DOWNDRAFTING PLASMA-FLOW IN EQUATORIAL BUBBLES

The electric field experiment carried aboard the San Marco D equatoria l ionospheric satellite regularly measured updrafting in plasma deplet ion channels or ''equatorial bubbles'' which form on the bottomside of the nightside equatorial F region. We report here observations of dow ndrafting vertical plasma velocities inside such depletion regions in the nightside equatorial ionosphere. Both updrafting and downdrafting motions can be expected on the basis of a generalized gradient drift/c ollisional Rayleigh-Taylor instability process in the ionospheric F re gion. Although the gravitation can only drive upward plasma flow in pl asma depletion regions, both background westward zonal electric fields and upward vertical neutral winds can cause an occurrence of downdraf ting (i.e., a downward motion of the plasma within the bubble) if thos e parameters are strong enough. We show that as the background zonal e lectric field becomes westward (often after approximately 2100 LT) in the equatorial ionosphere, the plasma interior to an existing bubble a t altitudes of approximately 400 km and less at the magnetic equator m ay assume a downdrafting motion, while at higher altitudes in the same bubble channel, the plasma flow remains upward. Such a simultaneous o ccurrence of the updrafting and downdrafting plasma flow in a single b ubble channel may lead to the pinching off of the upper part of the de pletion region from the lower altitude regions, causing the decay of a bubble or the formation of a ''dead'' bubble.