission by a Fabry-Perot interferometer at Shigaraki. From these parameters, we performed model calculations of MSTIDs generated by gravity waves and by an oscillating electric field. We found that for the case of gravity waves, the estimated vertical wavelength was too small to explain the observed amplitudes of airglow intensity. For the case of the electric field, we found that an electric field oscillation of similar to1.2 mV/m was sufficient to reproduce the observed airglow amplitudes. This modeled electric field was comparable to that observed by the DMSP F15 satellite as it passed over Shigaraki during our observing period on 17 May 2001. The DMSP ion drift data show that the oscillation of the polarization electric field correlated with the MSTID structure in the airglow image, suggesting that the polarization electric field plays an important role in the generation of MSTIDs.

DOI： 10.1029/2002JA009639

Web of Science

Damping of large-scale traveling ionospheric disturbances detected with GPS networks during the geomagnetic storm Reviewed

T Tsugawa, A Saito, Y Otsuka, M Yamamoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 108 ( A3 )   2003.3

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[1] Large-scale traveling ionospheric disturbances (LSTIDs) during the geomagnetic storm on 22 September 1999 were studied using total electron content (TEC) data from the GPS Earth Observation Network (GEONET) in Japan, International GPS Service (IGS), and Continuously Operating Reference Stations (CORS) in the United States. The damping rates of the LSTIDs were precisely derived in several local time sectors and were found to depend on values of the background TEC. This indicates that the dominant physical mechanism of the LSTIDs' damping is the ion-drag effect by the background ionosphere. The high-resolution TEC data from GEONET revealed that two successive LSTIDs were damped significantly as they traveled equatorward in the dawn sector. The ratio of the perturbation component of TEC to the background component (DeltaI/I-0) decreased exponentially with the damping rate of 0.89/1000 km and 0.77/1000 km. We studied also the amplitude of DeltaI/I-0 at high latitudes using IGS data and found that the damping rates of LSTIDs at high latitudes tended to be smaller than those at midlatitudes. Global TEC observations during this geomagnetic storm by the IGS and CORS networks detected that several LSTIDs propagated also equatorward in the afternoon sector and in the night sector. The LSTIDs in the afternoon sector were most damped with the damping rate of 1.04/1000 km, which corresponds to the e-folding length of 961 km. The damping rate of LSTIDs in the night sector was found to be small. The LSTIDs had a tendency to be damped rapidly in the regions where background TEC was large. This dependence of the damping rate on latitude and local time indicates that this intense damping of LSTIDs was caused mainly by the ion-drag effect that is proportional to the ion collision frequency. The relation between the damping rates and the background TEC derived from the observation are consistent with those estimated with a theoretical calculation of the gravity wave damping by the ion-drag effect. The worldwide distribution of GPS receivers enabled us to estimate the longitudinal extent of these LSTIDs. The zonal width of LSTIDs in the dawn sector was narrower than 45degrees in longitude (2,900 km) around 55degreesN and wider than 20degrees in longitude ( 1,800 km) around 40degreesN. These widths were narrower than those reported in previous studies.

DOI： 10.1029/2002JA009433

Web of Science

Statistical study of nighttime medium-scale traveling ionospheric disturbances using midlatitude airglow images Reviewed

K Shiokawa, C Ihara, Y Otsuka, T Ogawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 108 ( A1 )   2003.1

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[1] We have investigated statistical characteristics of the nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed in 630-nm airglow images at two stations, Rikubetsu (43.5degreesN, 34.8degreesMLAT) and Shigaraki (34.9degreesN, 25.4degreesMLAT), in Japan for 1998-2000 near the solar maximum period. Most of the observed MSTIDs propagate southwestward in the images. The typical wavelength, velocity, period, and amplitude are 100-300 km, 50-100 m/s, 0.5-1.5 h, and 5-15%, respectively. Seasonal variations in these parameters are not clear. The occurrence rate has a major peak (50-60%) in summer that appears similar to2 months earlier at lower latitudes and a minor peak in winter. Similar occurrence characteristics are obtained from midlatitude spread-F signatures using multipoint ionosonde data in Japan, though the coincidence of the spread-F and the MSTIDs in airglow images is only 10-15%.

DOI： 10.1029/2002JA009491

Web of Science

Optical and radio measurements of a 630-nm enhancement over Japan on September 9, 1999 Reviewed

Otsuka, Y., T. Kadota, K. Shiokawa, T. Ogawa, S. Kawamura,, S. Fukao, and S. -R. Zhang

J. Geophys. Res.   Vol. 108(A6) ( 1252 ) page： doi:10.1029/2002JA009594   2003

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Using a comprehensive data set from optical and radio instruments, we investigate a midnight brightness wave that appeared in 630-nm airglow images over Japan on the night of September 9, 1999. This may be the first such observation of the brightness wave with an all-sky imager in the East-Asian longitudinal sector. The imager at Shigaraki (35.6N, 136.1E) tracked a north-northeastward propagation of the wave with an apparent velocity of 500 m/s after midnight. Ionosonde observations at five stations in Japan showed that rapid descent of the F2 layer propagated northward beyond 35N with decreasing amplitude. Incoherent scatter observations with the MU radar at Shigaraki also revealed that the F2 peak altitude decreased from 360 km to 280 km during the event. During the F2 layer descent, the altitude profile of the electron density became sharp, enhancing the F2 peak electron density. After the F2 layer altitude reached 280 km, electron density in the F2 layer rapidly decreased because of increased neutral density at low altitude. A Fabry-Perot interferometer (FPI) at Shigaraki observed northward neutral winds of 10--70 m/s during the event. A model calculation demonstrates that the meridional winds estimated from the MU radar electron density profiles are fairly well consistent with those observed with the FPI. From these results, we conclude that the observed northward wind enhancements, probably caused by the the midnight temperature maximum, pushed down the plasma in the F2 layer to lower altitudes along the geomagnetic field to cause the
630-nm airglow intensity enhancement.

