Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

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Optical and radio measurements of a 630-nm airglow enhancement over Japan on 9 September 1999 Reviewed

Y Otsuka, T Kadota, K Shiokawa, T Ogawa, S Kawamura, S Fukao, Zhang, SR

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 108 ( A6 ) 2003.6

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

Using a comprehensive data set from optical and radio instruments, we investigate a midnight brightness wave that appeared in 630-nm airglow images over Japan on the night of 9 September 1999. This may be the first such observation of the brightness wave with an all-sky imager in the East-Asian longitudinal sector. The imager at Shigaraki (35.6degreesN, 136.1degreesE) tracked a north-northeastward propagation of the wave with an apparent velocity of 500 m s(-1) after midnight. Ionosonde observations at five stations in Japan showed that rapid descent of the F-2 layer propagated northward beyond 35degreesN with decreasing amplitude. Incoherent scatter observations with the MU radar at Shigaraki also revealed that the F-2 peak altitude decreased from 360 to 280 km during the event. During the F-2 layer descent the altitude profile of the electron density became sharp, enhancing the F-2 peak electron density. After the F-2 layer altitude reached 280 km, electron density in the F-2 layer rapidly decreased because of increased neutral density at low altitude. A Fabry-Perot interferometer (FPI) at Shigaraki observed northward neutral winds of 10-70 m s(-1) during the event. A model calculation demonstrates that the meridional winds estimated from the MU radar electron density profiles are fairly consistent with those observed with the FPI. From these results we conclude that the observed northward wind enhancements, probably caused by the midnight temperature maximum, pushed down the plasma in the F-2 layer to lower altitudes along the geomagnetic field to cause the 630-nm airglow intensity enhancement.

DOI： 10.1029/2002JA009594

Ground and satellite observations of nighttime medium-scale traveling ionospheric disturbance at midlatitude Reviewed

K Shiokawa, Y Otsuka, C Ihara, T Ogawa, FJ Rich

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 108 ( A4 ) 2003.4

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

[1] We have investigated a nighttime medium-scale traveling ionospheric disturbance (MSTID) observed by an airglow imager at Shigaraki (34.9degreesN, 25.4degreesMLAT), Japan, on 17 May 2001. The structure was identified in the airglow images of OI (630.0 nm and 777.4 nm) as NW-SE band structures (horizontal wavelength: 230 km) moving southwestward with a velocity of 50 m/s. Neutral wind velocity was measured simultaneously from the Doppler shift of the 630.0-nm emission by a Fabry-Perot interferometer at Shigaraki. From these parameters, we performed model calculations of MSTIDs generated by gravity waves and by an oscillating electric field. We found that for the case of gravity waves, the estimated vertical wavelength was too small to explain the observed amplitudes of airglow intensity. For the case of the electric field, we found that an electric field oscillation of similar to1.2 mV/m was sufficient to reproduce the observed airglow amplitudes. This modeled electric field was comparable to that observed by the DMSP F15 satellite as it passed over Shigaraki during our observing period on 17 May 2001. The DMSP ion drift data show that the oscillation of the polarization electric field correlated with the MSTID structure in the airglow image, suggesting that the polarization electric field plays an important role in the generation of MSTIDs.

DOI： 10.1029/2002JA009639

Damping of large-scale traveling ionospheric disturbances detected with GPS networks during the geomagnetic storm Reviewed

T Tsugawa, A Saito, Y Otsuka, M Yamamoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 108 ( A3 ) 2003.3

