Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

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Equatorial ionospheric scintillations and zonal irregularity drifts observed with closely-spaced GPS receivers in Indonesia Reviewed Open Access

Yuichi Otsuka, Kazuo Shiokawa, Tadahiko Ogawa

JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN Vol. 84A page： 343 - 351 2006.7

Language：English Publishing type：Research paper (scientific journal) Publisher：METEOROLOGICAL SOC JAPAN

To investigate drift velocities of a few hundred meter-scale irregularities associated with equatorial plasma bubbles, we used three single-frequency GPS receivers at the Equatorial Atmosphere Radar (EAR) site at Kototabang, Indonesia (0.20 degrees S, 100.32 degrees E; geomagnetic latitude 10.6 degrees S), since January 2003. The GPS receivers sampled GPS signal intensity at a rate of 20 Hz. Distances between the receivers were 116, 127, and 152 m. An analysis of scintillation index (S-4) in two years (2003-2004) revealed that the scintillations often occurred between 2000-0100 LT at equinoxes and that their occurrence rate was higher during March-April than during September-October. Drift velocities of irregularities were measured using cross-correlation analysis with the time series of the GPS signal intensity obtained from the three receivers. From a statistical analysis of the drift velocities, the eastward component of drift velocity just after sunset is found to be greater during March-April than during September-October. Based on these results, for the first time, we suggest that the east-west component of plasma drift velocity (or vertical electric field) may be related to the evolution of plasma irregularities causing scintillations throughout the mechanism causing the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocity could be attributed to the equinoctial asymmetry of neutral winds in the thermosphere.

DOI： 10.2151/jmsj.84A.343

Web of Science

Ionospheric disturbances over Indonesia and their possible association with atmospheric gravity waves from the troposphere Reviewed Open Access

Tadahiko Ogawa, Yuichi Otsuka, Kazuo Shiokawa, Akinori Saito, Michi Nishioka

JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN Vol. 84A page： 327 - 342 2006.7

Language：English Publishing type：Research paper (scientific journal) Publisher：METEOROLOGICAL SOC JAPAN

As part of the CPEA (Coupling Processes in the Equatorial Atmosphere) project, we have been conducting ground-based optical and radio observations of the ionosphere and thermosphere at Kototabang in Indonesia, Japan, and Australia. First, this paper gives a brief overview of some results, paying special attention to 100-1000 km scale plasma disturbances in the F region ionosphere, i.e., medium-scale traveling ionospheric disturbances (MSTIDs) and plasma bubbles. MSTIDs over the equator are observed within and in the south of the F region equatorial anomaly crest and have, on the average, a phase velocity of 300 m s(-1) toward the south, a period of 40 min, and a wavelength of 700 km. Plasma bubbles move to the east at similar to 100 m s(-1), have a scale of about 100 km with spacings of 200-250 km, and are embedded within plasma structures with a scale of about 1000 km. Interestingly, giant plasma bubbles can be simultaneously detected at lower midlatitudes in southern and northern hemispheres that are connected by the geomagnetic field line, and are very identical in appearance in the both hemispheres. Then, we present newly-obtained characteristics of equatorial ionospheric scintillations of 1.6-GHz GPS radio waves associated with plasma bubbles. Continuous scintillation observations for two years at Kototabang indicate that the scintillations appear predominantly from sunset to midnight in equinoctial months. Such a seasonal variation is also recognized from a statistical study of bubble occurrences over the Philippines, Singapore, and Indonesia. To investigate possible dynamical coupling between the ionosphere/thermosphere and troposphere over the equator, we compare the scintillation (bubble) activity and Earth's brightness temperature variation over the Indian Ocean measured by meteorological satellites. The results indicate that there can exist meaningful correlations between the scintillation occurrence and tropospheric disturbance at 80 degrees-95 degrees E longitudes, i.e. 5 degrees-20 degrees west of Kototabang. Possible processes to seed plasma bubbles are discussed.

