Longitudinal variability of annual changes in activity of equatorial spread <i>F</i> and plasma bubbles

Global distribution maps of equatorial spread F (ESF) activity for various annual periods were derived from the topside soundings by the Ionosphere Sounding Satellite b (ISS‐b) in 1978–1980. The ESF activity during the northern winter period reveals maximum enhancement at the Atlantic longitudes of large westward geomagnetic declination, and during the northern summer at the Pacific longitudes of large eastward declination. On several orbits passing over the region of the ESF activity enhancement, abrupt depletions of the electron density or equatorial plasma bubbles were observed at the satellite altitude of about 1100 km. It has also been found that the background electron density distributions tend to be symmetric with respect to the magnetic equator in the region of the enhanced ESF activity and asymmetric in the region of the suppressed ESF activity. These observations are taken account of by the influence of a transequatorial thermospheric wind upon the suppression of the Rayleigh‐Taylor type plasma instability.