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

[1] Large-scale traveling ionospheric disturbances (LSTIDs) during the geomagnetic storm on 22 September 1999 were studied using total electron content (TEC) data from the GPS Earth Observation Network (GEONET) in Japan, International GPS Service (IGS), and Continuously Operating Reference Stations (CORS) in the United States. The damping rates of the LSTIDs were precisely derived in several local time sectors and were found to depend on values of the background TEC. This indicates that the dominant physical mechanism of the LSTIDs' damping is the ion-drag effect by the background ionosphere. The high-resolution TEC data from GEONET revealed that two successive LSTIDs were damped significantly as they traveled equatorward in the dawn sector. The ratio of the perturbation component of TEC to the background component (DeltaI/I-0) decreased exponentially with the damping rate of 0.89/1000 km and 0.77/1000 km. We studied also the amplitude of DeltaI/I-0 at high latitudes using IGS data and found that the damping rates of LSTIDs at high latitudes tended to be smaller than those at midlatitudes. Global TEC observations during this geomagnetic storm by the IGS and CORS networks detected that several LSTIDs propagated also equatorward in the afternoon sector and in the night sector. The LSTIDs in the afternoon sector were most damped with the damping rate of 1.04/1000 km, which corresponds to the e-folding length of 961 km. The damping rate of LSTIDs in the night sector was found to be small. The LSTIDs had a tendency to be damped rapidly in the regions where background TEC was large. This dependence of the damping rate on latitude and local time indicates that this intense damping of LSTIDs was caused mainly by the ion-drag effect that is proportional to the ion collision frequency. The relation between the damping rates and the background TEC derived from the observation are consistent with those estimated with a theoretical calculation of the gravity wave damping by the ion-drag effect. The worldwide distribution of GPS receivers enabled us to estimate the longitudinal extent of these LSTIDs. The zonal width of LSTIDs in the dawn sector was narrower than 45degrees in longitude (2,900 km) around 55degreesN and wider than 20degrees in longitude ( 1,800 km) around 40degreesN. These widths were narrower than those reported in previous studies.

DOI： 10.1029/2002JA009433

Statistical study of nighttime medium-scale traveling ionospheric disturbances using midlatitude airglow images Reviewed

K Shiokawa, C Ihara, Y Otsuka, T Ogawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 108 ( A1 ) 2003.1

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

[1] We have investigated statistical characteristics of the nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed in 630-nm airglow images at two stations, Rikubetsu (43.5degreesN, 34.8degreesMLAT) and Shigaraki (34.9degreesN, 25.4degreesMLAT), in Japan for 1998-2000 near the solar maximum period. Most of the observed MSTIDs propagate southwestward in the images. The typical wavelength, velocity, period, and amplitude are 100-300 km, 50-100 m/s, 0.5-1.5 h, and 5-15%, respectively. Seasonal variations in these parameters are not clear. The occurrence rate has a major peak (50-60%) in summer that appears similar to2 months earlier at lower latitudes and a minor peak in winter. Similar occurrence characteristics are obtained from midlatitude spread-F signatures using multipoint ionosonde data in Japan, though the coincidence of the spread-F and the MSTIDs in airglow images is only 10-15%.

DOI： 10.1029/2002JA009491

Optical and radio measurements of a 630-nm enhancement over Japan on September 9, 1999 Reviewed

Otsuka, Y., T. Kadota, K. Shiokawa, T. Ogawa, S. Kawamura,, S. Fukao, and S. -R. Zhang

J. Geophys. Res. Vol. 108(A6) ( 1252 ) page： doi:10.1029/2002JA009594 2003

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Using a comprehensive data set from optical and radio instruments, we investigate a midnight brightness wave that appeared in 630-nm airglow images over Japan on the night of September 9, 1999. This may be the first such observation of the brightness wave with an all-sky imager in the East-Asian longitudinal sector. The imager at Shigaraki (35.6N, 136.1E) tracked a north-northeastward propagation of the wave with an apparent velocity of 500 m/s after midnight. Ionosonde observations at five stations in Japan showed that rapid descent of the F2 layer propagated northward beyond 35N with decreasing amplitude. Incoherent scatter observations with the MU radar at Shigaraki also revealed that the F2 peak altitude decreased from 360 km to 280 km during the event. During the F2 layer descent, the altitude profile of the electron density became sharp, enhancing the F2 peak electron density. After the F2 layer altitude reached 280 km, electron density in the F2 layer rapidly decreased because of increased neutral density at low altitude. A Fabry-Perot interferometer (FPI) at Shigaraki observed northward neutral winds of 10--70 m/s during the event. A model calculation demonstrates that the meridional winds estimated from the MU radar electron density profiles are fairly well consistent with those observed with the FPI. From these results, we conclude that the observed northward wind enhancements, probably caused by the the midnight temperature maximum, pushed down the plasma in the F2 layer to lower altitudes along the geomagnetic field to cause the
630-nm airglow intensity enhancement.

