Faculty Profiles - OTSUKA, Yuichi

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 Open Access

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Publishing type：Research paper (scientific journal)

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

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

Toshihiko Iyemori, Michi Nishioka, Yuichi Otsuka, Atsuki Shinbori

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

Publishing type：Research paper (scientific journal)

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

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

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

Publishing type：Research paper (scientific journal)

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

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

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

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

Publishing type：Research paper (scientific journal)

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

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

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

EARTH PLANETS AND SPACE Vol. 74 ( 1 ) 2022.4

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

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

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 Open Access

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

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

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

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