VERTICAL NEUTRAL WIND IN THE EQUATORIAL F-REGION DEDUCED FROM ELECTRIC-FIELD AND ION DENSITY-MEASUREMENTS

DC electric field and ion density measurements near density depletion regions (that is, equatorial plasma bubbles) are used to estimate the vertical neutral wind speed. The measured zonal electric field in a se ries of density depletions crossed by the San Marco D satellite at 01. 47-01.52 UT on 25 October 1988, can be explained if a downward neutral wind of 15-30 m s(-1) exists. Simultaneously, the F-region plasma was moving downward at a speed of 30-50 m s(-1). These events appear in t he local lime sector of 23.00-23.15 in which strong downward neutral w inds may occur. Indeed, airglow measurements suggest that downward neu tral velocities of 25-50 m s(-1) are possible at times near midnight i n the equatorial F-region.