A two-channel Fabry-Perot interferometer with thermoelectric-cooled CCD detectors for neutral wind measurement in the upper atmosphere Reviewed

K Shiokawa, T Kadota, Y Otsuka, T Ogawa, T Nakamura, S Fukao

EARTH PLANETS AND SPACE   Vol. 55 ( 5 ) page： 271 - 275   2003

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We describe performance of a new two-channel Fabry-Perot interferometer (FPI) with two thermoelectric-cooled CCD detectors to measure neutral winds for both the 558-nm (mesopause region) and 630-nm (thermosphere) airglow emissions simultaneously at Shigaraki (34.8degreesN, 136.1degreesE), Japan. The employment of the thermoelectric cooling system enables us continuous automatic operation of the FPI since October 2000. The random errors of wind measurement are estimated to be similar to5-8 m/s and similar to10-50 m/s for 558 nm and 630 nm, respectively. The wind velocities obtained by the FPI (558 and 630 nm) fit well with those obtained by the collocated Middle and Upper (MU) atmosphere radar.

DOI： 10.1186/BF03351759

Web of Science

Simultaneous middle and upper atmosphere radar and ionospheric sounder observations of midlatitude E region irregularities and sporadic E layer Reviewed

T Ogawa, O Takahashi, Y Otsuka, K Nozaki, M Yamamoto, K Kita

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 107 ( A10 )   2002.10

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[1] We made middle and upper atmosphere (MU) radar observations of midlatitude E region field- aligned irregularities (FAIs) in the summer of 1999 and 2000. Sporadic E (E-s) layer was monitored with a routine ionosonde, and its altitude was measured with an FMCW sounder (FCS). In this paper we draw attention to two findings. First, we show that quasiperiodic (QP) radar echoes appearing before 0200 LT are more enhanced with increasing f(o)E(s) - f(b)E(s), which means that the FAI generation is closely related to localized density gradients within E-s, and extend from 100 to 130 km in altitude, while E-s altitudes determined from the FCS soundings are between 100 and 110 km. The latter fact suggests that existing models for the QP echo generation, which require a deep modulation of E-s altitude, are not applicable to our observational results. We propose a new working model for generating QP echoes in which polarization electric fields originated from high- density plasma clouds within E-s are mapped upward along the geomagnetic field to produce relatively weak irregularities above the E-s layer. Second, we show new findings obtained from the current observations, namely, two types of QP echoes that occur below 100 km in the morning: one is the morning QP (MQP) echoes with periods of 4- 8 min, and the other is the QP echoes with periods of similar to1 min. The latter type can be categorized as low- altitude QP echoes that were found from previous nighttime MU radar observations. Until now the MU radar QP echoes have been believed to occur above 100 km for the period from sunset to midnight. Although we do not know the generation mechanisms of the low- altitude MQP echoes, we suppose that these echoes might be caused by a weak E-s that exists below 100 km.

DOI： 10.1029/2001JA900176

Web of Science

Annual and semiannual variations of the midlatitude ionosphere under low solar activity Reviewed

S Kawamura, N Balan, Y Otsuka, S Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 107 ( A8 )   2002.8

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[1] The annual and semiannual variations of the midlatitude ionosphere under low solar activity are studied using middle and upper (MU) radar (135degreesE, 35degreesN) incoherent scatter observations and Sheffield University plasmasphere-ionosphere model (SUPIM). The variations of the daytime electron density (Ne) and electron and ion temperatures (Te and Ti) at 200-600 km altitudes measured by the radar under low solar activity (F-10.7 less than or equal to 120) are satisfactorily reproduced for the first time by incorporating the radar measured values of the magnetic meridional neutral wind velocity (U-theta) and northward perpendicular plasma drift velocity (V-perpendicular to) into SUPIM that uses mass spectrometer incoherent scatter 1986 (MSIS-86) for neutral densities and neutral temperatures. The study shows that the annual and semiannual variations of the midlatitude ionosphere during daytime at altitudes near and above the ionospheric peak under low solar activity depend more on the direct effect of the neutral wind arising through the changes in ionospheric height than on the indirect effect arising through the changes in thermospheric composition (or atomic to molecular concentration ratio); the indirect effect, however, predominates on the variations at altitudes below the ionospheric peak. The electron temperature (Te) undergoes similar but almost opposite seasonal and semiannual variations as the electron density. The ion temperature (Ti) is closer to neutral temperature than to electron temperature at altitudes up to similar to400 km and shows comparatively weak annual and semiannual variations.

DOI： 10.1029/2001JA999267

Web of Science

Geomagnetic conjugate observations of equatorial airglow depletions Reviewed

Y Otsuka, K Shiokawa, T Ogawa, P Wilkinson

GEOPHYSICAL RESEARCH LETTERS   Vol. 29 ( 15 )   2002.8

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We report for the first time large-scale equatorial F-region airglow depletions extending to low-midlatitudes in both hemispheres. The observational sites were located at low-midlatitude geomagnetic conjugate points. Clear depletions of 630.0-nm airglow intensity due to equatorial plasma bubbles were simultaneously observed with two all-sky imagers at Sata, Japan (magnetic latitude 24degreesN), and its geomagnetic conjugate point, Darwin, Australia (magnetic latitude 22degreesS), on the night of November 12, 2001. Airglow depletion regions with east-west scale sizes of 40-100 km extend poleward. The maximum apex altitude of the plasma bubbles is about 1,700 km over the geomagnetic equator. The depletions move eastward at about 100 m/s, without changing their structures. The Darwin depletion structures mapped onto the northern hemisphere along the geomagnetic field coincide closely with structures in the Sata images, even for the 40-km structure. These observations indicate that plasma depletions in the equatorial ionosphere elongate along the geomagnetic field lines.

DOI： 10.1029/2002GL015347

Web of Science

A large-scale traveling ionospheric disturbance during the magnetic storm of 15 September 1999 Reviewed

K Shiokawa, Y Otsuka, T Ogawa, N Balan, K Igarashi, AJ Ridley, DJ Knipp, A Saito, K Yumoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 107 ( A6 )   2002.6

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[1] Using a comprehensive data set and model calculations, we have investigated a prominent large-scale traveling ionospheric disturbance (LSTID) observed in Japan (similar to37degrees-16degrees MLAT) on 15 September 1999, during a recovery phase of sequential storms. The LSTID was detected at 2300-2400 LT (1400-1500 UT) as an enhancement of the 630-nm airglow intensity (50 --&gt; 350 R), a decrease in the F layer virtual height (at 2 MHz, 360 --&gt; 200 km), an enhancement of f(o)F(2) (6 --&gt; 8 MHz), and an enhancement of GPS total electron content (similar to1.0 x 10(16) m(-2)). Multipoint and imaging observations of these parameters show that the LSTID moved equatorward over Japan with a velocity of similar to400-450 m/s. From a comparison with the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) we conclude that an enhancement (250-300 m/s) of poleward neutral wind (that is propagating equatorward) caused these observational features of the LSTID at midlatitudes. To investigate generation of the LSTID by auroral energy input, we have used auroral images obtained by the Polar UVI instrument, magnetic field variations obtained at multipoint ground stations, and the empirical Joule heating rate calculated by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. Intense auroral energy input was observed at 0800-1100 UT (4-6 hours before the LSTID), probably causing equatorward neutral wind at lower latitudes. It is likely that the poleward wind pulse that caused the observed LSTID was generated associated with the cessation of this equatorward wind. The effect of Lorentz force is also discussed.