DOI： 10.2151/jmsj.84A.327

Web of Science

Quasiperiodic southward moving waves in 630-nm airglow images in the equatorial thermosphere Reviewed Open Access

K. Shiokawa, Y. Otsuka, T. Ogawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 111 ( A6 ) 2006.6

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

We report quasiperiodic southward moving waves, which are commonly observed in the OI 630-nm airglow images (emission altitudes of 200-300 km) near the equator, in 2-year airglow observations at Kototabang, Indonesia (0.2 degrees S, 100.3 degrees E, geomagnetic latitude of -10.4 degrees). The waves have predominantly east-west phase fronts and repeatedly propagate southward with a velocity of 310 +/- 110 m/s and a period of 40 +/- 15 min. They are frequently observed in May-July with an occurrence rate of 53% and are also observed in other seasons with occurrences of similar to 20%. The waves are observed in and to the south (geomagnetically poleward) of the equatorial ionospheric anomaly, which is identified as an airglow enhancement region moving gradually to lower geomagnetic latitudes at the premidnight local times. We suggest that gravity waves in the lower thermosphere below similar to 300 km are a plausible cause of the observed quasiperiodic waves in the airglow images.

DOI： 10.1029/2005JA011406

Web of Science

Characteristics and implications of Doppler spectra of E region quasi-periodic echoes observed by the multibeam middle and upper atmosphere radar Reviewed Open Access

Ogawa, T, Otsuka, Y, Yamamoto, M

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 111 ( A5 ) 2006.5

Climatological study of GPS total electron content variations caused by medium-scale traveling ionospheric disturbances Reviewed

N Kotake, Y Otsuka, T Tsugawa, T Ogawa, A Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 111 ( A4 ) 2006.4

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

Using global positioning system (GPS) data taken from the International GNSS Service (IGS), we investigated total electron content (TEC) perturbations associated with medium-scale traveling ionospheric disturbances (MSTIDs). We analyzed TEC data taken from four or five GPS receivers in each of six regions ( Japan, Europe, eastern United States, western United States, Australia, and South America) in 3 years ( 1998, 2000, and 2001). To derive perturbation components of TEC (I'), we subtracted the 1-hour running average form the time sequence of TEC for each satellite-receiver pair. Standard deviation of I' within 1 hour, delta I, was calculated every hour, and MSTIDs activity were defined as delta I/(I) over bar, where (I) over bar is the 1-hour average of absolute vertical TEC. We found that MSTIDs activity during daytime is different from that during nighttime with respect to seasonal, solar activity, longitudinal, and latitudinal dependences. Daytime MSTIDs activity are high in winter in all six regions. On the other hand, seasonal variation of nighttime MSTIDs activity is coupled with its longitudinal variation. In the Japanese and Australian longitudinal sector, nighttime MSTIDs are most active near the June solstice, whereas it is most active near the December solstice in the European longitudinal sector. Nighttime MSTIDs activity at the Japanese and Australian longitudinal sector shows negative correlation with solar activity, whereas solar activity dependence is not seen in daytime MSTIDs activity. These results suggest that mechanisms causing MSTIDs could be different between daytime and nighttime.

DOI： 10.1029/2005JA011418

Web of Science

Geomagnetic conjugate observations of large-scale traveling ionospheric disturbances using GPS networks in Japan and Australia Reviewed

T Tsugawa, K Shiokawa, Y Otsuka, T Ogawa, A Saito, M Nishioka

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 111 ( A2 ) 2006.2

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

The geomagnetic conjugacy of large-scale traveling ionospheric disturbances (LSTIDs) was studied using total electron content (TEC) data derived from GPS networks in Japan and Australia. The number of simultaneous occurrences of LSTIDs within +/- 1 hour in both hemispheres was 5 out of 20 (21) events in Japan (Australia) when Kp >= 5- and 0 out of 15 (10) when Kp <= 4+. As for the LSTIDs observed simultaneously in both hemispheres, the propagation velocities of equatorward LSTID were comparable between the two hemispheres, with differences of 10-40%. The crossing times at 30 degrees geomagnetic latitude of the simultaneous LSTIDs over Japan and Australia were also different by several tens of minutes for all five events. These observational results indicate that the LSTIDs observed almost simultaneously in both hemispheres are not connected electromagnetically through the geomagnetic field but are generated by atmospheric gravity waves propagating to the equator independently in the two hemispheres.

