Satellite Studies of Quiet and Disturbed Equatorial Vertical Plasma Drifts

The generation and evolution of equatorial plasma density structures and spread F have been important topics of ionospheric research for over four decades. Recent satellite studies have confirmed the fundamentally important role of the equatorial vertical plasma drift velocity on the generation and longitudinal distribution of equatorial spread F during geomagnetically quiet times. We use vertical plasma drift observations on board of the ROCSAT- 1 satellite during quiet and disturbed conditions to illustrate the strong longitudinal variations of the equatorial evening and nighttime drifts and their strong dependence on the ionospheric conductivity. The evening prereversal velocity enhancements have strongest longitudinal variations in the American sector during December solstice. Our data suggest that the equatorial prompt penetration electric fields resulting from sudden increases in the high latitude convection field are season dependent and have largest magnitudes during June solstice. The disturbance dynamo electric fields generate largest evening downward drift perturbations during equinox; the postmidnight upward disturbance dynamo drifts do change much with season. These empirical perturbation electric field patterns will be compared with results from numerical models.