F-Region Plasma Distribution seen from TIMED/GUVI and its Effect on the Equatorial Spread F Activity

The nighttime plasma distribution in the low-latitude F region was investigated using the measurements of O I 135.6-nm intensity from the Global Ultraviolet Imager (GUVI) on board TIMED satellite. The 135.6-nm disk scan images of the Earth showed the equatorial ionization anomaly (EIA) features around +/-12^o-15^o magnetic latitudes. The EIA strength was maximum during equinox periods and minimum during northern summer at most of longitude regions. The most distinguishing feature was a suppression in 135.6-nm intensity in the southern American-Atlantic sector during northern summer. Those observations indicate an existence of season-longitudinal variations in the F-region plasma density that may affect the equatorial spread F (ESF) activity. That is, it is suggested that an occurrence of low plasma density is responsible for the suppressed ESF activity during northern summer in the American-Atlantic sector, whereas an occurrence of high plasma density is responsible for the strong ESF activity during equinox periods in most of longitude regions. We will further investigate the growth condition of ESF by calculating the growth rate of Rayleigh-Taylor instability using the GUVI limb data.