DOI： 10.1029/2001JA000245

Web of Science

*A new technique for mapping of total electron content using GPS network in Japan Reviewed

Otsuka, Y., T. Ogawa, A. Saito, T. Tsugawa, S. Fukao, and S. Miyazaki

Earth, Planets, and Space   Vol. 54   page： 63-70   2002

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Geomagnetic conjugate observations of equatorial airglow depletions Reviewed

Otsuka, Y., K. Shiokawa, T. Ogawa, and P. Wilkinson

Geophys. Res. Lett.   Vol. 29   page： 10.1029/2002GL015347   2002

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A new technique for mapping of total electron content using GPS network in Japan Reviewed

Y Otsuka, T Ogawa, A Saito, T Tsugawa, S Fukao, S Miyazaki

EARTH PLANETS AND SPACE   Vol. 54 ( 1 ) page： 63 - 70   2002

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The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15degrees x 0.15degrees in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.

DOI： 10.1186/BF03352422

Web of Science

Imaging observations of the equatorward limit of midlatitude traveling ionospheric disturbances Reviewed

K Shiokawa, Y Otsuka, MK Ejiri, Y Sahai, T Kadota, C Ihara, T Ogawa, K Igarashi, S Miyazaki, A Saito

EARTH PLANETS AND SPACE   Vol. 54 ( 1 ) page： 57 - 62   2002

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This paper reports the first attempt to observe the equatorward limit of medium-scale traveling ionospheric disturbances (TIDs) in the middle latitudes. The TIDs usually propagate southwestward in the northern hemisphere. An all-sky cooled-CCD imager measured 630-nm airglow at a southern island of Japan, Okinawa (26.9degreesN, 128.3degreesE, geomagnetic latitude (MLAT) = 17.0degrees), during the FRONT-2 campaign of August 4-15, 1999. The TIDs were detected at the mainland of Japan (similar to21degrees-36degrees MLAT) by the total electron content (TEC) observations of more than 1000 GPS receivers. In the August 4 event, the TIDs moving southwestward was seen only in the northern sky of Okinawa as a depletion band in the 630-nm airglow images. In the August 6 event, the TIDs were not seen in the 630-nm images at Okinawa, although weak TID activity was observed by the GPS network at the mainland of Japan. The TEC data also showed weakening of the TID activity below 18degrees MLAT. Based on these observations, we suggest that there is a possible limit of medium-scale TID propagation around similar to18degrees MLAT.

DOI： 10.1186/BF03352421

Web of Science

Observations and modeling of 630 nm airglow and total electron content associated with traveling ionospheric disturbances over Shigaraki, Japan Reviewed

T Ogawa, N Balan, Y Otsuka, K Shiokawa, C Ihara, T Shimomai, A Saito

EARTH PLANETS AND SPACE   Vol. 54 ( 1 ) page： 45 - 56   2002

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Southwestward-propagating medium-scale traveling ionospheric disturbances (MSTIDs) observed over Shigaraki (34.85degreesN, 136.10degreesE) in Japan on the night of May 22, 1998 are analyzed in detail. The MSTIDs were detected with a 630.0 nm (01) all-sky imager at Shigaraki and a large number of GPS (Global Positioning System) receivers distributed around Shigaraki. Each GPS receiver provided total electron content (TEC) between the GPS altitude (20,200 km) and the ground. MSTID amplitudes varied in space and time, and showed decay and enhancement during the southwestward propagation, suggesting that amplitudes of atmospheric gravity waves and the interaction process between gravity waves and F region plasma were highly variable. It is found that spatial and temporal fluctuations of the 630 nm intensity are well correlated with those of GPS-TEC except for a certain period of time. The Scheffield University Plasmasphere Ionosphere Model (SUPIM) is used to obtain theoretical relationships between the 630 nm airglow intensity and GPS-TEC and between their fluctuation amplitudes. The results indicate that the fluctuation amplitudes observed in weak airglow regions are caused by an electron density fluctuation of about +/-20% occurring around an altitude of 250 km, where the 630 nm emission rate reaches a maximum, below the F layer peak altitude. Highly enhanced 630 nm intensity and GPS-TEC within a bright airglow region are due to an electron density enhancement of about 150% occurring at altitudes below 300 km.

DOI： 10.1186/BF03352420

Web of Science

Regional ionosphere map over Japanese Islands Reviewed

J Ping, Y Kono, K Matsumoto, Y Otsuka, A Saito, C Shum, K Heki, N Kawano

EARTH PLANETS AND SPACE   Vol. 54 ( 12 ) page： E13 - E16   2002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

Based on the high time and spatial resolution total electron content (TEC) data, which is estimated from the phase and code observables obtained by using GPS (Global Positioning System) Earth Observation Network (GEONET), the TEC distribution and its time variation over Japanese Islands are scaled into 0.5degrees x 0.5degrees grid data for each 10 minutes. The TEC daily map time series are arranged in an array to show the TEC evolution. Based on the spherical harmonics expansion of global ionospheric TEC model (GIM), which is estimated from global GPS observation, the TEC maps are expanded firstly through as high as 60 degrees and orders for a spherical harmonic function as a regional ionosphere map (RIM). The evolution history of medium scale traveling ionospheric disturbances (MSTIDs), i.e. polarward intense TEC enhancement and pre-noon rapid irregular fluctuations near the geomagnetic equator, are identified and confirmed in a quiet geomagnetic period.