DOI： 10.1029/2005JA011300

Web of Science

GPS detection of total electron content variations over Indonesia and Thailand following the 26 December 2004 earthquake Reviewed Open Access

Otsuka, Y., N. Kotake, T. Tsugawa, K. Shiokawa, T. Ogawa, Effendy, S. Saito, M. Kawamura, T. Maruyama, N. Hemmakorn, and T. Komolmis

Earth, Planets and Space Vol. 58 page： 159-165 2006

Equatorial ionospheric scintillations and zonal irregularity drifts observed with closely-spaced GPS receivers in Indonesia Reviewed Open Access

Otsuka, Y., K. Shiokawa, and T. Ogawa

J. Meteor. Soc. Japan Vol. 84A page： 343-351 2006

A Statistical Study of Ionospheric Irregularities Observed with a GPS Network in Japan

Y. Otsuka, T. Aramaki, T. Ogawa, A. Saito

Recurrent Magnetic Storms: Corotating Solar Wind Streams Vol. 167 page： 271 - + 2006

© 2006 by the American Geophysical Union. All rights reserved. The Geographical Survey Institute of Japan has installed a network of about 1000 dual-frequency GPS receivers in Japan with their mutual distance of about 25 km. Phases and pseudo ranges of dual-frequency GPS signals are recorded every 30 s. The dense distribution of the GPS receivers allows us to reveal twodimensional structures of the ionospheric plasma density irregularities with scale sizes of the order of several kilometers. We analyzed TEC data obtained from the GPS network of Japan in 2000. It was found that the irregularity characteristics over Japan depended on latitude. The results are as follows: (1) at the northern part, the irregularities appeared only during geomagnetic storms. (2) At the middle part, they had the most frequent occurrence in summer nighttime and were usually accompanied by Medium-Scale Traveling Ionospheric Disturbances (MSTID). (3) At the southern part, they were associated with the equatorial plasma bubbles and their occurrences were highest in the equinoctial nighttime.

DOI： 10.1029/167GM21

Web of Science

Scopus

GPS observations of post-storm TEC enhancements at low latitudes Reviewed Open Access

Ivan Kutiev, Yuichi Otsuka, Akinori Saito, Shigeto Watanabe

EARTH PLANETS AND SPACE Vol. 58 ( 11 ) page： 1479 - 1486 2006

Language：English Publishing type：Research paper (scientific journal) Publisher：TERRA SCIENTIFIC PUBL CO

In a previous work (J. Geophys. Res., 110(A01308), 1-11, 2005), the authors developed an original approach to the processing of total electron content (TEC) data obtained by GPS signals from the Japan receiver network. This approach includes removing the diurnal and seasonal variation carried by 27-day medians and the solar rotation periodicity. The relative deviations of TEC from the median-from all measured locations at a given hour-were then approximated by a regression line along the main prolongation of the Japan islands, between latitudes 24 degrees and 45 degrees N. The two variables of the regression line, the average value at the center and the slope were obtained as a time series, and their behavior during geomagnetic storms in the period 2000-2002 were analyzed. One interesting result was the observed enhancement of TEC at the end of the recovery phase of the storms. The slope variations clearly showed that this enhancement started from the south and was interpreted as a poleward expansion of equatorial crest. In the present paper we further analyze this post-storm phenomenon, adding foF2 data from Japanese Kokubunji and Okinawa ionosondes. We also show the latitude extension of the poleward expansion by using lat/UT contour plots. The results confirm that most of the post-storm TEC enhancements are part of the equatorial crest region which extends poleward during nighttime. In some cases, the enhanced TEC structures develop by separating from the crest region. Daytime TEC enhancements were also observed. Their structures are not confined to the equatorial crests region, but occupy the whole latitude range considered in this study. TEC post-storm enhancements were generally found to be in agreement with foF2 variations.

