Statistical Features of Fast Rising Ionospheric Bubbles From ROCSAT- 1 Observations

A large set of F-region ionospheric irregularities (bubbles) at low latitudes has been detected b y the Ionospheric Plasma a nd Electrodynamics Instrument (IPEI) onboard the ROCSAT-1 satellite during March 1999 to June 2004. Over those years IPEI has been collecting d ata by alternating operations of the NORMAL mode with a sampling rate of 32 Hz and the FAST/AUTO mode with a sampling rate of 1024 Hz. The simultaneously high-resolution d ata of ion d ensity and two cross-track ion velocity components allow us, never before from satellite ion drift data, to study the spectral characteristics of the irregularities to the smallest scale of 15 m along the satellite track. Here, we c oncentrate on the bubble structures, within which the ion upward motions exceed 500 m/s. We report some statistical features associated with these fast rising bubble (FRB) events. In particular, we a re interested in the followings: (1) the pre-conditionings s uch as horizontal density gradients and p re- reversal upward drift enhancements that trigger instabilities thus FRB structures; (2) the dependencies of their occurrences and sizes on local time, longitude (geomagnetic field configuration), season, as well as the e volution of storm-time disturbed electric fields; and (3) the detailed spectral features such as spectral index, wavelengths of spectral breaks that differentiate various wave modes, and the correlation b etween density and velocity spectra for the FRB events. Based on these characteristic features we hope to identify the underlying processes (instabilities) that