Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

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Characteristics of Seasonal Variation and Solar Activity Dependence of the Geomagnetic Solar Quiet Daily Variation Reviewed

Atsuki Shinbori, Yukinobu Koyama, Masahito Nosé, Tomoaki Hori, Yuichi Otsuka

Journal of Geophysical Research: Space Physics Vol. 122 ( 10 ) page： 10796 - 10810 2017.10

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

Characteristics of seasonal variation and solar activity dependence of the X and Y components of the geomagnetic solar quiet (Sq) daily variation at Memambetsu in midlatitudes and Guam near the equator have been investigated using long-term geomagnetic field data with 1 h time resolution from 1957 to 2016. The monthly mean Sq variation in the X and Y components (Sq-X and Sq-Y) shows a clear seasonal variation and solar activity dependence. The amplitude of seasonal variation increases significantly during high solar activities and is proportional to the solar F10.7 index. The pattern of the seasonal variation is quite different between Sq-X and Sq-Y. The result of the correlation analysis between the solar F10.7 index and the Sq-X and Sq-Y shows an almost linear relationship, but the slope of the linear fitted line varies as a function of local time and month. This implies that the sensitivity of Sq-X and Sq-Y to the solar activity is different for different local times and seasons. The pattern of the local time and seasonal variations of Sq-Y at Guam shows good agreement with that of a magnetic field produced by interhemispheric field-aligned currents (FACs), which flow from the summer to winter hemispheres in the dawn and dusk sectors and from the winter to summer hemispheres in the prenoon to afternoon sectors. The direction of the interhemispheric FAC in the dusk sector is opposite to the concept of Fukushima's model.

DOI： 10.1002/2017JA024342

Geomagnetically conjugate observations of ionospheric and thermospheric variations accompanied by a midnight brightness wave at low latitudes Reviewed Open Access

D. Fukushima, K. Shiokawa, Y. Otsuka, M. Kubota, T. Yokoyama, M. Nishioka, S. Komonjinda, C. Y. Yatini

EARTH PLANETS AND SPACE Vol. 69 2017.8

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

We conducted geomagnetically conjugate observations of 630-nm airglow for a midnight brightness wave (MBW) at Kototabang, Indonesia [geomagnetic latitude (MLAT): 10.0 degrees S], and Chiang Mai, Thailand (MLAT: 8.9 degrees N), which are geomagnetically conjugate points at low latitudes. An airglow enhancement that was considered to be an MBW was observed in OI (630-nm) airglow images at Kototabang around local midnight from 2240 to 2430 LT on February 7, 2011. This MBW propagated south-southwestward, which is geomagnetically poleward, at a velocity of 290 m/s. However, a similar wave was not observed in the 630-nm airglow images at Chiang Mai. This is the first evidence of an MBW that does not have geomagnetic conjugacy, which also implies generation of MBW only in one side of the hemisphere from the equator. We simultaneously observed thermospheric neutral winds observed by a co-located Fabry-Perot interferometer at Kototabang. The observed meridional winds turned from northward (geomagnetically equatorward) to southward (geomagnetically poleward) just before the wave was observed. This indicates that the observed MBW was generated by the poleward winds which push ionospheric plasma down along geomagnetic field lines, thereby increasing the 630-nm airglow intensity. The bottomside ionospheric heights observed by ionosondes rapidly decreased at Kototabang and slightly increased at Chiang Mai. We suggest that the polarization electric field inside the observed MBW is projected to the northern hemisphere, causing the small height increase observed at Chiang Mai. This implies that electromagnetic coupling between hemispheres can occur even though the original disturbance is caused purely by the neutral wind.