DOI： 10.1186/BF03352647

Open Access

Web of Science

GPS detection of total electron content variations over Indonesia and Thailand following the 26 December 2004 earthquake Reviewed Open Access

Yuichi Otsuka, N. Kotake, Takuya Tsugawa, Kazuo Shiokawa, Tadahiko Ogawa, E. Effendy, Susumu Saito, Masabumi Kawamura, Takashi Maruyama, N. Hemmakorn, T. Komolmis

Earth, Planets and Space Vol. 58 ( 2 ) page： 159 - 165 2006

Language：English Publishing type：Research paper (scientific journal) Publisher：The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan , The Geodetic Society of Japan , The Japanese Society for Planetary Sciences

We report the response of the ionosphere to the large earthquake that occurred in West Sumatra, Indonesia, at 0058 UT on December 26, 2004. We have analyzed Global Positioning System (GPS) data obtained at two sites in Sumatra and at three sites in Thailand to investigate total electron content (TEC) variations. Between 14 and 40 min after the earthquake, TEC enhancements of 1.6-6.9 TEC units (TECU) were observed at subionospheric points located 360-2000 km north of the epicenter. From the time delays of the observed TEC enhancements, we find that the TEC enhancements propagated northward from the epicenter. The time delays between the earthquake and rapid increases in TEC, which occurred near the epicenter, are consistent with the idea that acoustic waves generated by the earthquake propagated into the ionosphere at the speed of sound to cause the TEC variations. A small TEC enhancement of 0.6 TECU was observed south of the epicenter, while no TEC enhancements were seen east of the epicenter. From a model calculation, we find that this directivity of the TEC variations with respect to the azimuth from the epicenter could be caused partially by the directivity in the response of the electron density variation to the acoustic waves in the neutral atmosphere. © 2006, The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences. All rights reserved.

DOI： 10.1186/BF03353373

Ionospheric variations at midlatitude detected by a dense GPS receiver array in Japan Reviewed

A. Saito, Y. Otsuka, T. Tsugawa, K. Matsunaga, K. Hoshinoo

Proceedings of the Annual Meeting - Institute of Navigation page： 425 - 428 2005.12

Language：English Publishing type：Research paper (international conference proceedings)

Ionospheric variations over Japan were investigated with high spatial and temporal resolution data of GEONET, which consists of about 1,200 GPS receivers. Total Electron Content (TEC) data derived from GEONET clarified that various ionospheric phenomena cause the variation of the ionosphere in the region around 30 degrees of the geomagnetic latitude, and affect on the navigation system. The large day-to-day variation of TEC gradient over Japan was found to be attributed to the day-to-day variation of the Equatorial Ionization Anomaly (EIA) because Japan is located on the northern slope of EIA. The amplitude and location of EIA have large variability because they are controlled by a complex system among the neutral and the ionized atmospheres, lower and upper atmospheres, and low-latitude and high-latitude ionospheres. As a result, the variation of EIA is difficult to be predicted. Meso-scale ionospheric structures, such as medium-scale traveling ionospheric disturbances (MSTID) and plasma bubbles, drift in about 100m/s of velocity and cause variations of short temporal scale. They were often observed to be accompanied by smaller scale structures that generate phase and amplitude scintillations of the GPS radio waves. The activity of MSTID has maxima in summer and winter. During these maximum periods, they were detected by GEONET almost everyday. Plasma bubbles were detected in the lowest latitude area of Japan, and found to reach the central part of Japan in a few days in a year. Most of them were in the geomagnetically disturbed periods. The geomagnetic disturbances also generate large-scale traveling ionospheric disturbances and reduction of TEC.

Scopus

Efficacy of STI571 for a patient with metastatic gastrointestinal stromal tumor Reviewed

Sasaki, Y, Niwa, Y, Ando, N, Otsuka, Y, Ohmiya, N, Hirooka, Y, Itoh, A, Furuta, S, Goto, H

HEPATO-GASTROENTEROLOGY Vol. 52 ( 66 ) page： 1764 - 1767 2005.11

Observations of equatorial plasma bubbles using broadcast VHF radio waves Reviewed Open Access

H Nakata, Nagashima, I, K Sakata, Y Otsuka, Y Akaike, T Takano, S Shimakura, K Shiokawa, T Ogawa

