Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

Division display 441 - 460 of about 496 ／ All the affair displays >>

Spatial relationship of equatorial plasma bubbles and field-aligned irregularities observed with an all-sky airglow imager and the Equatorial Atmosphere Radar Reviewed

Y Otsuka, K Shiokawa, T Ogawa, T Yokoyama, M Yamamoto, S Fukao

GEOPHYSICAL RESEARCH LETTERS Vol. 31 ( 20 ) 2004.10

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

We report, for the first time, simultaneous two-dimensional observations of 630-nm airglow depletions and radar backscatter from field-aligned irregularities (FAI) associated with equatorial plasma bubbles. Spatial distributions of backscatter were obtained by performing east-west scans with the 47-MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia. A 630-nm airglow depletion, caused by a plasma bubble, was simultaneously observed with an all-sky airglow imager. Both the airglow depletion and backscatter region appeared as band-like structure elongated in the meridional direction with a zonal width of about 100 km. To compare the spatial structures of backscatter with that of airglow depletion, the backscatter was projected onto a horizontal plane at 250-km altitude. Backscatter was found to occur within the entire airglow-depleted region.

DOI： 10.1029/2004GL020869

Web of Science

Statistical characteristics of gravity waves observed by an all-sky imager at Darwin, Australia Reviewed Open Access

Shin Suzuki, K. Shiokawa, Y. Otsuka, T. Ogawa, P. Wilkinson

Journal of Geophysical Research D: Atmospheres Vol. 109 ( D20 ) page： D20 - S07 2004.8

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

An all-sky airglow imager with a cooled charge-coupled device camera in place at Darwin (12.4°S, 131.0°E), Australia, since October 2001 has been used to obtain two-dimensional gravity wave images in the mesopause region. Using airglow images of the OI (557.7 nm, emission altitude ∼96 km) and OH band (720-910 nm, ∼86 km) emissions obtained for October 2001 to August 2002, we investigated the wavelengths, phase velocities, and propagation directions of gravity waves. Wave occurrence in OH images (60-90%) is higher than in OI images (30-70%) for all seasons. The waves have wavelengths of less than 90 km (peak: 30-50 km) and phase velocities ofless than 90 m/s (peak: 30-60 m /s). Most of the waves propagate in the meridional direction, and the directionality strongly depends on the season. In winter, waves propagate both poleward and equatorward, while in summer almost all waves propagate poleward. An examination of airglow-imaging statistics at Adelaide (35°S, 138°E), Australia, obtained by Walterscheid et al. [1999], leads us to conclude that this clear directionality is caused by the location of the wave sources and by the wave ducting processes
that is, poleward waves in summer come from an equatorial convective source through a thermal duct structure. The effect of wind filtering on the waves is also discussed for zonal wave propagation. Copyright 2004 by the American Geophysical Union.

DOI： 10.1029/2003JD004336

Web of Science

Scopus

Geomagnetic conjugate observations of medium-scale traveling ionospheric disturbances at midlatitude using all-sky airglow imagers Reviewed

Y Otsuka, K Shiokawa, T Ogawa, P Wilkinson

GEOPHYSICAL RESEARCH LETTERS Vol. 31 ( 15 ) 2004.8

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

We report for the first time simultaneous observations of medium-scale traveling ionospheric disturbances (MSTIDs) at geomagnetic conjugate points in both hemispheres, using two all-sky airglow imagers at midlatitudes. A 630-nm all-sky CCD imager at Sata, Japan, detected MSTIDs with a wavefront elongated from NW to SE on the night of August 9, 2002. During this event, MSTIDs with a wavefront elongated from SW to NE were observed at the geomagnetic conjugate point, Darwin, Australia. To investigate geomagnetic conjugacy of the MSTID structures, the Darwin images were mapped The MSTID structures mapped from Darwin to its magnetic conjugate points along the geomagnetic field lines ( B) coincide closely with those in the Sata images. This result suggests that polarization electric field (E-p) plays an important role in the generation of MSTIDs. E-p maps along B and moves the F region plasma upward or downward by E x B drifts, causing plasma density perturbations with structures mirrored in the northern and southern hemispheres.

