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 bottomside 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.
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A 1997 study studied this question.