GEOPHYSICAL RESEARCH LETTERS Vol. 32 ( 17 ) 2005.9

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

The propagation of VHF radio waves affected by field-aligned irregularities within equatorial plasma bubbles is examined. Continuous observation of VHF radio waves at Tateyama, Japan, shows that broadcast radio waves transmitted from Southeast Asia propagate to Japan. Using a ray-tracing calculation combined with a model of scattering by field-aligned irregularities, we determined scatter points suitable for the reception of these radio waves. The results of the calculation show that radio waves transmitted from the Philippines can propagate to Japan owing to field-aligned irregularities within equatorial plasma bubbles located west of Japan. Comparing with all-sky imager data observed at Sata, Japan, we found that the radio waves are received at Tateyama when the plasma bubbles are observed at Sata. The results of this study show that the reception of these radio waves is closely correlated with the equatorial plasma bubbles.

DOI： 10.1029/2005GL023243

Web of Science

Geomagnetic conjugate observation of nighttime medium-scale and large-scale traveling ionospheric disturbances: FRONT3 campaign - art. no. A05303 Open Access

Shiokawa, K; Otsuka, Y; Tsugawa, T; Ogawa, T; Saito, A; Ohshima, K; Kubota, M; Maruyama, T; Nakamura, T; Yamamoto, M; Wilkinson, P

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 110 ( A5 ) page： doi:10.1029/2004JA010845 2005.5

Total electron content behavior over Japan during geomagnetic storms Reviewed

Kutiev, I, S Watanabe, Y Otsuka, A Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 110 ( A1 ) 2005.1

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

[ 1] The total electron content (TEC) obtained from GPS signals is used to study ionospheric dynamics over Japan during geomagnetically disturbed conditions. The numerous TEC measurements are averaged in cells with a size 1.5degrees x 1.5degrees geographic scale and formatted as time series within the years 2000 - 2002. To extract the storm time changes of TEC, the diurnal and 27-day periodicities are subsequently removed. Diurnal variations are removed by replacing absolute TEC values in each cell with their relative deviations (RTEC) from medians. The hourly RTEC values from all cells within the central 4degrees-wide band over Japan area are then approximated by a plane surface. This surface is represented by two parameters: its value at the center (rt) and the slope (b) along the main axis, taken as constants of the linear regression. The 27-day periodicity was approximated by Fourier waves with main period of 640 hours and two harmonics separately for rt and b and subtracted from them. The analysis of rt and b behavior during a number of geomagnetic storms allowed us to reveal several repeatable features of average TEC behavior. It was found that TEC behavior during the storms is similar to that of f(o)F(2) at the F region and was local time - dependent. A marked poleward expansion of the equatorial ionosphere ( crest region) at the end of recovery phase is persistently observed feature, produced probably by intensified eastward zonal winds. Such an expansion of equatorial ionosphere is observed also during isolated substorms, outside main geomagnetic storms. An oscillation-like change of positive and negative disturbances with period of 24 hours is observed during a 4-day period, following a moderate storm. In the absence of geomagnetic activity driver that effect is probably caused by the alternative expansion and contraction of equatorial ionosphere.

DOI： 10.1029/2004JA010586

Web of Science

Relationship between propagation direction of gravity waves in OH and OI airglow images and VHF radar echo occurrence during the SEEK-2 campaign Reviewed Open Access

F Onoma, Y Otsuka, K Shiokawa, T Ogawa, M Yamamoto, S Fukao, S Saito

ANNALES GEOPHYSICAE Vol. 23 ( 7 ) page： 2385 - 2390 2005

Language：English Publishing type：Research paper (scientific journal) Publisher：EUROPEAN GEOSCIENCES UNION

We report simultaneous observations of atmospheric gravity waves (AGW) in OI (557.7 nm) and OH air-glow images and VHF radar backscatter from field-aligned irregularities (FAI) in the E-region during the SEEK-2 (Sporadic-E Experiment over Kyushu 2) campaign period from 29 July to 9 August 2002. An all-sky imager was operated at Nishino-Omote (30.5 degrees N, 130.1 degrees E), Japan. On 14 nights. 17 AGW events were detected in OI and OH air-glow images. AGW propagated mostly toward the northeast or southeast. From comparison with the E-region FAI occurrence, which is detected by a nearby VHF radar (31.57 MHz), we found that AGW tended to propagate southeastward during FAI events. This result suggests that the interaction between AGW and E-region plasma plays an important role in generating FAI. Furthermore, polarization electric fields generated directly by AGW may contribute to the FAI generation.

