Earth Planets Space, Vol. 61 (No. 4), pp. 397-410, 2009
Tadahiko Ogawa1, Yasunobu Miyoshi2, Yuichi Otsuka1, Takuji Nakamura3, and Kazuo Shiokawa1
1Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi 442-8507, Japan
2Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan
3Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Kyoto 611-0011, Japan
(Received October 5, 2007; Revised January 25, 2008; Accepted March 12, 2008; Online published May 14, 2009)
Using Global Positioning System (GPS) satellites, we have been conducting equatorial ionospheric scintillation observations at Kototabang, Indonesia since January 2003. Scintillations caused by equatorial plasma bubbles appear between 2000 and 0100 LT in equinoctial months with a seasonal asymmetry, and their activity decreases with decreasing solar activity. A comparison between scintillation index (S4) and Earth's brightness temperature (Tbb) variations suggests that the scintillation activity can be related to tropospheric disturbances over the Indian Ocean to the west of Kototabang. To understand better the reasons of day-to-day variability of S4, we analyze S4, Tbb and lower thermospheric neutral wind (u′2) data. The results show that S4 fluctuates with periods of about 2.5, 5, 8, 14 and 25 days, possibly due to atmospheric waves from below and that similar periods are also found in the Tbb and u′2 variations. Using a general circulation model, we made numerical simulations to determine the behavior of neutral wind in the equatorial thermosphere. The results indicate the following: (1) 2- to 20-day waves dissipate rapidly above about an altitude of 125 km, and 0.5- to 3-hour waves become predominant above 100 km, (2) zonal winds above 200 km altitude are, on the whole, eastward during sunset-sunrise, (3) zonal wind patterns due to short-period (1-4 h) atmospheric gravity waves (AGWs) above 120 km altitude change day by day, exhibit wavy structures with scale lengths of about 30-1000 km and, as a whole, move eastward at about 100-1 while changing patterns over time. These simulations suggest that the Rayleigh-Taylor instability responsible for plasma bubble generation can be seeded by AGWs with short periods of about 0.5-3 h, and that background conditions necessary for this instability are modulated by planetary-scale atmospheric waves propagating up to an altitude of about 120 km from below.
Key words: Equatorial ionosphere, GPS scintillation, plasma bubble, atmospheric wave, tropospheric disturbance.