A two-channel Fabry-Perot interferometer with thermoelectric-cooled CCD detectors for neutral wind measurement in the upper atmosphere Reviewed

K Shiokawa, T Kadota, Y Otsuka, T Ogawa, T Nakamura, S Fukao

EARTH PLANETS AND SPACE Vol. 55 ( 5 ) page： 271 - 275 2003

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

We describe performance of a new two-channel Fabry-Perot interferometer (FPI) with two thermoelectric-cooled CCD detectors to measure neutral winds for both the 558-nm (mesopause region) and 630-nm (thermosphere) airglow emissions simultaneously at Shigaraki (34.8degreesN, 136.1degreesE), Japan. The employment of the thermoelectric cooling system enables us continuous automatic operation of the FPI since October 2000. The random errors of wind measurement are estimated to be similar to5-8 m/s and similar to10-50 m/s for 558 nm and 630 nm, respectively. The wind velocities obtained by the FPI (558 and 630 nm) fit well with those obtained by the collocated Middle and Upper (MU) atmosphere radar.

DOI： 10.1186/BF03351759

Simultaneous middle and upper atmosphere radar and ionospheric sounder observations of midlatitude E region irregularities and sporadic E layer Reviewed

T Ogawa, O Takahashi, Y Otsuka, K Nozaki, M Yamamoto, K Kita

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 107 ( A10 ) 2002.10

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

[1] We made middle and upper atmosphere (MU) radar observations of midlatitude E region field- aligned irregularities (FAIs) in the summer of 1999 and 2000. Sporadic E (E-s) layer was monitored with a routine ionosonde, and its altitude was measured with an FMCW sounder (FCS). In this paper we draw attention to two findings. First, we show that quasiperiodic (QP) radar echoes appearing before 0200 LT are more enhanced with increasing f(o)E(s) - f(b)E(s), which means that the FAI generation is closely related to localized density gradients within E-s, and extend from 100 to 130 km in altitude, while E-s altitudes determined from the FCS soundings are between 100 and 110 km. The latter fact suggests that existing models for the QP echo generation, which require a deep modulation of E-s altitude, are not applicable to our observational results. We propose a new working model for generating QP echoes in which polarization electric fields originated from high- density plasma clouds within E-s are mapped upward along the geomagnetic field to produce relatively weak irregularities above the E-s layer. Second, we show new findings obtained from the current observations, namely, two types of QP echoes that occur below 100 km in the morning: one is the morning QP (MQP) echoes with periods of 4- 8 min, and the other is the QP echoes with periods of similar to1 min. The latter type can be categorized as low- altitude QP echoes that were found from previous nighttime MU radar observations. Until now the MU radar QP echoes have been believed to occur above 100 km for the period from sunset to midnight. Although we do not know the generation mechanisms of the low- altitude MQP echoes, we suppose that these echoes might be caused by a weak E-s that exists below 100 km.

DOI： 10.1029/2001JA900176

Annual and semiannual variations of the midlatitude ionosphere under low solar activity Reviewed

S Kawamura, N Balan, Y Otsuka, S Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 107 ( A8 ) 2002.8