DOI： 10.5194/angeo-23-2385-2005

Open Access

Web of Science

Transition region of TEC enhancement phenomena during geomagnetically disturbed periods at mid-latitudes Reviewed Open Access

K Unnikrishnan, A Saito, Y Otsuka, M Yamamoto, S Fukao

ANNALES GEOPHYSICAE Vol. 23 ( 11 ) page： 3439 - 3450 2005

Language：English Publishing type：Research paper (scientific journal) Publisher：EUROPEAN GEOSCIENCES UNION

Large-scale TEC perturbations/enhancements observed during the day sectors of major storm periods, 1213 February 2000, 23 September 1999, 29 October 2003, and 21 November 2003, were studied using a high resolution GPS network over Japan. TEC enhancements described in the present study have large magnitudes (>= 25 x 10(16) electrons/m(2)) compared to the quiet-time values and long periods (>= 120 min). The sequential manner of development and the propagation of these perturbations show that they are initiated at the northern region and propagate towards the southern region of Japan, with velocities > 350 m/s. On 12 February 2000, remarkably high values of TEC and background content are observed at the southern region, compared to the north, because of the poleward expansion of the equatorial anomaly crest, which is characterized by strong latitudinal gradients near 35 degrees N (26 degrees N geomagnetically). When the TEC enhancements, initiating at the north, propagate through the region 39-34 degrees N (30-25 degrees N geomagnetically), they undergo transitions characterized by a severe decrease in amplitude of TEC enhancements. This may be due to their interaction with the higher background content of the expanded anomaly crest. However, at the low-latitude region, below 34 degrees N, an increase in TEC is manifested as an enhanced ionization pattern (EIP). This could be due to the prompt penetration of the eastward electric field, which is evident from high values of the southward Interplanetary Magnetic Field component (IMF B-z) and AE index. The TEC perturbations observed on the other storm days also exhibit similar transitions, characterized by a decreasing magnitude of the perturbation component, at the region around 39-34 degrees N. In addition to this, on the other storm days, at the low-latitude region, below 34 degrees N, an increase in TEC (EIP feature) also indicates the repeatability of the above scenario. It is found that, the latitude and time at which the decrease in magnitude of the perturbation component/amplitude of the TEC enhancement are matching with the latitude and time of the appearance of the high background content. In the present study, on 12 February 2000, the F-layer height increases at Wakkanai and Kokubunji, by exhibiting a typical dispersion feature of LSTID, or passage of an equator-ward surge, which is matching with the time of occurrence of the propagating TEC perturbation component. Similarly, on 29 October 2003, the increase in F-layer heights by more than 150 km at Wakkanai and 90 km at Kokubunji around 18:00 JST, indicates the role of the equatorward neutral wind. On that day, TEC perturbation observed at the northern region, after 18:30 JST, which propagates towards south, could be caused mainly by the equatorward neutral wind, leading to an F-layer height increase. These observations imply the role of the equatorward neutral wind, which increases the F-layer height, by lifting the ionization to the regions of lower loss during daytime, when production is still taking place, which, in turn, increases the TEC values.
Large-scale traveling ionospheric disturbances (LSTIDs) are considered as ionospheric manifestations of the passage of Atmospheric Gravity Waves (AGWs) that are generated at the high latitude by energy input from the magnetosphere to the low-latitude ionosphere. This study shows that large-scale TEC perturbations observed here are produced at the northern region due to the combined effects of the equatorward neutral wind, the subsequent F-layer height increase, and LSTIDs. When these perturbation components propagate through the region, 39-34 degrees N, they undergo transitions characterised by a decrease in magnitude. Also, at the low-latitude region, below 34 degrees N, an increase in the TEC exhibits EIP feature, due to the prompt penetration of the eastward electric field.