DOI： 10.1186/s40623-017-0698-z

Daytime zonal drifts in the ionospheric 150 km and E regions estimated using EAR observations Reviewed

P. Pavan Chaitanya, A. K. Patra, Y. Otsuka, T. Yokoyama, M. Yamamoto, R. A. Stoneback, R. A. Heelis

Journal of Geophysical Research: Space Physics Vol. 122 ( 8 ) page： 9045 - 9055 2017.8

Sixteen year variation of horizontal phase velocity and propagation direction of mesospheric and thermospheric waves in airglow images at Shigaraki, Japan Reviewed Open Access

D. Takeo, K. Shiokawa, H. Fujinami, Y. Otsuka, T. S. Matsuda, M. K. Ejiri, T. Nakamura, M. Yamamoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 122 ( 8 ) page： 8770 - 8780 2017.8

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

We analyzed the horizontal phase velocity of gravity waves and medium-scale traveling ionospheric disturbances (MSTIDs) by using the three-dimensional fast Fourier transform method developed by Matsuda et al. (2014) for 557.7 nm (altitude: 90-100 km) and 630.0 nm (altitude: 200- 300 km) airglow images obtained at Shigaraki MU Observatory (34.8 degrees N, 136.1 degrees E, dip angle: 49 degrees) over similar to 16 years from 16 March 1999 to 20 February 2015. The analysis of 557.7 nm airglow images shows clear seasonal variation of the propagation direction of gravity waves in the mesopause region. In spring, summer, fall, and winter, the peak directions are northeastward, northeastward, northwestward, and southwestward, respectively. The difference in east-west propagation direction between summer and winter is probably caused by the wind filtering effect due to the zonal mesospheric jet. Comparison with tropospheric reanalysis data shows that the difference in north-south propagation direction between summer and winter is caused by differences in the latitudinal location of wave sources due to convective activity in the troposphere relative to Shigaraki. The analysis of 630.0 nm airglow images shows that the propagation direction of MSTIDs is mainly southwestward with a minor northeastward component throughout the 16 years. A clear negative correlation is seen between the yearly power spectral density of MSTIDs and F-10.7 solar flux. This negative correlation with solar activity may be explained by the linear growth rate of the Perkins instability and secondary wave generation of gravity waves in the thermosphere.

DOI： 10.1002/2017JA023919

Equinoctial asymmetry in the zonal distribution of scintillation as observed by GPS receivers in Indonesia Reviewed Open Access

P. Abadi, Y. Otsuka, K. Shiokawa, A. Husin, Huixin Liu, S. Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 122 ( 8 ) page： 8947 - 8958 2017.8

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

We investigate the azimuthal distribution of amplitude scintillation observed by Global Positioning System (GPS) ground receivers at Pontianak (0.0 degrees S, 109.3 degrees E; magnetic latitude: 9.8 degrees S) and Bandung (6.9 degrees S, 107.6 degrees E; magnetic latitude: 16.7 degrees S) in Indonesia in March and September from 2011 to 2015. The scintillation is found to occur more to the west than to the east in March at both stations, whereas no such zonal difference is found in September. We also analyze the zonal scintillation drift as estimated using three closely spaced single-frequency GPS receivers at Kototabang (0.2 degrees S, 100.3 degrees E; magnetic latitude: 9.9 degrees S) in Indonesia during 2003-2015 and the zonal thermospheric neutral wind as measured by the CHAMP satellite at longitudes of 90 degrees-120 degrees E during 2001-2008. We find that the velocities of both the zonal scintillation drift and the neutral wind decrease with increasing latitudes. Interestingly, the latitudinal gradients of both the zonal scintillation drift and the neutral wind are steeper in March than in September. These steeper March gradients may be responsible for the increased westward altitudinal and latitudinal tilting of plasma bubbles in March. This equinoctial asymmetry could be responsible for the observed westward bias in scintillation in March, because the scintillation is more likely to occur when radio waves pass through longer lengths of plasma irregularities in the plasma bubbles.

DOI： 10.1002/2017JA024146

Sixteen-year variation of horizontal phase velocity and propagation direction of mesospheric and thermospheric waves in airglow images at Shigaraki Reviewed Open Access

Takeo, D, K. Shiokawa, H. Fujinami, Y. Otsuka, T. S. Matsuda, M. K. Ejiri, T. Nakamura, M. Yamamoto

Journal of Geophysical Research -Atmospheres Vol. 122 page： 8770-8780 2017.8

Coordinated observations of postmidnight irregularities and thermospheric neutral winds and temperatures at low latitudes Reviewed Open Access

Tam Dao, Yuichi Otsuka, Kazuo Shiokawa, Michi Nishioka, Mamoru Yamamoto, Suhaila M. Buhari, Mardina Abdullah, Asnawi Husin