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

[1] The annual and semiannual variations of the midlatitude ionosphere under low solar activity are studied using middle and upper (MU) radar (135degreesE, 35degreesN) incoherent scatter observations and Sheffield University plasmasphere-ionosphere model (SUPIM). The variations of the daytime electron density (Ne) and electron and ion temperatures (Te and Ti) at 200-600 km altitudes measured by the radar under low solar activity (F-10.7 less than or equal to 120) are satisfactorily reproduced for the first time by incorporating the radar measured values of the magnetic meridional neutral wind velocity (U-theta) and northward perpendicular plasma drift velocity (V-perpendicular to) into SUPIM that uses mass spectrometer incoherent scatter 1986 (MSIS-86) for neutral densities and neutral temperatures. The study shows that the annual and semiannual variations of the midlatitude ionosphere during daytime at altitudes near and above the ionospheric peak under low solar activity depend more on the direct effect of the neutral wind arising through the changes in ionospheric height than on the indirect effect arising through the changes in thermospheric composition (or atomic to molecular concentration ratio); the indirect effect, however, predominates on the variations at altitudes below the ionospheric peak. The electron temperature (Te) undergoes similar but almost opposite seasonal and semiannual variations as the electron density. The ion temperature (Ti) is closer to neutral temperature than to electron temperature at altitudes up to similar to400 km and shows comparatively weak annual and semiannual variations.

DOI： 10.1029/2001JA999267

Geomagnetic conjugate observations of equatorial airglow depletions Reviewed

Y Otsuka, K Shiokawa, T Ogawa, P Wilkinson

GEOPHYSICAL RESEARCH LETTERS Vol. 29 ( 15 ) 2002.8

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

We report for the first time large-scale equatorial F-region airglow depletions extending to low-midlatitudes in both hemispheres. The observational sites were located at low-midlatitude geomagnetic conjugate points. Clear depletions of 630.0-nm airglow intensity due to equatorial plasma bubbles were simultaneously observed with two all-sky imagers at Sata, Japan (magnetic latitude 24degreesN), and its geomagnetic conjugate point, Darwin, Australia (magnetic latitude 22degreesS), on the night of November 12, 2001. Airglow depletion regions with east-west scale sizes of 40-100 km extend poleward. The maximum apex altitude of the plasma bubbles is about 1,700 km over the geomagnetic equator. The depletions move eastward at about 100 m/s, without changing their structures. The Darwin depletion structures mapped onto the northern hemisphere along the geomagnetic field coincide closely with structures in the Sata images, even for the 40-km structure. These observations indicate that plasma depletions in the equatorial ionosphere elongate along the geomagnetic field lines.

DOI： 10.1029/2002GL015347

A large-scale traveling ionospheric disturbance during the magnetic storm of 15 September 1999 Reviewed

K Shiokawa, Y Otsuka, T Ogawa, N Balan, K Igarashi, AJ Ridley, DJ Knipp, A Saito, K Yumoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 107 ( A6 ) 2002.6

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

[1] Using a comprehensive data set and model calculations, we have investigated a prominent large-scale traveling ionospheric disturbance (LSTID) observed in Japan (similar to37degrees-16degrees MLAT) on 15 September 1999, during a recovery phase of sequential storms. The LSTID was detected at 2300-2400 LT (1400-1500 UT) as an enhancement of the 630-nm airglow intensity (50 --> 350 R), a decrease in the F layer virtual height (at 2 MHz, 360 --> 200 km), an enhancement of f(o)F(2) (6 --> 8 MHz), and an enhancement of GPS total electron content (similar to1.0 x 10(16) m(-2)). Multipoint and imaging observations of these parameters show that the LSTID moved equatorward over Japan with a velocity of similar to400-450 m/s. From a comparison with the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) we conclude that an enhancement (250-300 m/s) of poleward neutral wind (that is propagating equatorward) caused these observational features of the LSTID at midlatitudes. To investigate generation of the LSTID by auroral energy input, we have used auroral images obtained by the Polar UVI instrument, magnetic field variations obtained at multipoint ground stations, and the empirical Joule heating rate calculated by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. Intense auroral energy input was observed at 0800-1100 UT (4-6 hours before the LSTID), probably causing equatorward neutral wind at lower latitudes. It is likely that the poleward wind pulse that caused the observed LSTID was generated associated with the cessation of this equatorward wind. The effect of Lorentz force is also discussed.