DOI： 10.5194/angeo-23-3439-2005

Open Access

Web of Science

The first coordinated observations of mid-latitude E-region quasi-periodic radar echoes and lower thermospheric 557.7-nm airglow Reviewed Open Access

T Ogawa, Y Otsuka, F Onoma, K Shiokawa, M Yamamoto

ANNALES GEOPHYSICAE Vol. 23 ( 7 ) page： 2391 - 2399 2005

Language：English Publishing type：Research paper (scientific journal) Publisher：EUROPEAN GEOSCIENCES UNION

We present the first coordinated observations of quasi-periodic (QP) radar echoes from sporadic-E (E-S) field-aligned irregularities (FAIs), OI 557.7-nm airglow, and neutral winds in a common volume over Shigaraki, Japan (34.9 degrees N. 136.1 degrees E) on the night of 5 August 2002 during the SEEK-2 campaign. QP echo altitudes of 90-110 km were lower than usual by 10 km, enabling us to make a detailed comparison among QP echoes, airglow intensity, and neutral wind at around 96 km altitude. Eastward movement of the QP echo regions is consistent with the motions of neutral winds, airglow structures, and FAIs, suggesting that the electrodynamics of E-S-layers is fundamentally controlled by the neutral atmospheric dynamics. During the QP echo event, the echo altitudes clearly went up (down) in harmony with an airglow enhancement (subsidence) that also moved to the east. This fact suggests that the eastward-moving enhanced airglow region included an upward (downward) component of neutral winds to raise (lower) the altitude of the wind-shear node responsible for the E-S formation. The airglow intensity, echo intensity, and Doppler velocity of FAIs at around 96km altitude fluctuated with periods from 10min to 1 h, indicating that these parameters were modulated with short-period atmospheric disturbances. Some QP echo regions below 100 km altitude contained small-scale QP structures in which very strong neutral winds exceeding 100 m/s existed. The results are compared with recent observations, theories, and simulations of QP echoes.

DOI： 10.5194/angeo-23-2391-2005

Open Access

Web of Science

Simultaneous ground- and satellite-based airglow observations of geomagnetic conjugate plasma bubbles in the equatorial anomaly Reviewed Open Access

T Ogawa, E Sagawa, Y Otsuka, K Shiokawa, TI Immel, SB Mende, P Wilkinson

EARTH PLANETS AND SPACE Vol. 57 ( 5 ) page： 385 - 392 2005

Language：English Publishing type：Research paper (scientific journal) Publisher：TERRA SCIENTIFIC PUBL CO

We compare, for the first time, geomagnetically-conjugate plasma bubbles observed by ground-based 01 630.0-nm all-sky imagers at Shigaraki, Japan (34.8 degrees N, 136.1 degrees E; magnetic latitude 25.4 degrees N) and Darwin, Australia (12.4 degrees S, 131.0 degrees E; magnetic latitude 22.0 degrees S), with global-scale plasma structures (similar to 10,000 km in longitude) in the equatorial anomaly simultaneously detected with an OI 135.6-nm imager on the IMAGE satellite at similar to 7 earth radii. As found previously, global-scale plasma structures in both hemisphere imaged by IMAGE consist of an array of geomagnetically-conjugate small- to medium-scale (a few hundreds to 1000 km in longitude) wavy structures that move to the east at similar to 100 in s(-1). We find the following: 1) plasma bubbles detected with the all-sky imagers reach an apex altitude of 1800 km over the geomagnetic equator while moving to the east at - 100 m s(-1) with spacings of 200-250 km. 2) Bubbles observed with the all-sky imagers and IMAGE are embedded within the small- to medium-scale wavy structures, and some of them are located near the crest of ail enhanced electron density region associated with the wavy structures. 3) The bubbles and wavy structures that are generated near sunset slant to the west with increasing latitude in both hemispheres, and tilts do not change with longitude (i.e., local time). The results suggest that the generation and evolution of plasma bubbles are closely related to those of the small- to medium-scale plasma structures.

DOI： 10.1186/BF03351822

Open Access

Web of Science

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Papers - OTSUKA, Yuichi