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 122 ( 7 ) page： 7504 - 7518 2017.7

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

We investigated a postmidnight field-aligned irregularity (FAI) event observed with the Equatorial Atmosphere Radar at Kototabang (0.2 degrees S, 100.3 degrees E, dip latitude 10.4 degrees S) in Indonesia on the night of 9 July 2010 using a comprehensive data set of both neutral and plasma parameters. We examined the rate of total electron content change index (ROTI) obtained from GPS receivers in Southeast Asia, airglow images detected by an all-sky imager, and thermospheric neutral winds and temperatures obtained by a Fabry-Perot interferometer at Kototabang. Altitudes of the F layer (h'F) observed by ionosondes at Kototabang, Chiang Mai, and Chumphon were also surveyed. We found that the postmidnight FAIs occurred within plasma bubbles and coincided with kilometer-scale plasma density irregularities. We also observed an enhancement of the magnetically equatorward thermospheric neutral wind at the same time as the increase of h'F at low-latitude stations, but h'F at a station near the magnetic equator remained invariant. Simultaneously, a magnetically equatorward gradient of thermospheric temperature was identified at Kototabang. The convergence of equatorward neutral winds from the Northern and Southern Hemispheres could be associated with a midnight temperature maximum occurring around the magnetic equator. Equatorward neutral winds can uplift the F layer at low latitudes and increase the growth rate of Rayleigh-Taylor instabilities, causing more rapid extension of plasma bubbles. The equatorward winds in both hemispheres also intensify the eastward Pedersen current, so a large polarization electric field generated in the plasma bubble might play an important role in the generation of postmidnight FAIs.

DOI： 10.1002/2017JA024048

Detection of Plasma Bubble in Ionosphere using GPS Receivers in Southeast Asia Reviewed Open Access

Suhaila M. Buhari, Mardina Abdullah, Yuichi Otsuka, Tatsuhiro Yokoyama, Michi Nishioka, Alina Marie Hasbi, Takuya Tsugawa

SAINS MALAYSIANA Vol. 46 ( 6 ) page： 879 - 885 2017.6

Language：Malay Publishing type：Research paper (scientific journal) Publisher：UNIV KEBANGSAAN MALAYSIA

The equatorial ionosphere most often shows a nighttime plasma irregularity that is commonly referred as equatorial plasma bubble (PBB). The occurrence of PBB could cause rapid fluctuations in the amplitude and phase of the propagation radio signals and crucial to communication and navigation systems. The PBB normally occur successively where one structure rising after another during the sunset time. However, the onset time and location of the PBB are ubiquitous because the seed of the initial perturbation is not completely understood. Although various observation systems have been developed to capture the EPB, each of the measurement is limited with space and time resolution. This study aims to observe 2D structure of the PBB using high-density GPS receivers in Southeast Asia. GPS data was collected from 127 GPS receivers in Southeast Asia with the spacing distances of 30-120 km from each other. Total electron content (TEC) was derived from the difference between two signals from each GPS satellite. The signature of the PBB was detected using rate of TEC change index (ROTI) for all the available satellites to receiver paths. The 2D structure of the PBB was obtained by averaging GPS ROTI into 0.45 degrees latitude x 0.45 degrees longitude grid and projected at 300 km altitude. A case study on the night of 18 Mac 2011 showed the births of six PBB structures during the passage of the solar terminator along the 95 degrees E to 120 degrees E longitude. The separation distance between the PBB structures varied from 300 to 600 km. The separation distance between the EPB structures play an important role in determining the source of the seeding mechanism that believed in a form of wavelike structure.