DOI： 10.1186/BF03352450

Web of Science

Plasmaspheric electron content in the GPS ray paths over Japan under magnetically quiet conditions at high solar activity Reviewed

N Balan, Y Otsuka, T Tsugawa, S Miyazaki, T Ogawa, K Shiokawa

EARTH PLANETS AND SPACE   Vol. 54 ( 1 ) page： 71 - 79   2002

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Vertical total electron content (GPS-TEC) data obtained from the dual-frequency GPS receiver network (GEONET) in Japan are compared with those calculated using the Sheffield University plasmasphere-ionosphere model (SUPIM). The model is also used to estimate the electron content in the plasmaspheric sections of GPS ray paths for the three seasons of high solar activity (F10.7 = 165) under magnetically quiet conditions. According to the estimates, the plasmaspheric sections of vertical GPS ray paths over Japan at altitudes above the O+ to H+ transition height and above the upper altitude (2500 km) of Faraday rotation contain up to I I and 9 TEC units (I TEC unit = 1016 electrons m(-2)) of free electrons, respectively. The free electrons present above the Faraday rotation altitude can cause propagation errors of up to 4.9 ns in time delay and 1.6 m in range at the GPS L1 (1.57542 GHz) frequency. The plasmaspheric electron content, PEC, changes appreciably with season and latitude and very little with the time of the day. However, the percentage contribution of PEC to GPS-TEC changes most significantly with the time of the day; the contribution varies from a minimum of about 12% during daytime at equinox to a maximum of about 60% at night in winter.

DOI： 10.1186/BF03352423

Web of Science

Observations of traveling ionospheric disturbances and 3-m scale irregularities in the nighttime F-region ionosphere with the MU radar and a GPS network Reviewed

A Saito, M Nishimura, M Yamamoto, S Fukao, T Tsugawa, Y Otsuka, S Miyazaki, MC Kelley

EARTH PLANETS AND SPACE   Vol. 54 ( 1 ) page： 31 - 44   2002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

The nighttime traveling ionospheric disturbances (TIDs) and the F-region 3-m scale field-aligned irregularities were simultaneously observed with the MU radar and GEONET, a GPS network, during the FRONT (F-region Radio and Optical measurement of the Nighttime TID) campaign periods in May 1998 and August 1999. The vertical profile of electron density detected by the incoherent scatter observation of the MU radar clarified that ionized atmosphere on the bottomside of the ionospheric F-region was deeply modulated by TIDs. which would cause the variations of the 630 nm band airglow luminosity. The coherent echoes from the 3-m scale field-aligned irregularities were detected also on the bottomside of the F-region in the nights when TIDs were intense in amplitude and the ionosphere was uplifted, Two-dimensional structures of the field-aligned irregularities detected by the multi-beam observation of the MU radar revealed that the 3-m scale irregularities formed band-like structures and traveled to the southwest in several nights. Their wave vector and traveling velocity were coincident with those of the nighttime TIDs that were simultaneously detected by the TEC observation of GEONET. The intense Doppler velocities of the coherent echoes indicate that the polarization electric field is generated inside the TIDs. We consider that the horizontal gradient of the electric conductivity associated by TIDs and the vertical gradient of the conductivity on the bottomside of the F-region ionosphere generates the 3-m scale irregularities through the gradient-drift instability process. The anti-correlation of the occurrence rate of the F-region field-aligned irregularities to the solar activity would be caused by the anti-correlation of the amplitude of TIDs and of the vertical gradient of the Pedersen conductivity.

DOI： 10.1186/BF03352419

Web of Science

Annual and semiannual variations of the midlatitude ionosphere under low solar activity Reviewed

S. Kawamura, N. Balan, Y. Otsuka, S. Fukao

Journal of Geophysical Research: Space Physics   Vol. 107 ( 8 )   2002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

The annual and semiannual variations of the midlatitude ionosphere under low solar activity are studied using middle and upper (MU) radar (135°E, 35°N) incoherent scatter observations and Sheffield University plasmasphere-ionosphere model (SUPIM). The variations of the daytime electron density (Ne) and electron and ion temperatures (Te and Ti) at 200-600 km altitudes measured by the radar under low solar activity (F10.7 ≤ 120) are satisfactorily reproduced for the first time by incorporating the radar measured values of the magnetic meridional neutral wind velocity (U 0) and northward perpendicular plasma drift velocity (V ⊥) into SUPIM that uses mass spectrometer incoherent scatter 1986 (MSIS-86) for neutral densities and neutral temperatures. The study shows that the annual and semiannual variations of the midlatitude ionosphere during daytime at altitudes near and above the ionospheric peak under low solar activity depend more on the direct effect of the neutral wind arising through the changes in ionospheric height than on the indirect effect arising through the changes in thermospheric composition (or atomic to molecular concentration ratio)
the indirect effect, however, predominates on the variations at altitudes below the ionospheric peak. The electron temperature (Te) undergoes similar but almost opposite seasonal and semiannual variations as the electron density. The ion temperature (Ti) is closer to neutral temperature than to electron temperature at altitudes up to ∼400 km and shows comparatively weak annual and semiannual variations. Copyright 2002 by the American Geophysical Union.

DOI： 10.1029/2001JA000267

Scopus

Imaging observations of midlatitude ionospheric disturbances during the geomagnetic storm of February 12, 2000 Reviewed

Y Sahai, K Shiokawa, Y Otsuka, C Ihara, T Ogawa, K Igarashi, S Miyazaki, A Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 106 ( A11 ) page： 24481 - 24492   2001.11

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Regular all-sky imaging observations of the F region OI 630 nm nightglow emission are carried out at Rikubetsu (43.5 degreesN) and Shigaraki (34.8 degreesN), Japan since 1998. In this paper, we present observations of the OI 630 nm emission images from these two sites with a cluster of other observations during the geomagnetic storm of February 12, 2000. The airglow observations were obtained between 1500 UT and 2000 UT (0000-0500 LT) on this night. Images from both Rikubetsu and Shigaraki show the presence of mesoscale-enhanced airglow bands moving slowly (similar to 20-30 m s(-1)) to the southwest direction. The mesoscale band structures were also detected by an extensive network of GPS signal observations over Japan. The airglow observations at Shigaraki also show intense OI 630 nm emission in the south with several small-scale intensity depletion structures. We suggest that these small-scale intensity depletion structures (scale size 30-50 km) are caused by possible nonlinear interaction between the mesoscale TIDs and enhanced regions of the equatorial ionospheric anomaly. The ionosonde observations from Wakkanai (45.4 degreesN) and Kokubunji (35.7 degreesN) show spread F signatures between 1130 and 2100 UT and 1215 and 1715 UT, respectively, whereas no spread F was observed at Yamagawa (31.2 degreesN) and Okinawa (26.3 degreesN). The phase fluctuations of GPS signal were observed at latitudes higher than 30 degreesN for 1400-1900 UT. These observations suggest enhanced storm time ionospheric disturbances were confined in the midlatitudes.