DOI： 10.1029/2001JA000245

*A new technique for mapping of total electron content using GPS network in Japan Reviewed

Otsuka, Y., T. Ogawa, A. Saito, T. Tsugawa, S. Fukao, and S. Miyazaki

Earth, Planets, and Space Vol. 54 page： 63-70 2002

Geomagnetic conjugate observations of equatorial airglow depletions Reviewed

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

Geophys. Res. Lett. Vol. 29 page： 10.1029/2002GL015347 2002

A new technique for mapping of total electron content using GPS network in Japan Reviewed

Y Otsuka, T Ogawa, A Saito, T Tsugawa, S Fukao, S Miyazaki

EARTH PLANETS AND SPACE Vol. 54 ( 1 ) page： 63 - 70 2002

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

The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15degrees x 0.15degrees in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.

DOI： 10.1186/BF03352422

Imaging observations of the equatorward limit of midlatitude traveling ionospheric disturbances Reviewed

K Shiokawa, Y Otsuka, MK Ejiri, Y Sahai, T Kadota, C Ihara, T Ogawa, K Igarashi, S Miyazaki, A Saito

EARTH PLANETS AND SPACE Vol. 54 ( 1 ) page： 57 - 62 2002

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

This paper reports the first attempt to observe the equatorward limit of medium-scale traveling ionospheric disturbances (TIDs) in the middle latitudes. The TIDs usually propagate southwestward in the northern hemisphere. An all-sky cooled-CCD imager measured 630-nm airglow at a southern island of Japan, Okinawa (26.9degreesN, 128.3degreesE, geomagnetic latitude (MLAT) = 17.0degrees), during the FRONT-2 campaign of August 4-15, 1999. The TIDs were detected at the mainland of Japan (similar to21degrees-36degrees MLAT) by the total electron content (TEC) observations of more than 1000 GPS receivers. In the August 4 event, the TIDs moving southwestward was seen only in the northern sky of Okinawa as a depletion band in the 630-nm airglow images. In the August 6 event, the TIDs were not seen in the 630-nm images at Okinawa, although weak TID activity was observed by the GPS network at the mainland of Japan. The TEC data also showed weakening of the TID activity below 18degrees MLAT. Based on these observations, we suggest that there is a possible limit of medium-scale TID propagation around similar to18degrees MLAT.

DOI： 10.1186/BF03352421

Observations and modeling of 630 nm airglow and total electron content associated with traveling ionospheric disturbances over Shigaraki, Japan Reviewed

T Ogawa, N Balan, Y Otsuka, K Shiokawa, C Ihara, T Shimomai, A Saito

EARTH PLANETS AND SPACE Vol. 54 ( 1 ) page： 45 - 56 2002

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

Southwestward-propagating medium-scale traveling ionospheric disturbances (MSTIDs) observed over Shigaraki (34.85degreesN, 136.10degreesE) in Japan on the night of May 22, 1998 are analyzed in detail. The MSTIDs were detected with a 630.0 nm (01) all-sky imager at Shigaraki and a large number of GPS (Global Positioning System) receivers distributed around Shigaraki. Each GPS receiver provided total electron content (TEC) between the GPS altitude (20,200 km) and the ground. MSTID amplitudes varied in space and time, and showed decay and enhancement during the southwestward propagation, suggesting that amplitudes of atmospheric gravity waves and the interaction process between gravity waves and F region plasma were highly variable. It is found that spatial and temporal fluctuations of the 630 nm intensity are well correlated with those of GPS-TEC except for a certain period of time. The Scheffield University Plasmasphere Ionosphere Model (SUPIM) is used to obtain theoretical relationships between the 630 nm airglow intensity and GPS-TEC and between their fluctuation amplitudes. The results indicate that the fluctuation amplitudes observed in weak airglow regions are caused by an electron density fluctuation of about +/-20% occurring around an altitude of 250 km, where the 630 nm emission rate reaches a maximum, below the F layer peak altitude. Highly enhanced 630 nm intensity and GPS-TEC within a bright airglow region are due to an electron density enhancement of about 150% occurring at altitudes below 300 km.

