Earth Planets Space, Vol. 61 (No. 4), pp. 471-478, 2009
Yasunobu Miyoshi1 and Hitoshi Fujiwara2
1Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, 6-10-1, Hakozaki, Fukuoka, Japan
2Department of Geophysics, Faculty of Science, Tohoku University, Sendai, Japan
(Received September 27, 2007; Revised June 23, 2008; Accepted June 24, 2008; Online published May 14, 2009)
Using a general circulation model that contains the region from the ground surface to the upper thermosphere, we have examined characteristics of gravity waves in the equatorial thermosphere. At an altitude of 150 km, the dominant periods of gravity waves for zonal wave number 20 (zonal wavelength λx ≈ 2000 km), 40 (λx ≈ 1000 km) and 80 (λx ≈ 500 km) are 3, 1.5 and 1 h, respectively. For individual zonal wave numbers, the corresponding dominant period becomes shorter at higher altitudes due to dissipation processes in the thermosphere, such as molecular viscosity and ion drag force, indicating that gravity waves with a larger horizontal phase velocity (larger vertical wavelength) can penetrate into the thermosphere. The longitudinal variation of gravity wave activity in the equatorial thermosphere and upward propagation of gravity waves from the lower atmosphere were also studied. The longitudinal distribution of gravity wave activity in the thermosphere is quite similar to that of gravity wave activity in the lower atmosphere and the cumulus convective activity in the tropical troposphere. Our results indicate that the strong energy flux due to gravity waves from the enhanced cumulus convective activity propagates upward into the upper thermosphere. The relation between the wind fluctuation associated with gravity waves and the ionospheric variation is discussed. Fluctuations of the neutral zonal wind with periods of 1-2 h are significant in the 200- to 300-km height region, and its amplitude sometimes exceeds 50 m s-1. These results suggest that upward propagating gravity waves can affect the ionospheric variation in the F-region.
Key words: Gravity waves, equatorial thermosphere, upward propagation, wind fluctuation, ionospheric variation.