Ionospheric height variations observed by ionosondes along magnetic meridian and plasma bubble onsets

Since October 2004, a Frequency Modulated– Continuous Wave (FM–CW) ionosonde chain along the magnetic meridian has been operating in Southeast Asia, in Kototabang (0.2 S, 100.3 E), Indonesia, Chumphon (10.7 N, 99.4 E), Thailand, and Chiang Mai (18.8 N, 98.9 E), Thailand. Variations in the virtual height of the bottomside of the F-region (hF ) at 2.5 MHz were analyzed, in order to study the day-to-day variability of plasma bubble occurrence for the periods of October 2004 and March–April 2005. When plasma bubbles were generated, hF was enhanced at the three stations. However, even when hF at the equatorial station, Chumphon, was largely enhanced, plasma bubbles were not generated when a noticeable north-south asymmetry of hF existed. This asymmetry could be attributed to the transequatorial thermospheric wind. Our results show that the strong transequatorial thermospheric wind can suppress the plasma bubble generation and is one of the important factors which controls the day-to-day variability of plasma bubble occurrences.