Faculty Profiles - OTSUKA, Yuichi

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

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

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

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

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

Otsuka Yuichi

PROGRESS IN EARTH AND PLANETARY SCIENCE Vol. 5 2018.9

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

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

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

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

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

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

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

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 Open Access

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

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

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

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

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

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