DOI： 10.17576/jsm-2017-4606-06

Measurement of thermospheric temperatures using OMTI Fabry-Perot interferometers with 70-mm etalon Reviewed Open Access

Y. Nakamura, K. Shiokawa, Y. Otsuka, S. Oyama, S. Nozawa, T. Komolmis, S. Komonjida, Dave Neudegg, Colin Yuile, J. Meriwether, H. Shinagawa, H. Jin

EARTH PLANETS AND SPACE Vol. 69 2017.4

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

Fabry-Perot interferometer (FPI) is an instrument that can measure the temperature and wind velocity of the thermosphere through observations of airglow emission at a wavelength of 630.0 nm. The Solar-Terrestrial Environment Laboratory/Institute for Space-Earth Environmental Research, Nagoya University, has recently developed four new ground-based FPIs. One of those FPIs, possessing a large-aperture etalon (diameter: 116 mm), was installed in Tromso (FP01), Norway, in 2009. The other three small FPIs, using 70-mm-diameter etalons, were installed in Thailand (FP02), Indonesia (FP03) and Australia (FP04) in 2010-2011. They use highly sensitive cooled-CCD cameras with 1024 x 1024 pixels to obtain interference fringes. However, appropriate temperature has not been obtained from the interference fringes using these new small-aperture FPIs. In the present study we improved the analysis procedure of temperature determination using these FPIs. Each of FPIs measures north, south, east and west directions repeatedly by rotating two mirrors mounted on top of the FPI. We estimated center pixel of laser fringe and airglow fringes for each direction and found significant differences in the center pixel locations (a few pixels) among the measurement directions. These differences are considered to be caused by movement of the scanning mirror on the top of the optics, resulting in mechanical distortion of the optics body. By calculating the fringe center separately for each direction, we could correct these center pixel variations and determine the temperature with random errors of 10-40 K. This new method was employed to the all measurements from four FPIs after 2009 and provided temperatures with reasonably small errors. However, we found that temperatures below 400 K were obtained associated with weak airglow intensities and concluded using a model calculation that they are due to contamination of OH line emissions in the upper mesosphere. By defining an appropriate threshold of the fringe peak count, we successfully eliminated these unrealistic temperature values, and the corrected temperature values became comparable to those provided by the MSIS-90E and GAIA models.

DOI： 10.1186/s40623-017-0643-1

Large-scale traveling ionospheric disturbances observed by GPS dTEC maps over North and South America on Saint Patrick's Day storm in 2015 Reviewed Open Access

C. A. O. B. Figueiredo, C. M. Wrasse, H. Takahashi, Y. Otsuka, K. Shiokawa, D. Barros

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 122 ( 4 ) page： 4755 - 4763 2017.4

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

Large-scale traveling ionospheric disturbances (LSTIDs) were detected in both Northern and Southern Hemispheres over American sector during the geomagnetic storm on 17-18 March 2015, also known as the Saint Patrick's Day storm. Detrended total electronic content (dTEC) maps were made using dense GNSS network receiver data. The retrieved LSTIDs showed wavelengths of 1000 to 2000 km, phase velocity of similar to 300-1000 m/s, and period of similar to 30-50 min. Among them, three couples of LSTIDs were observed propagating from the polar regions to low latitudes. Two wave events observed in daytime showed the propagation direction of southwest in the Northern Hemisphere and northeast in the Southern Hemisphere, which means an asymmetric propagation against the geographic equator. The other wave event observed during the evening hour showed symmetric propagation direction, i.e., southwest in the Northern Hemisphere and northwest in the Southern Hemisphere, whereas their wavelength and phase velocity are significantly different between NH and SH. These observations indicate that the two groups of LSTID have different propagation conditions from polar to low-latitude regions. The observed asymmetric/symmetric propagation forms suggest asymmetric/symmetric auroral current activity between the northern and southern polar regions.

DOI： 10.1002/2016JA023417

Climatology of successive equatorial plasma bubbles observed by GPS ROTI over Malaysia Reviewed Open Access

S. M. Buhari, M. Abdullah, T. Yokoyama, Y. Otsuka, M. Nishioka, A. M. Hasbi, S. A. Bahari, T. Tsugawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 122 ( 2 ) page： 2174 - 2184 2017.2

