Faculty Profiles - OTSUKA, Yuichi

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

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

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

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

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

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

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

Satoru Katsuda, Hiroyuki Shinagawa, Hitoshi Fujiwara, Hidekatsu Jin, Yasunobu Miyoshi, Yoshizumi Miyoshi, Yuko Motizuki, Motoki Nakajima, Kazuhiro Nakazawa, Kumiko K. Nobukawa, Yuichi Otsuka, Atsushi Shinbori, Takuya Sori, Chihiro Tao, Makoto S. Tashiro, Yuuki Wada, Takaya Yamawaki

Geophysical Research Letters Vol. 51 ( 20 ) 2024.10

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 Open Access

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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