DOI： 10.1029/2004GL020262

Web of Science

Comparison of OH rotational temperatures measured by the spectral airglow temperature imager (SATI) and by a tilting-filter photometer Reviewed

K Shiokawa, Y Otsuka, T Ogawa, H Takahashi, T Nakamura, T Shimomai

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS Vol. 66 ( 11 ) page： 891 - 897 2004.7

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Measurements of OH rotational temperatures have been carried out at Shigaraki, Japan(34.8degreesN, 136.1degreesE), for 8 nights in September-December:2000, using the Spectral Airglow Temperature Imager (SATI) and a tilting-filter zenith photometer (MC4), simultaneously. Clear positive correlation was obtained between the two temperatures. However, the absolute temperatures obtained by SATI were unusually low (similar to130-180 K) with differences of similar to60 K from the MC4 temperatures. We discuss possible causes of the temperature differences, particularly for SATI, which newly uses OH(6-2) Q-branch lines (MC4 uses OH(6-2) P-branch lines) and imaging optics for the temperature measurement. (C) 2004 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2004.03.003

Web of Science

A statistical study of large-scale traveling ionospheric disturbances using the GPS network in Japan Reviewed Open Access

Tsugawa, T, Saito, A, Otsuka, Y

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 109 ( A6 ) page： A06302 2004.6

Simultaneous mesosphere/lower thermosphere and thermospheric F region observations during geomagnetic storms Reviewed

Balan, N, Kawamura, S, Nakamura, T, Yamamoto, M, Fukao, S, Igarashi, K, Maruyama, T, Shiokawa, K, Otsuka, Y, Ogawa, T, Alleyne, H, Watanabe, S, Murayama, Y

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 109 ( A4 ) 2004.4

*Geomagnetic conjugate observations of medium-scale traveling ionospheric disturbances at midlatitude using all-sky airglow imagers Reviewed

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

Geophys. Res. Lett. Vol. 31 ( L15803 ) page： doi:10.1029/2004GL020262 2004

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

We report for the first time simultaneous observations of medium-scale traveling ionospheric disturbances (MSTIDs) at geomagnetic conjugate points in both hemispheres, using two all-sky airglow imagers at midlatitudes. A 630-nm all-sky CCD imager at Sata, Japan, detected MSTIDs with a wavefront elongated from NW to SE on the night of August 9, 2002. During this event, MSTIDs with a wavefront elongated from SW to NE were observed at the geomagnetic conjugate point, Darwin, Australia. The MSTID structures mapped from Darwin to its magnetic conjugate points along the geomagnetic field lines B coincide closely with those in the Sata images. This result suggests that polarization electric field (Ep) plays an important role in the generation of MSTIDs. Ep maps along B and moves the F region plasma upward or downward by ExB drifts, causing plasma density perturbations with structures mirrored in the northern and southern hemispheres.

Spatial relationship of equatorial plasma bubbles and field-aligned irregularities observed with an all-sky airglow imager and the Equatorial Atmosphere Radar Reviewed

Otsuka, Y., K. Shiokawa, T. Ogawa, T. Yokoyama, M. Yamamoto, and S. Fukao

Geophys. Res. Lett. Vol. 31 ( L20802 ) page： 10.1029/2004GL020869 2004

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

We report, for the first time, simultaneous two-dimensional observations of 630-nm airglow depletions and radar backscatter from field-aligned irregularities (FAI) associated with equatorial plasma bubbles. Spatial distributions of backscatter were obtained by performing
east-west scans with the 47-MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia. A 630-nm airglow depletion, caused by a plasma bubble, was simultaneously
observed with an all-sky airglow imager.
Both the airglow depletion and backscatter region appeared as band-like structure elongated in the meridional direction with a zonal width of about 100 km. To compare the spatial structures of backscatter with that of airglow depletion,
the backscatter was projected onto a horizontal plane at 250-km altitude. Backscatter was found to occur within the entire airglow-depleted region.

DOI： 10.1029/2004GL020869

Generation of large-scale equatorial F-region plasma depletions during low range spread-F season Reviewed Open Access

Y Sahai, PR Fagundes, Abalde, JR, AA Pimenta, JA Bittencourt, Y Otsuka, VH Rios

ANNALES GEOPHYSICAE Vol. 22 ( 1 ) page： 15 - 23 2004

Language：English Publishing type：Research paper (scientific journal) Publisher：EUROPEAN GEOPHYSICAL SOC

All-sky imaging observations of the F-region OI 630nm nightglow emission allow us to visualize large-scale equatorial plasma depletions, generally known as trans- equatorial plasma bubbles. Strong range type spread-F is the radio signature of these (magnetically) north-south aligned plasma depletions. An extensive database of the 01 630nm emission all-sky imaging observations has been obtained at Cachoeira Paulista (22.7degreesS, 45.0degreesW; dip latitude similar to16degreesS), Brazil, between the years 1987 and 2000. An analysis of these observations revealed that relatively few large-scale ionospheric plasma depletions occur during the months of May to August (southern winter, June solstice) in the Brazilian sector. Of the few that are observed during, these months, some occur in association with geomagnetic storms and some do not. In this paper, a detailed analysis of the events when large-scale ionospheric plasma depletions were initiated and evolved during the June solstice periods are presented and discussed.