Web of Science

Three-channel imaging Fabry-Perot interferometer for measurement of mid-latitude airglow Reviewed

K Shiokawa, T Kadota, MK Ejiri, Y Otsuka, Y Katoh, M Satoh, T Ogawa

APPLIED OPTICS   Vol. 40 ( 24 ) page： 4286 - 4296   2001.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

We have developed a three-channel imaging Fabry-Perot interferometer with which to measure atmospheric wind and temperature in the mesosphere and thermosphere through nocturnal airglow emissions. The interferometer measures two-dimensional wind and temperature for wavelengths of 630.0 nm (OI, altitude, 200-300 km), 557.7 nm (OI, 96 km), and 839.9 mn (OH, 86 km) simultaneously with a time resolution of 20 min, using three cooled CCD detectors with liquid-N-2 Dewars. Because we found that the CCD sensor moves as a result of changes in the level of liquid N-2 in the Dewars, the cooling system has been replaced by thermoelectric coolers. The fringe drift that is due to changes in temperature of the etalon is monitored with a frequency-stabilized He-Ne laser. We also describe a data-reduction scheme for calculating wind and temperature from the observed fringes. The system is fully automated and has been in operation since June 1999 at the Shigaraki Observatory (34.8N, 136.1E), Shiga, Japan. (C) 2001 Optical Society of America.

DOI： 10.1364/AO.40.004286

Web of Science

PubMed

Traveling ionospheric disturbances detected in the FRONT campaign Reviewed

A Saito, M Nishimura, M Yamamoto, S Fukao, M Kubota, K Shiokawa, Y Otsuka, T Tsugawa, T Ogawa, M Ishii, T Sakanoi, S Miyazaki

GEOPHYSICAL RESEARCH LETTERS   Vol. 28 ( 4 ) page： 689 - 692   2001.2

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The F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign was conducted to clarify the non-classical features of traveling ionospheric disturbances (TIDs) at mid-latitudes in May, 1998 and August, 1999. A cluster of all-sky CCD cameras and a GPS receiver network observed a wide area of the ionosphere over Japan to detect the spatial structure and temporal evolution of TIDs. The propagation direction of the nighttime TID detected during the FRONT campaign periods is restricted to the southwest. The time evolution of their amplitude indicates that the TID structure is intensified as it travels from high-latitudes to low-latitudes. The significant coincidence between the structures of 630 nm band airglow and total electron content indicates that the perturbations take place in the bottomside of the ionospheric F region. Coherent echoes from the field-aligned irregularities were observed by the MU radar in the nights when the TID activity was high.

DOI： 10.1029/2000GL011884

Web of Science

Multi-point observation of short-period mesospheric gravity waves over Japan during the FRONT campaign Reviewed

K Shiokawa, MK Ejiri, Y Otsuka, T Ogawa, M Kubota, K Igarashi, A Saito, T Nakamura

GEOPHYSICAL RESEARCH LETTERS   Vol. 27 ( 24 ) page： 4057 - 4060   2000.12

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Simultaneous observations of short-period gravity waves were carried out using three all-sky cooled-CCD imagers of OH airglow at Moshiri (northern edge of Japan) and at Shigaraki and Bisei (middle part of Japan) during the FRONT campaign of May 1998. These stations were separated by horizontal distances of 250 - 1300 km, and they offered a unique opportunity to investigate the spatial extent of gravity waves in the mesopause region. The images from Shigaraki and Bisei showed short-period gravity waves (horizontal wavelength: 20-40 km, phase front: similar to east-west aligned) moving northward (phase velocity: 20-40 m/s) on May 19, 21 and 22. At Moshiri, similar east-west waves moving northward were observed on May 19 and 21, indicating a broad horizontal extent of the gravity waves. Waves at Moshiri were not evident on May 22, however. The horizontal wind measured by the MF radar at Wakkanai (near Moshiri) showed height profiles that were similar to those at Yamagawa (southern edge of Japan) on May 10 and 21 but quite dissimilar oil May 22. On the basis of these data, we suggest that short-period gravity waves in the mesopause region carl have a I,road spatial extent of more than 1000 km. The generation and propagation of internal gravity waves are discussed as a cause of the spatial extent of thc waves.

DOI： 10.1029/2000GL011917

Web of Science

Traveling ionospheric disturbances observed in the OI 630-nm nightglow images over Japan by using a multipoint imager network during the FRONT campaign Reviewed

M Kubota, K Shiokawa, MK Ejiri, Y Otsuka, T Ogawa, T Sakanoi, H Fukunishi, M Yamamoto, S Fukao, A Saito

GEOPHYSICAL RESEARCH LETTERS   Vol. 27 ( 24 ) page： 4037 - 4040   2000.12

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Pilot observations using a network of five all-sky imagers (ASIs) were conducted during the new moon period of May 19-22, 1998 as part of the F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign. The network observation enabled us to track propagation of medium-scale traveling ionospheric disturbances (TIDs) in the OI 630-nm nightglow over a distance of more than 2500 km. The TIDs were observed every night during the campaign period, but occurrence was limited from evening to midnight. They have horizontal wavelengths of 200-600 km, travel a horizontal distance of more than 1000 km, and last for more than three hours. In every case, the TIDs moved southwestward with a velocity of 83-137 m/s. Using dual-site TID images, the altitude of the TID structures in the 630-nm nightglow was calculated to be similar to 260 km, which corresponds to the bottom side of the mid-latitude ionospheric F layer.

