研究者詳細 - 大山　伸一郎

論文 - 大山　伸一郎

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Relations between proton auroras, intense electric field and ionospheric electron density depletion 査読有り

Fujii, R., Y. Iwata, S. Oyama, S. Nozawa, and Y. Ogawa

J. Geophysical Research 頁： 114, A09304, doi:10.1029/2009JA014319 2009年9月

Spatial evolution of frictional heating and the predicted thermospheric-wind effects in the vicinity of an auroral arc measured with the Sondrestrom incoherent-scatter radar and the Reimei satellite 査読有り

Oyama, S., T. T. Tsuda, T. Sakanoi, Y. Obuchi, K. Asamura, M. Hirahara, A. Yamazaki, Y. Kasaba, R. Fujii, S. Nozawa, and B. J. Watkins

J. Geophysical Research 114 巻 ( A07311 ) 頁： doi:10.1029/2009JA014091 2009年7月

Acceleration mechanism of high-speed neutral wind observed in the polar lower thermosphere 査読有り

Tsuda, T. T., S. Nozawa, S. Oyama, T. Motoba, Y. Ogawa, H. Shinagawa, N. Nishitani, K. Hosokawa, N. Sato, M. Lester, and R. Fujii

J. Geophysical Research 2009年4月

*Generation of the lower-thermospheric vertical wind estimated with the EISCAT KST radar at high latitudes during periods of moderate geomagnetic disturbance 査読有り

Oyama, S., B. J. Watkins, S. Maeda, H. Shinagawa, S. Nozawa, Y. Ogawa, A. Brekke, C. Lathuillere, and W. Kofman

Ann. Geophysicae 26 巻頁： 1491-1505 2008年6月

Excitation and diagnosis of cascading Langmuir waves in ionospheric plasmas at Gakona, Alaska, 査読有り

Burton, L. M., J. A. Cohen, R. Pradipta, A. Labno, M. C. Lee, O. Batishchev, D. L. Rokusek, S. P. Kuo, B. J. Watkins, and S. Oyama

Phys. Scr. 2008年

Temporal evolution of pump beam self-focusing at the High-Frequency Active Auroral Research Program 査読有り

Kosch M. J., T. Pedersen, E. Mishin, M. Starks, E. Gerken-Kendall, D. Sentman, S. Oyama, B. Watkins

J. Geophysical Research 2007年8月

Coordinated optical and radar observations of ionospheric pumping for a frequency pass through the second electron gyroharmonic at HAARP 査読有り

Kosch M. J., T. Pedersen, E. Mishin, S. Oyama, J. Hughes, A. Senior, B. Watkins, B. Bristow

J. Geophysical Research 2007年6月

A two-dimensional simulation of thermospheric vertical winds in the vicinity of an auroral arc 査読有り

Shinagawa, H., and Oyama, S

Earth Planets and Space 58 巻 ( 9 ) 頁： 58(9), 1173-1181 2006年9月

Persistent enhancement of the HF pump-induced plasma line measured with a UHF diagnostic radar at HAARP 査読有り

Oyama, S., B. J. Watkins, F. T. Djuth, M. J. Kosch, P. Bernhardt, and C. J. Heinselman

J. Geophysical Research 111 巻 ( A06309 ) 頁： 2005JA011363 2006年6月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

Plasma lines excited by a powerful, high-frequency (HF) radio wave are studied using
data obtained with an ultrahigh frequency (UHF) radar at HAARP (High Frequency
Active Auroral Research Program) from 3 to 5 February 2005. Of particular interest is
persistent enhancement of the radar backscatter power during HF on at several HF
frequencies. The persistent enhancement is induced with the HF frequency slightly lower
than foF2 by a few hundred kHz; by contrast the persistent enhancement does not
appear when the HF frequency is equal to and higher than foF2 or lower than foF2 by
more than 500 kHz. When persistent enhancements of the radar backscatter power
appear, two case studies show that the local plasma frequency at the reflection height of
the O-mode polarization wave is close to the second or third electron gyroharmonic
frequencies, but one case study shows that the local plasma frequency at the reflection
height is significantly different from the third electron gyroharmonic frequency.

DOI： 10.1029/2005JA011363

*Application of a new beam configuration to estimate lower thermospheric vertical velocities at high latitudes with mono-static incoherent scatter radars 査読有り

Oyama, S., B. J. Watkins, S. Maeda, and J. Watermann

Radio Science 40 巻 ( RS4005 ) 頁： 2004RS003205 2005年7月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

We have developed a new beam configuration for monostatic incoherent scatter (IS)
radars at high latitudes in order to estimate the vertical component of the neutral wind
velocity in the lower thermosphere (from 100 to 120 km). This method has been applied to
experiments with the Sondrestrom IS radar, Greenland, on 5 and 8 March 2004. The
measurement and the analysis methods provide various temporal and altitudinal structures
of the vertical neutral wind velocity with amplitude of a few tens of meters per second at
3 km height resolution. This paper also demonstrates how to evaluate assumptions
adopted in calculations of the vertical neutral wind velocity using experimental data on
5 and 8 March 2004. Caution should be exercised in inferring effects of variations in
the thermospheric density or the ion-neutral collision frequency in association with
vertical thermospheric motions, in particular above 112 km.

