| dc.description.abstract | Quiet time ion density depletions observed by the ROCSAT-1 IPEI payload during the solar maximum years of 2000-2002 were used to statistically study the effects of many parameters which are responsible for the generation of plasma bubbles. We examine how the occurrence rates of plasma depletions (bubbles) vary with local time, season, longitude, solar activity, magnetic activity, local magnetic declination, and weather system.
Based on the morphology of plasma bubbles, we investigate the dependencies of bubble occurrence rates on season, latitude, longitude, local time, solar flux activity of topside ionosphere. In particular, we focus on the SAA longitude sector (270°E-360°E) and the Pacific longitude sector (150?E - 240?E), where the seasonal and longitudinal variations of the bubbles are most pronounced. The characteristic differences between the two longitude sectors will be helpful to reveal the mechanisms that generate plasma bubbles.
We have examined the relationship between plasma depletions and upward ion drifts and found that the time of bubble onset is typically at local time ~ 1900 LT, about half to one hour after the post-sunset enhancement. We further studied the correlation between the pre-reversal peaks and the local times for all seasons to determine the threshold values of the upward ion drift (? 20 m/s) required for the occurrence of large-scale bubble structures.
By investigation of the relationship between topside ionospheric plasma depletions and Inter-Tropical Convergence Zone (ITCZ), we found that the maximum occurrences of seasonal bubble structure are almost all collocated with the most intense rainfall regions of ITCZ. Such spatial correlation exists not only in the seasonal averaged data but also in the monthly averaged patterns, which is demonstrated by comparing the monthly bubble occurrence pattern of 2001 January with that of 2002 January. Significant differences in the bubble morphology between the two months were found to closely relate to the precipitation anomalies along the northeast coast of Brazil during the two different years, which provides further evidence that low altitude atmosphere can significantly affect the occurrence of the topside ionospheric plasma bubbles. The positive correlations among the spatial distributions of ion density, ion velocity and ITCZ suggest that both the location of ITCZ and the magnitude of upward ion drift (> 20 m/s) are the main factors to affect the occurrence of large-scale bubbles at early post sunset hours. | en_US |