DOI： 10.1029/2000GL011858

Web of Science

Comprehensive imaging observations of midlatitude ionospheric disturbances during storm time substorms Reviewed

K Shiokawa, Y Otsuka, T Ogawa, K Igarashi, S Miyazaki, FJ Rich, A Saito, Y Yumoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 105 ( A12 ) page： 27067 - 27080   2000.12

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This paper reports comprehensive imaging observations of 630-nm airglow enhancements and related phenomena in the nighttime F region over Japan. We found two events during which the airglow intensity increases up to 300 R at the lower-latitude side of Japan associated with storm time substorms of August 26; 1998, and February 18, 1999. Total electron content (TEC) observations by more than 900 GPS receivers over Japan show a clear TEC enhancement of similar to 10-40 Ya at latitudes below 40 degrees, which coincides with the airglow enhancements and the substorms. On the other hand, 2-MHz virtual heights obtained by bottomside sounding at four ionosonde stations show equatorward moving large-scale structures! which do not coincide with the air glow enhancements. Vertical ion drift data at an altitude of similar to 800 km obtained by the Defense Meteorological Satellite Program satellites show upward ion motion in midlatitudes just. after the substorm onset for the February event. These observations suggest, that the midlatitude airglow and TEC enhancements are caused by upward electron motion (fountain) in the low latitudes due to penetration of eastward electric field associated with the storm time substorms.

DOI： 10.1029/2000JA900099

Web of Science

A study of the forenoon ionospheric F-2 layer behavior over the middle and upper atmospheric radar Reviewed

Zhang, SR, WL Oliver, S Fukao, Y Otsuka

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 105 ( A7 ) page： 15823 - 15833   2000.7

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The middle and upper atmosphere (MU) radar incoherent Scatter observations of electron density N-e profiles for 1986-1995 at Shigaraki (34.85 degrees N, 136.10 degrees E) show a forenoon density depression (bite-out) in summer bo th on individual days and in statistical averages. The depression takes place generally between 0700 and 1200 LT, above the F-2 peak for low solar activity and around the F-2 peak for high solar activity. The maximum electron density NmF2 shows, in both percentage and time duration, a greater depression for low solar activity than for high solar activity. With increasing altitude, the forenoon N-e depression develops earlier for low solar activity. In combination with the MU radar observations of ion drifts and plasma; temperatures, our modeling indicates the following: (1) Transport processes such as the enhancement of electron temperature T-e, the poleward neutral wind, and the reduced upper boundary ionization density help to reduce N-e and result in a negative partial derivative N-e/partial derivative t in the morning hours in summer. Among those processes the morning enhancement of T-e is found to be most important for the presence of the forenoon bite-out, since it greatly increases the O+ diffusion such that ionization outflow is significantly intensified. (2) The forenoon bite-out occurs only in summer, not in other seasons, because the smaller atmospheric composition ratio [O]/[N-2] in summer leads to a smaller daytime background N-e and thus to an enhanced T-e, and the after-sunrise photochemical contributions (mainly the O+ production) are so small that partial derivative Ne/partial derivative t also tends to be smaller. These two conditions give prominence to the transport contributions. (3) The solar activity dependences of the forenoon bite-out can be explained basically in terms of the low-electron-density background. (4) The height of formation of the forenoon bite-out is also related to the sunrise effect on photoionization.

DOI： 10.1029/2000JA000007

Web of Science

A climatology of middle and upper atmosphere radar observations of thermospheric winds Reviewed

S Kawamura, Y Otsuka, Zhang, SR, S Fukao, WL Oliver

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 105 ( A6 ) page： 12777 - 12788   2000.6

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Shigaraki middle and upper atmosphere (MU) radar observations of horizontal thermospheric winds in the magnetic meridian plane over the period September 1986 to September 1996 are reported as climatological averages in the form of time-of-day variations for several combinations of seasonal and solar activity conditions and are compared with winds predicted by the horizontal wind model (HWM) anti with winds measured at Saint Santin and Millstone Hill. The dominant feature of the MU wind behavior is its mean diurnal variation of northward flow by day and southward flow by night, with the nighttime wind smoothly approaching and receding from a midnight maximum, while the daytime wind tends to show two peaks, a strong. one in the early daylight hours and a weak one in the afternoon-evening. HWM shows the same unimodal nighttime and bimodal daytime behavior, but the HWM pattern is shifted about 2 hours later in time. The amplitude of the diurnal harmonic decreases from 78 m/s at solar minimum to 45 m/s at solar maximum, while: HWM shows a corresponding increase from 53 to 62 m/s. The diurnal amplitude is remarkably stable with season but is superposed on a steady willd of 41 m/s southward in summer, 15 m/s northward in winter, and midway between these limits at the equinoxes. HWM shows a symmetric pattern of 30 m/s southward in summer and 30 m/s northward ill winter. Ion drag appears to be the main regulator of wind speed, and the seasonal wind patterns have a profound effect on the seasonal behavior of the ionosphere.

Web of Science

The IRI's B parameters measured by the MU radar Reviewed

Zhang, SR, S Fukao, XY Huang, Y Otsuka

LOWER IONOSPHERE: MEASUREMENTS AND MODELS   Vol. 25 ( 1 ) page： 101 - 104   2000

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON PRESS LTD  

The IS measurements of power profile by the MU radar at Shigaraki (34.9 degrees N, 136.1 degrees E) are analyzed to give IRI's B-parameters, B0 B1 for 1986-1991. Empirical formulas are obtained to represent their variations as functions of local time and season at high solar activity. They are compared also with ionosonde ones for Wuchang (30.5 degrees N, 114.4 degrees E). (C) 1999 COSPAR. Published by Elsevier Science Ltd.

Web of Science

Annual variations of the ionosphere: A review based on MU radar observations Reviewed

N Balan, Y Otsuka, S Fukao, MA Abdu, GJ Bailey

LOWER IONOSPHERE: MEASUREMENTS AND MODELS   Vol. 25 ( 1 ) page： 153 - 162   2000

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON PRESS LTD  

A review of the annual variations of the ionosphere, which focuses on the physical mechanisms causing the well-known seasonal anomaly and equinoctial asymmetry, is presented. In this review, the electron density (Nc), electron and ion temperatures (Te and Ti). and field-parallel and field-perpendicular plasma velocities (V-parallel to and V-perpendicular to), measured by the MU radar in the 180-600 km altitude range during 1986-1994, are analysed to study the altitude dependence of the seasonal anomaly and equinoctial asymmetry. The meridional component of the thermospheric neutral wind velocity (U-theta) derived from V-parallel to and neutral densities obtained from MSIS-86 are used to investigate the relative importance of the chemical and dynamical processes causing the anomaly and asymmetry. The review concludes that, although the anomaly and asymmetry involve chemical and dynamical processes, the dynamical processes (mainly through the neutral wind) predominate in the asymmetry while the chemical processes predominate in the anomaly. (C) 1999 COSPAR. Published by Elsevier Science Ltd.