DOI： 10.1029/2004RS003205

*Vertical ion motions observed with incoherent-scatter radars in the polar lower ionosphere 査読有り

Oyama, S., B. J. Watkins, S. Nozawa, S. Maeda, and M. Conde

J. Geophysical Research 110 巻 ( A04302 ) 頁： 2004JA010705 2005年4月

Summer-winter dependences of day-night differences in the ion temperature in the polar upper F region 査読有り

Oyama, S., C. Lathuillere, S. Maeda, and B. J. Watkins

Geophysical Research Letter 31 巻 ( L05806 ) 頁： 2003GL018820 2004年3月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

The daily variation of ion temperature observed with the
European Incoherent Scatter (EISCAT) Tromsø UHF radar in
the upper F region (from 300 to 500 km) has been found to
be significantly dependent on season. Of particular interest
is the statistics that the ion temperature at local night is
higher than that at local daytime for near solar maximum
summer conditions; by contrast no differences for winter.
Using the ion energy equation estimates of the heat
exchange with electrons suggest these local time and
seasonal differences. A case study using summer-time
data from the Sondrestrom incoherent-scatter radar
(ISR) also shows local-time dependences similar to
that observed with the Tromsø ISR. The case study
using a summer-time data from the EISCAT Svalbard
radar (ESR) shows the opposite sense. These seasonal
characteristics of the ion temperature that depend on the
observation site suggest that the day-night difference of ion
temperature in the polar upper F region has complicated
magnetic-latitudinal dependences.

DOI： 10.1029/2003GL018820

The quasi 2-day wave observed in the polar mesosphere (2): Comparison of the characteristics observed at Tromso and Poker Flat 査読有り

Nozawa, S., H. Iwahashi, A. Brekke, C. M. Hall, C. Meek, A. Manson, S. Oyama, Y. Murayama, and R. Fujii

J. Geophysical Research 頁： 108 , 101029/2002JD003221 2003年12月

Thermospheric and ionosphric dynamics in the auroral region 査読有り

Shinagawa, H., S. Oyama, S. Nozawa, S. C. Buchert, R. Fujii, and M. Ishii

Advances in Space Research 31 巻 ( 4 ) 頁： 951-956 2003年9月

Field-aligned ion motions in the polar E-F transition region: Mean characteristics 査読有り

Oyama, S., S. Nozawa, S. Maeda, Y. Murayama, R. Fujii, and H. Shinagawa

J. Geophysical Research 頁： 108(A8), 1334, doi:10.1029/2003JA009830 2003年8月

The quasi 2-day wave observed in the polar mesosphere 査読有り

Nozawa, S., S. Imaida, A. Brekke, C. Hall, C. Meek, A. Manson, S. Oyama, K. Dobashi, and R. Fujii

J. Geophysical Research 頁： 108, 10.1029/2002JD002440 2003年1月

A comparison study of the auroral lower thermopsheric neutral winds derived by the EISCAT UHF radar and the Tromso MF radar 査読有り

Nozawa, S., A. Brekke, A. Manson, C. M. Hall, C. Meek, K. Morise, S. Oyama, K. Dobashi, and R. Fujii

J. Geophysical Research 頁： 107, 10.1029/2000JA007581 2002年8月

Field-aligned ion motions in the E and F region 査読有り

Fujii, R., S. Oyama, S. C. Buchert, S. Nozawa, and N. Matuura

J. Geophysical Research 頁： 107, 10.1029/ 2001JA900148 2002年5月

Effects of auroral arc on the generation of atmospheric gravity waves in the polar F-region 査読有り

Oyama, S., H. Shinagawa, S. C. Buchert, R. Fujii, S. Nozawa, M. Ishii, Y, Murayama, and W. Kofman

Advances in Space Res. 27 巻頁： 1767-1772 2001年10月

*Generation of atmospheric gravity waves associated with auroral activity in the polar F region 査読有り

Oyama, S., M. Ishii, Y. Murayama, H. Shinagawa, S. C. Buchert, S. Nozawa, R. Fujii, and W. Kofman

J. Geophysical Research 106 巻頁： 18,543-18,554 2001年2月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

Relationships between the generation of atmospheric gravity waves and
auroral activity were investigated using data on neutral
oscillations, at periods of 20-30 min,
estimated from European Incoherent Scatter radar data in the polar
F-region (150-300 km).
The equation for the neutral dispersion relation gave
the values of the wave parameters,
such as the
phase velocity and wavelength, applicable to the estimated oscillations.
These values were used to estimate the distance between the observation
position and the region where the oscillations were generated.
The estimated distance superimposed on all-sky images
taken at Kilpisjarvi suggests
that the auroral arc extended in an almost zonal direction related to
the generation of the oscillations.
The oscillations were compared with ones calculated
using the Francis model.
The phase surface of the modeled atmospheric gravity waves showed fairly
good agreement with that of the observed oscillations,
suggesting that the estimated distance is acceptable
for reproducing the estimated oscillations with the model.
It is concluded that auroral activities significantly relate to the AGW
generation in the polar thermosphere as investigated with theoretical
studies.

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