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

The occurrence rate of the equatorial plasma bubble (EPB) with season, solar activity, and geomagnetic conditions are investigated using long-term data sets of Malaysia Real-Time Kinematics Network (MyRTKnet) from 2008 to 2013. The rate of TEC (total electron content) change index (ROTI) in 5 min was derived from MyRTKnet data to detect the EPB with scale sizes around tens of kilometers. Then, the daily east-west cross sections of 2-D ROTI maps were used to examine the EPB features over 100 degrees E-119 degrees E longitudes. The EPBs tend to occur successively in one night along the observational coverage of MyRTKnet during equinoxes in high solar activity years. The perturbations in a form of wavelike structures along the observed longitudes might be responsible for the development of successive EPBs due to high growth rate of the Rayleigh-Taylor instability (RTI) process. On the contrary, the occurrence of successive EPBs is infrequent and the occurrence day of EPB remains active during equinoctial months in low solar activity years. The small growth rate of the RTI process during low solar activity years might require a strong seed perturbation to generate the EPB structure. The occurrence probability of the EPB was found to be similar during quiet and disturbed geomagnetic conditions. The results imply that the strong perturbations play an important role in the development of the EPB in low solar activity years. Nonetheless, the high growth rate of the RTI could cause the successive occurrence of the EPB in high solar activity years.

DOI： 10.1002/2016JA023202

STATISTICAL ANALYSIS OF THE OCCURRENCES OF MSTIDs OBSERVED BY ALL-SKY IMAGER IN LOW MAGNETIC LATITUDE Reviewed

2017 IEEE INTERNATIONAL GEOSCIENCE AND REMOTE SENSING SYMPOSIUM (IGARSS) page： 4425 - 4428 2017

The first long-term all-sky imager observation of lunar sodium tail Reviewed

Masaki N. Nishino, Kazuo Shiokawa, Yuichi Otsuka

ICARUS Vol. 280 page： 199 - 204 2016.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE

The Moon possesses a long tail of neutral sodium atoms that are emitted from the lunar surface and transported anti-sunward by the solar radiation pressure. Since the earth crosses the lunar sodium tail for a few days around the new moon, the resonant light emission from sodium atoms can be detected from the ground. Here we show the first long-term (16 years) observation of the lunar sodium tail, using an all-sky imager at Shigaraki Observatory (35 degrees N, 136 degrees E), Japan. We have surveyed our database of all-sky sodium images at a wavelength of 5893 nm to find more than 20 events in which a bright spot emerges around the anti-lunar point during the new moon periods. We could not find any clear correlation between the sodium brightness and solar wind parameters (density, speed, dynamic pressure, and F10.7 index). In particular, no enhancement of the sodium spot brightness is detected even under very high density solar wind conditions (70 cm(-3); an order-of-magnitude higher than usual), which means that solar wind sputtering is not a principal mechanism of the formation of the lunar sodium tail. (C) 2016 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.icarus.2016.08.004

Three years of concentric gravity wave variability in the mesopause as observed by IMAP/VISI Reviewed

S. Perwitasari, T. Sakanoi, T. Nakamura, M. K. Ejiri, M. Tsutsumi, Y. Tomikawa, Y. Otsuka, A. Yamazaki, A. Saito

GEOPHYSICAL RESEARCH LETTERS Vol. 43 ( 22 ) page： 11528 - 11535 2016.11

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

We report a statistical study on concentric gravity waves (CGWs) in the mesopause (similar to 95 km) using 3 years nightglow data obtained by Ionosphere, Mesosphere, upper Atmosphere and Plasmasphere/Visible and near-Infrared Spectral Imager. The 235 CGWs events were found with horizontal wavelength ranging from 40 to 250 km and maximum radius of 200 to 3000 km. The latitudinal distribution of the CGWs centers had peaks in mid latitude (40 degrees N and 40 degrees S) and minimum at low latitudes (10 degrees S). More events were found in the summer hemisphere midlatitudes, with a rapid transition between northern and Southern Hemisphere around the equinoxes. The occurrence probability was significantly higher during nonsolstice months (February-May and August-November) than solstice months (June-July and December-January), suggesting that there was a little breaking or critical level absorption so the waves could reach the mesopause more often during these periods. The global distribution showed several preferable regions but very few events over tropical convective regions.

