Faculty Profiles - OTSUKA, Yuichi

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

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

Otsuka Yuichi, Seki Kazuhiro, Sorella Sandro, Yunoki Seiji

PHYSICAL REVIEW B Vol. 98 ( 3 ) 2018.7

Equatorial plasma bubble seeding by MSTIDs in the ionosphere Reviewed Open Access

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

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

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

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

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

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 Open Access

Mamoru Yamamoto, Yuichi Otsuka, Hidekatsu Jin, Yasunobu Miyoshi

PROGRESS IN EARTH AND PLANETARY SCIENCE Vol. 5 2018.5

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

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

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

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

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

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

PROGRESS IN EARTH AND PLANETARY SCIENCE Vol. 5 2018.3

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 Open Access

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

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

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

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