DOI： 10.5194/angeo-22-15-2004

Open Access

Web of Science

Time evolution of high-altitude plasma bubbles imaged at geomagnetic conjugate points Reviewed Open Access

K Shiokawa, Y Otsuka, T Ogawa, P Wilkinson

ANNALES GEOPHYSICAE Vol. 22 ( 9 ) page： 3137 - 3143 2004

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

Temporal and spatial evolution of two high-altitude plasma bubbles (evening and midnight) was observed on 4 April 2002, at geomagnetic conjugate points at Sata, Japan (magnetic latitude 24degrees N), and Darwin, Australia (magnetic latitude 22degrees S), using two 630-nm airglow imagers. The apex height of the bubbles reached similar to1500km. The upward velocity of the evolution was faster in the evening (similar to170m/s at 20:00-21:00 LT) than around midnight (similar to28 m/s at 23:00-00:00 LT). Bifurcating features of the bubbles into a smaller scale size of similar to50 km were clearly seen for both the evening and midnight bubbles, showing fairly good conjugacy between the Northern and Southern Hemispheres.

DOI： 10.5194/angeo-22-3137-2004

Open Access

Web of Science

Ray-tracing Calculation of VHF Radio Waves Scattered by Field-aligned Irregularities Associated with Equatorial Plasma Bubbles Reviewed

Hiroyuki Nakata, Yoshiaki Akaike, Yuichi Otsuka, Toshiaki Takano, Satoshi Ujigawa, Ikuo Nagashima

IEEJ Transactions on Fundamentals and Materials Vol. 124 ( 12 ) page： 1253 - 1254 2004

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

In order to explain the propagation of VHF radio waves for TV broadcasting transmitted from Southeast Asia associated with equatorial plasma bubbles, we have examined ray paths of the radio waves scattered by field-aligned irregularities in equatorial plasma bubbles. In determining the ray paths of the radio waves, a ray tracing calculation combined with a model of the scattering by field-aligned irregularities is used. It is found that VHF radio waves transmitted from Philippines can propagate to Japan due to scattering by field-aligned irregularities located above the East China Sea. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.124.1253

Scopus

Thermospheric wind during a storm-time large-scale traveling ionospheric disturbance Reviewed Open Access

K Shiokawa, Y Otsuka, T Ogawa, S Kawamura, M Yamamoto, S Fukao, T Nakamura, T Tsuda, N Balan, K Igarashi, G Lu, A Saito, K Yumoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 108 ( A12 ) 2003.12

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

[1] A prominent large-scale traveling ionospheric disturbance (LSTID) was observed in Japan during the major magnetic storm (Dst similar to -358 nT) of 31 March 2001. It was detected as enhancements of the 630-nm airglow and foF2, GPS-TEC variations, and a decrease in F-layer virtual height at 1700 - 1900 UT (0200 - 0400 LT). It moved equatorward with a speed of similar to600 m/s. The decrease in the F-layer height was also detected by the MU radar at Shigaraki. Thermospheric wind variations were observed by the MU radar through ion drift measurement and by a Fabry-Perot interferometer (FPI) through a Doppler shift of the 630-nm airglow line at Shigaraki. The wind data show a turn of the meridional wind from -94 m/s (equatorward) to + 44 m/s (poleward) during the LSTID, indicating that an intense poleward wind in the thermosphere passed over Shigaraki as an atmospheric gravity wave and caused the observed ionospheric features of the LSTID. Intense poleward wind was also detected at mesospheric altitudes (95 - 100 km) by the MU radar ( through meteor echoes) and by the FPI ( through the 558-nm airglow) with a delay of similar to2 hours from the thermospheric wind, indicating downward phase progression of the wave. Generation of the observed poleward wind in the auroral zone was investigated using magnetic field data and auroral energy input estimated by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. We suggest that simple atmospheric heating and/or the Lorentz force in the auroral zone do not explain the observed poleward wind enhancement.

DOI： 10.1029/2003JA010001

Web of Science

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

Web of Science

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

Web of Science

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

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

Web of Science

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

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

Web of Science

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 Open Access

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

Open Access

Web of Science

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

Web of Science

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

Web of Science

　 PREV - NEXT 　

<OTSUKA, Yuichi>

Papers - OTSUKA, Yuichi