DOI： 10.1186/BF03352420

Regional ionosphere map over Japanese Islands Reviewed

J Ping, Y Kono, K Matsumoto, Y Otsuka, A Saito, C Shum, K Heki, N Kawano

EARTH PLANETS AND SPACE Vol. 54 ( 12 ) page： E13 - E16 2002

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

Based on the high time and spatial resolution total electron content (TEC) data, which is estimated from the phase and code observables obtained by using GPS (Global Positioning System) Earth Observation Network (GEONET), the TEC distribution and its time variation over Japanese Islands are scaled into 0.5degrees x 0.5degrees grid data for each 10 minutes. The TEC daily map time series are arranged in an array to show the TEC evolution. Based on the spherical harmonics expansion of global ionospheric TEC model (GIM), which is estimated from global GPS observation, the TEC maps are expanded firstly through as high as 60 degrees and orders for a spherical harmonic function as a regional ionosphere map (RIM). The evolution history of medium scale traveling ionospheric disturbances (MSTIDs), i.e. polarward intense TEC enhancement and pre-noon rapid irregular fluctuations near the geomagnetic equator, are identified and confirmed in a quiet geomagnetic period.

DOI： 10.1186/BF03352450

Plasmaspheric electron content in the GPS ray paths over Japan under magnetically quiet conditions at high solar activity Reviewed

N Balan, Y Otsuka, T Tsugawa, S Miyazaki, T Ogawa, K Shiokawa

EARTH PLANETS AND SPACE Vol. 54 ( 1 ) page： 71 - 79 2002

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

Vertical total electron content (GPS-TEC) data obtained from the dual-frequency GPS receiver network (GEONET) in Japan are compared with those calculated using the Sheffield University plasmasphere-ionosphere model (SUPIM). The model is also used to estimate the electron content in the plasmaspheric sections of GPS ray paths for the three seasons of high solar activity (F10.7 = 165) under magnetically quiet conditions. According to the estimates, the plasmaspheric sections of vertical GPS ray paths over Japan at altitudes above the O+ to H+ transition height and above the upper altitude (2500 km) of Faraday rotation contain up to I I and 9 TEC units (I TEC unit = 1016 electrons m(-2)) of free electrons, respectively. The free electrons present above the Faraday rotation altitude can cause propagation errors of up to 4.9 ns in time delay and 1.6 m in range at the GPS L1 (1.57542 GHz) frequency. The plasmaspheric electron content, PEC, changes appreciably with season and latitude and very little with the time of the day. However, the percentage contribution of PEC to GPS-TEC changes most significantly with the time of the day; the contribution varies from a minimum of about 12% during daytime at equinox to a maximum of about 60% at night in winter.

DOI： 10.1186/BF03352423

Observations of traveling ionospheric disturbances and 3-m scale irregularities in the nighttime F-region ionosphere with the MU radar and a GPS network Reviewed

A Saito, M Nishimura, M Yamamoto, S Fukao, T Tsugawa, Y Otsuka, S Miyazaki, MC Kelley

EARTH PLANETS AND SPACE Vol. 54 ( 1 ) page： 31 - 44 2002

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

The nighttime traveling ionospheric disturbances (TIDs) and the F-region 3-m scale field-aligned irregularities were simultaneously observed with the MU radar and GEONET, a GPS network, during the FRONT (F-region Radio and Optical measurement of the Nighttime TID) campaign periods in May 1998 and August 1999. The vertical profile of electron density detected by the incoherent scatter observation of the MU radar clarified that ionized atmosphere on the bottomside of the ionospheric F-region was deeply modulated by TIDs. which would cause the variations of the 630 nm band airglow luminosity. The coherent echoes from the 3-m scale field-aligned irregularities were detected also on the bottomside of the F-region in the nights when TIDs were intense in amplitude and the ionosphere was uplifted, Two-dimensional structures of the field-aligned irregularities detected by the multi-beam observation of the MU radar revealed that the 3-m scale irregularities formed band-like structures and traveled to the southwest in several nights. Their wave vector and traveling velocity were coincident with those of the nighttime TIDs that were simultaneously detected by the TEC observation of GEONET. The intense Doppler velocities of the coherent echoes indicate that the polarization electric field is generated inside the TIDs. We consider that the horizontal gradient of the electric conductivity associated by TIDs and the vertical gradient of the conductivity on the bottomside of the F-region ionosphere generates the 3-m scale irregularities through the gradient-drift instability process. The anti-correlation of the occurrence rate of the F-region field-aligned irregularities to the solar activity would be caused by the anti-correlation of the amplitude of TIDs and of the vertical gradient of the Pedersen conductivity.

