FAST EQUATORIAL BUBBLES

Ion velocities associated with nighttime equatorial plasma depletions near 800 Irm altitude have been investigated utilizing data from the D efense Meteorological Satellite Program. Observations of upward ion dr ifts exceeding 800 m s(-1), within the depleted regions, are shown to have a strong dependence on solar zenith angle. Given the existence of bubble plasma, the probability of observing large upward drifts is as high as 40% just after sunset, when the solar zenith angle is 110 deg rees, but it decreases rapidly, falling below 5% for solar zenith angl es above 140 degrees. It is suggested that irregularity formation in t he bottom-side F region during a period of postsunset enhanced electri c fields will cause magnetic flux tubes of depleted plasma to accelera te upward, reaching a velocity maximum at apex heights corresponding t o the peak in the flux-tube-integrated conductivity of the back-ground plasma. In the presence of a postsunset enhanced electric field, this peak may exist at apex heights above 500 km.