Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

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

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

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

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 >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

ISS-IMAPによる大気光波状構造の観測 Open Access

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Statistical study of auroral fragmentation into patches Reviewed Open Access

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

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

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

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

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

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

Prayitno Abadi, Yuichi Otsuka, Takuya Tsugawa

EARTH PLANETS AND SPACE Vol. 67 2015.5

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

EARTH PLANETS AND SPACE Vol. 66 2014.12

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

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

CME front and severe space weather Reviewed Open Access

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

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

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) < -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

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

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

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

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

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