DOI： 10.1002/2016GL071511

Ionospheric TEC Weather Map Over South America Reviewed Open Access

H. Takahashi, C. M. Wrasse, C. M. Denardini, M. B. Padua, E. R. de Paula, S. M. A. Costa, Y. Otsuka, K. Shiokawa, J. F. Galera Monico, A. Ivo, N. Sant'Anna

SPACE WEATHER-THE INTERNATIONAL JOURNAL OF RESEARCH AND APPLICATIONS Vol. 14 ( 11 ) page： 937 - 949 2016.11

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

Ionospheric weather maps using the total electron content (TEC) monitored by ground-based Global Navigation Satellite Systems (GNSS) receivers over South American continent, TECMAP, have been operationally produced by Instituto Nacional de Pesquisas Espaciais's Space Weather Study and Monitoring Program (Estudo e Monitoramento Brasileiro de Clima Especial) since 2013. In order to cover the whole continent, four GNSS receiver networks, (Rede Brasileiro de Monitoramento Continuo) RBMC/Brazilian Institute for Geography and Statistics, Low-latitude Ionospheric Sensor Network, International GNSS Service, and Red Argentina de Monitoreo Satelital Continuo, in total similar to 140 sites, have been used. TECMAPs with a time resolution of 10min are produced in 12h time delay. Spatial resolution of the map is rather low, varying between 50 and 500km depending on the density of the observation points. Large day-to-day variabilities of the equatorial ionization anomaly have been observed. Spatial gradient of TEC from the anomaly trough (total electron content unit, 1TECU=10(16)elm(-2) (TECU) <10) to the crest region (TECU>80) causes a large ionospheric range delay in the GNSS positioning system. Ionospheric plasma bubbles, their seeding and development, could be monitored. This plasma density (spatial and temporal) variability causes not only the GNSS-based positioning error but also radio wave scintillations. Monitoring of these phenomena by TEC mapping becomes an important issue for space weather concern for high-technology positioning system and telecommunication.

DOI： 10.1002/2016SW001474

On the fresh development of equatorial plasma bubbles around the midnight hours of June solstice Reviewed

K. K. Ajith, S. Tulasi Ram, M. Yamamoto, Y. Otsuka, K. Niranjan

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 121 ( 9 ) page： 9051 - 9062 2016.9

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

Using the 47MHz Equatorial Atmosphere Radar (EAR) at Kototabang, Indonesia, the nocturnal evolution of equatorial plasma bubbles (EPBs) was examined during the moderate solar activity years 2011-2012. While the evolution of EPBs was mostly (86%) confined to post sunset hours (1900-2100LT) during equinoxes, in contrast, the majority of EPBs (similar to 71%) in June solstice found evolve around the midnight hours (2200-0300LT). The mechanisms behind the fresh evolution of summer time midnight EPBs were investigated, for the first time, through SAMI2 model simulations with a realistic input of background ExB drift variation derived from CINDI IVM on board C/NOFS satellite. The term-by-term analysis of linear growth rate of RT instability indicates that the formation of high flux tube electron content height gradient (K-F) (steep vertical gradient) region at higher altitudes is the key factor for the enhanced growth rate of RT instability. The responsible factors are discussed in light of relatively weak westward zonal electric field in the presence of equatorward neutral wind and bottomside recombination around the midnight hours of June solstice. The effects of neutral winds and weak westward electric fields on the uplift of equatorial F layer were examined separately using controlled SAMI2 simulations. The results indicate that relatively larger linear growth rate is more likely to occur around midnight during June solstice because of relatively weak westward electric field than other local times in the presence of equatorward meridional wind.

DOI： 10.1002/2016JA023024

Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes Reviewed Open Access

M. Ozaki, K. Shiokawa, Y. Miyoshi, R. Kataoka, S. Yagitani, T. Inoue, Y. Ebihara, C. -W Jun, R. Nomura, K. Sakaguchi, Y. Otsuka, M. Shoji, I. Schofield, M. Connors, V. K. Jordanova

GEOPHYSICAL RESEARCH LETTERS Vol. 43 ( 15 ) page： 7859 - 7866 2016.8

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

To understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave-particle interactions at subauroral latitudes, high-time-resolution (1/8s) images of proton-induced N-2(+) emissions were recorded using a new electron multiplying charge-coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising-tone elements with a spacing for each element of 100s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensity showed a similar repetition period of 100s and an unpredicted fast modulation of a few tens of seconds. These results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator.

