Ion velocities associated with nighttime equatorial plasma depletions near 800 km altitude have been investigated utilizing data from the Defense Meteorological Satellite Program. Observations of upward ion drifts 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¿, but it decreases rapidly, falling below 5% for solar zenith angles above 140¿. It is suggested that irregularity formation in the bottom-side F region during a period of postsunset enhanced electric fields will cause magnetic flux tubes of depleted plasma to accelerate upward, reaching a velocity maximum at apex heights corresponding to the peak in the flux-tube-integrated conductivity of the background plasma. In the presence of a postsunset enhanced electric field, this peak may exist at apex heights above 500 km.¿ 1997 American Geophysical Union