DOI： 10.1186/BF03352419

Annual and semiannual variations of the midlatitude ionosphere under low solar activity Reviewed

S. Kawamura, N. Balan, Y. Otsuka, S. Fukao

Journal of Geophysical Research: Space Physics Vol. 107 ( 8 ) 2002

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

The annual and semiannual variations of the midlatitude ionosphere under low solar activity are studied using middle and upper (MU) radar (135°E, 35°N) incoherent scatter observations and Sheffield University plasmasphere-ionosphere model (SUPIM). The variations of the daytime electron density (Ne) and electron and ion temperatures (Te and Ti) at 200-600 km altitudes measured by the radar under low solar activity (F10.7 ≤ 120) are satisfactorily reproduced for the first time by incorporating the radar measured values of the magnetic meridional neutral wind velocity (U 0) and northward perpendicular plasma drift velocity (V ⊥) into SUPIM that uses mass spectrometer incoherent scatter 1986 (MSIS-86) for neutral densities and neutral temperatures. The study shows that the annual and semiannual variations of the midlatitude ionosphere during daytime at altitudes near and above the ionospheric peak under low solar activity depend more on the direct effect of the neutral wind arising through the changes in ionospheric height than on the indirect effect arising through the changes in thermospheric composition (or atomic to molecular concentration ratio)
the indirect effect, however, predominates on the variations at altitudes below the ionospheric peak. The electron temperature (Te) undergoes similar but almost opposite seasonal and semiannual variations as the electron density. The ion temperature (Ti) is closer to neutral temperature than to electron temperature at altitudes up to ∼400 km and shows comparatively weak annual and semiannual variations. Copyright 2002 by the American Geophysical Union.

DOI： 10.1029/2001JA000267

Imaging observations of midlatitude ionospheric disturbances during the geomagnetic storm of February 12, 2000 Reviewed

Y Sahai, K Shiokawa, Y Otsuka, C Ihara, T Ogawa, K Igarashi, S Miyazaki, A Saito

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 106 ( A11 ) page： 24481 - 24492 2001.11

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

Regular all-sky imaging observations of the F region OI 630 nm nightglow emission are carried out at Rikubetsu (43.5 degreesN) and Shigaraki (34.8 degreesN), Japan since 1998. In this paper, we present observations of the OI 630 nm emission images from these two sites with a cluster of other observations during the geomagnetic storm of February 12, 2000. The airglow observations were obtained between 1500 UT and 2000 UT (0000-0500 LT) on this night. Images from both Rikubetsu and Shigaraki show the presence of mesoscale-enhanced airglow bands moving slowly (similar to 20-30 m s(-1)) to the southwest direction. The mesoscale band structures were also detected by an extensive network of GPS signal observations over Japan. The airglow observations at Shigaraki also show intense OI 630 nm emission in the south with several small-scale intensity depletion structures. We suggest that these small-scale intensity depletion structures (scale size 30-50 km) are caused by possible nonlinear interaction between the mesoscale TIDs and enhanced regions of the equatorial ionospheric anomaly. The ionosonde observations from Wakkanai (45.4 degreesN) and Kokubunji (35.7 degreesN) show spread F signatures between 1130 and 2100 UT and 1215 and 1715 UT, respectively, whereas no spread F was observed at Yamagawa (31.2 degreesN) and Okinawa (26.3 degreesN). The phase fluctuations of GPS signal were observed at latitudes higher than 30 degreesN for 1400-1900 UT. These observations suggest enhanced storm time ionospheric disturbances were confined in the midlatitudes.

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