Web of Science

The height of the maximum ionospheric electron density over the MU radar Reviewed

Zhang, SR, S Fukao, WL Oliver, Y Otsuka

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS   Vol. 61 ( 18 ) page： 1367 - 1383   1999.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Ionospheric F-2-layer peak height h(m)F(2) variations, as measured over 1986-1995 by the MU radar (34.85 degrees N, 136.1 degrees E) and as calculated with a theoretical model, are discussed. The diurnal variations of the measured peak height for different seasons and levels of solar activity are compared with those estimated from ionosonde M3000F(2) and IRI predictions. Also given are the measured ion drift velocities and meridional neutral winds needed to understand the dynamic behavior of the F-2-layer. It is found that: (1) h(m)F(2) is generally higher during periods of the solar maximum than during periods of the solar minimum, and higher in summer than in winter; (2) for the solar maximum, h(m)F(2) drops markedly in the morning and in the afternoon, while, for the solar minimum, the h(m)F(2) minimum occurs in the morning during summer and usually in the afternoon during winter. In general, the measured h(m)F(2) is well reproduced by our model when we use the observed drift velocities and plasma temperatures as inputs. Our modeling study shows that the neutral wind contributes strongly to the diurnal variation of h(m)F(2) in winter by lowering the ionization layer by day, particularly for the solar maximum; it also helps to enlarge the daynight difference of h(m)F(2) in summer. The northward electromagnetic drifts that usually cancel the neutral wind effect have only a minor effect for the location of the MU radar. Other features of the observed h(m)F(2) variations, e.g., the solar maximum-minimum difference, the summer-winter difference, and the morning and afternoon drops, are explained by the basic processes of O+ production, loss and diffusion, as influenced by the atomic oxygen concentration and neutral and plasma temperatures. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S1364-6826(99)00088-7

Web of Science

The height of the maximum ionospheric electron density over the MU radar Reviewed

Journal of Atmospheric and Solar-Terrestrial Physics   Vol. 61   page： 1367-83   1999

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MU radar observations of H+ ions in the topside ionosphere Reviewed

S Kawamura, Y Otsuka, WL Oliver, N Balan, S Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 103 ( A9 ) page： 20697 - 20704   1998.9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report the detection of H+ ions in the topside ionosphere above the middle and upper atmosphere (MU) radar in Japan, the first such detection by a mesosphere-stratosphere-troposphere radar extended to perform as an incoherent scatter radar. Owing to the very limited signal-to-noise ratios of similar to 1% achievable in the topside ionosphere with this radar, long integration times of 45 min and statistical analysis of 10 years of data are employed to yield useful results. This sensitivity restriction limits the top altitude of observation to only similar to 400 km during solar-minimum winter but up to similar to 650 km during solar-maximum summer. These measurements are possible only at night, when we may assume that the electrons and ions have the same temperature. While the data scatter is large, climatological averages constructed for various solar-activity, seasonal, and magnetic activity cases show less than similar to 5% H+ at most under conditions of detectability, although it is exactly those conditions of undetectability which are expected to correspond to the highest proportions of H+. At solar maximum we see no detectable amounts of H+ up to 650 km altitude, except perhaps for a faint amount in winter. At solar minimum the data are good up to similar to 550 km during summer and equinox but to only similar to 400 km during winter. The H+ profiles for solar-minimum summer and equinox closely resemble each other and reach similar to 5% at 550 km altitude. We see no clearly discernible H+ in winter up to our 400 km altitude limit of detectability; we expect that there is substantial HS present above 400 km, but the total ion density is so low that we cannot measure it. We see no detectable change in H+ between magnetically quiet and disturbed periods. The H+ proportions seen above the MU radar are much smaller than those seen above Arecibo, which has a similar magnetic latitude but much lower geographic latitude, and are much more comparable to those amounts seen by the higher-latitude radars at Saint Santin and Millstone Hill. Comparisons with models suggest that at these low altitudes, H+ may well be near its charge-exchange-equilibrium value.

DOI： 10.1029/98JA00898

Web of Science

Plasma temperature variations in the ionosphere over the middle and upper atmosphere radar Reviewed

Y Otsuka, S Kawamura, N Balan, S Fukao, GJ Bailey

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 103 ( A9 ) page： 20705 - 20713   1998.9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The temperature variations in the ionosphere over the middle and upper atmosphere radar at Shigaraki (34.85 degrees N, 136.10 degrees E, magnetic latitude 25 degrees N) in Japan are studied using the electron and ion temperature (T-e and T-i, respectively) data measured by the radar during nearly a full solar cycle (1986-1995). A comprehensive picture of the diurnal, seasonal, and solar activity variations of T-e and T-i is presented for the altitude range 200-550 km. The temperatures T-e and T-i are found to have similar diurnal and altitude variations and different seasonal and solar activity dependence. With season; while daytime T-e is highest in summer and lowest in equinox, daytime T-i is highest in equinox and lowest in summer. With solar activity, while daytime T-e decreases, the corresponding T-i increases. The diurnal variation of T-e is characterized by morning and evening peaks. The occurrence and strength of these peaks are found to depend on altitude, season, and solar activity. The peaks arise basically from the photoelectron heating of the morning and evening electron gas. However, neutral winds play a dominant role in the appearance of the peaks. A poleward wind, which reduces the electron density to a low value before sunset, is an essential requirement, especially for the evening peak. The mechanisms causing the morning and evening peaks in T-e are illustrated through model calculations using the Sheffield University plasmasphere-ionosphere model.

DOI： 10.1029/98JA01748

Web of Science

MST radar measurement of ionospheric F region winds: The "layer wind" technique Reviewed

WL Oliver, Y Otsuka, S Fukao

RADIO SCIENCE   Vol. 33 ( 4 ) page： 941 - 948   1998.7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We develop an improved application of the mesosphere-stratospheretroposphere (MST) radar technique for the measurement of winds at ionospheric F region altitudes. While the experiment is basically one of incoherent scatter (IS), which yields very weak signals at the usual operating frequencies of MST radars, we develop an analysis technique that allows the signals to be integrated over the entire F region of the ionosphere to gain sufficient statistical accuracy to compute the "layer wind" accurately. This method is contrasted with the normal IS method of determining winds at discrete heights. The method is applied to data collected with the middle and upper atmosphere radar in Japan, though it is applicable to any MST radar having a comparable or even somewhat smaller power-aperture product and may also be adapted for improved results from mainline IS radars. Using this method, we are able to discuss confidently the geophysical behavior of the wind on individual days, whereas previously we were reluctant to discuss anything but climatological features from data averages.