DOI： 10.1002/2016GL070008

Enhanced ionospheric plasma bubble generation in more active ITCZ Reviewed Open Access

Guozhu Li, Yuichi Otsuka, Baiqi Ning, M. A. Abdu, M. Yamamoto, Weixing Wan, Libo Liu, Prayitno Abadi

GEOPHYSICAL RESEARCH LETTERS Vol. 43 ( 6 ) page： 2389 - 2395 2016.3

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

A close link between the atmospheric Intertropical Convergence Zone (ITCZ) and ionospheric plasma bubble has been proposed since the last century. But this relationship has often appeared to be less than convincing due to the simultaneous roles played by several other factors in shaping the global distribution of ionospheric bubbles. From simultaneous collaborative radar multibeam steering measurements at Kototabang (0.2 degrees S, 100.3 degrees E) and Sanya (18.4 degrees N, 109.6 degrees E), conducted during September-October of 2012 and 2013, we find that the total numbers of nights with bubble (i.e., occurrence rates) at the two closely located longitudes (Kototabang and Sanya) are comparable. But interestingly, the total number of nights with locally generated bubble (i.e., generation rate) over Kototabang is clearly more than that over Sanya. Further analysis reveals that a more active ITCZ is situated around the longitude of Kototabang. We surmise that the enhanced ionospheric bubble generation at Kototabang longitude could be caused by a higher gravity wave activity associated with the more active ITCZ.

DOI： 10.1002/2016GL068145

Altitude development of postmidnight F region field-aligned irregularities observed using Equatorial Atmosphere Radar in Indonesia Reviewed Open Access

Tam Dao, Yuichi Otsuka, Kazuo Shiokawa, S. Tulasi Ram, Mamoru Yamamoto

GEOPHYSICAL RESEARCH LETTERS Vol. 43 ( 3 ) page： 1015 - 1022 2016.2

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

For the first time, vertical rise velocities of postmidnight field-aligned irregularities (FAIs) at low geomagnetic latitudes have been examined near the June solstice by using two-dimensional maps of F region FAI echoes observed with the Equatorial Atmosphere Radar in Indonesia for 3years starting in May 2010. We found 15 freshly growing FAIs at postmidnight between May and August during the 3years. The rise velocities of FAIs are smaller at postmidnight than at postsunset, and most postmidnight FAIs do not exceed an altitude of 450km. Based on the rise velocities, a lower limit for the creation time of the postmidnight FAIs is estimated to be between 21:30LT and 02:00LT for 14 of the 15 events, indicating that this class of FAIs is distinct from the postsunset FAIs.

DOI： 10.1002/2015GL067432

Pulsating proton aurora caused by rising tone Pc1 waves Reviewed Open Access

R. Nomura, K. Shiokawa, Y. Omura, Y. Ebihara, Y. Miyoshi, K. Sakaguchi, Y. Otsuka, M. Connors

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 121 ( 2 ) page： 1608 - 1618 2016.2

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

We found rising tone emissions with a dispersion of approximate to 1Hz per several tens of seconds in the dynamic spectrum of a Pc1 geomagnetic pulsation (Pc1) observed on the ground. These Pc1 rising tones were successively observed over approximate to 30min from 0250UT on 14 October 2006 by an induction magnetometer at Athabasca, Canada (54.7 degrees N, 246.7 degrees E, magnetic latitude 61.7 degrees N). Simultaneously, a Time History of Events and Macroscale Interactions during Substorms panchromatic (THEMIS) all-sky camera detected pulsations of an isolated proton aurora with a period of several tens of seconds, approximate to 10% variations in intensity, and fine structures of 3 degrees in magnetic longitudes. The pulsations of the proton aurora close to the zenith of ATH have one-to-one correspondences with the Pc1 rising tones. This suggests that these rising tones scatter magnetospheric protons intermittently at the equatorial region. The radial motion of the magnetospheric source, of which the isolated proton aurora is a projection, can explain the central frequency increase of Pc1, but not the shorter period (tens of seconds) frequency increase of approximate to 1Hz in Pc1 rising tones. We suggest that EMIC-triggered emissions generate the frequency increase of Pc1 rising tones on the ground and that they also cause the Pc1 pearl structure, which has a similar characteristic time.

DOI： 10.1002/2015JA021681

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