DOI： 10.1029/98RS00658

Web of Science

Equinoctial asymmetries in the ionosphere and thermosphere observed by the MU radar Reviewed

N Balan, Y Otsuka, GJ Bailey, S Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 103 ( A5 ) page： 9481 - 9495   1998.5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Annual variations of the ionosphere and thermosphere studied with the middle and upper atmosphere (MU) radar during the solar maximum period 1988-1992 show that the well-known seasonal anomaly in the electron density Ne exists only during daytime and at altitudes near the ionospheric peak and below. The observations also reveal the existence of equinoctial asymmetries in the ionosphere and thermosphere, with the asymmetry in the ionosphere changing its character with altitude during daytime. In the bottomside ionosphere the electron density Ne is slightly greater in September equinox than in March equinox. At higher altitudes the asymmetry reverses and becomes strong; the values of Ne in March equinox exceed those in September equinox by up to 100%. The electron temperature Te exhibits equinoctial asymmetries almost opposite those in Ne. The ion temperature Ti shows a weak asymmetry, in phase with the asymmetry in Ne. The field-aligned and field-perpendicular plasma velocities V-parallel to and V-perpendicular to are also different in the two equinoxes. In the thermosphere the neutral wind and composition show consistent equinoctial asymmetries. The meridional component of the daytime poleward wind velocity (U-theta) derived from the field-parallel plasma velocity is weaker in March equinox than in September equinox by up to 20 m s(-1), and the values of the daytime [O]/[N-2] ratio obtained from MSIS-86 are larger in September equinox than in March equinox by about 20%. Model calculations carried out by incorporating the measured V-perpendicular to and U-theta into the Sheffield University plasmasphere-ionosphere model that uses MSIS-86 for neutral atmosphere show that the equinoctial asymmetries in the ionosphere arise mainly from the corresponding asymmetries in the thermosphere, with major contributions from neutral winds and minor contributions from composition.

DOI： 10.1029/97JA03137

Web of Science

Plasma temperature variations in the ionosphereover the middle and upper atmosphere radar Reviewed

Journal of Geophysical Research   Vol. 103   page： 20,705-20,713   1998

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Equinoctial asymmetries in the ionosphere and thermosphere observed by the MU radar Reviewed

Journal of Geophysical Research   Vol. 103   page： 9,481-9,495   1998

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MU radar observations of H+ ions in the topside ionosphere Reviewed

Journal of Geophysical Research   Vol. 103   page： 20,697-20,704   1998

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MST radar measurement of ionospheric F region winds:The "layer-wind" technique Reviewed

Radio Science   Vol. 33   page： 941-948   1998

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New aspects in the annual variation of the ionosphere observed by the MU radar Reviewed

N Balan, Y Otsuka, S Fukao

GEOPHYSICAL RESEARCH LETTERS   Vol. 24 ( 18 ) page： 2287 - 2290   1997.9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Annual variations of the daytime ionosphere studied with the MU radar during a full solar cycle show equinoctial asymmetry in density, temperature, and plasma velocity. The electron density (Ne) in the bottomside ionosphere is slightly greater in September equinox compared to March equinox, which arises from the asymmetry in thermospheric composition. At higher altitudes, the asymmetry in Ne reverses and becomes strong; the values of Ne in March equinox exceed those in September equinox by up to 150%. The electron temperature (Te) shows an asymmetry, which is opposite to the asymmetry in Ne; the values of Te in the topside in March equinox are less than those in September equinox by up to 300K. The asymmetry in ion temperature (Ti) is weak (about 100K) and in phase with the asymmetry in Ne. The northward perpendicular plasma velocity (V-perpendicular to) is slightly greater in March equinox compared to September equinox. The meridional component of the thermospheric neutral wind velocity (U-theta), derived from the field-parallel plasma velocity V-parallel to, is up to 50% lower in March equinox compared to September equinox. The asymmetry in neutral winds accounts for the asymmetry in ionospheric density and temperature in the topside, with minor contribution from the asymmetry in plasma drifts.

DOI： 10.1029/97GL02184

Web of Science

A climatology of F region gravity wave propagation over the middle and upper atmosphere radar Reviewed

WL Oliver, Y Otsuka, M Sato, T Takami, S Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS   Vol. 102 ( A7 ) page： 14499 - 14512   1997.7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere radar, we have been able to track the passage of gravity waves and measure their propagation characteristics. Here we develop a climatology of wave propagation based on the observation of 58 daytime experiments conducted during 1986-1994. The thermosphere seems to be continuously swept by waves detectable by an incoherent scatter radar. These waves generally come for hours on end from a,consistent or slowly varying direction, which can be any direction on a given day. Statistically, the waves show a moderate preference for southward travel, with this preference being reduced or shifted to southeastward travel during disturbed times. On average, the horizontal phase trace speed remains near 240 m/s for all periods inspected (40-130 min). This speed matches the behavior expected for lossless waves with 150-200 km vertical wavelength. We find small variability in this relation for different times of day, seasons, solar and magnetic conditions, and directions of wave travel, though waves on disturbed days seem to travel moderately faster on solar minimum mornings.

DOI： 10.1029/97JA00491

Web of Science

A climatology of F region gravity wave propagation over the middle and upper atmosphere radar Reviewed

Journal of Geophysical Research   Vol. 102   page： 14,499-14,512   1997

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Language：English   Publishing type：Research paper (scientific journal)  

New aspects in the annual variation of the ionosphere observed by the MU radar Reviewed

Geophysical Research Letters   Vol. 24   page： 2,287-2,290   1997

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Language：English   Publishing type：Research paper (scientific journal)  

Middle and upper atmosphere radar observations of the dispersion relation for ionospheric gravity waves Reviewed

Journal of Geophysical Research   Vol. 100   page： 23,763-23,768   1995

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Language：English   Publishing type：Research paper (scientific journal)