MOTION OF DEPLETED PLASMA REGIONS IN THE EQUATORIAL IONOSPHERE

Depleted plasma regions in the equatorial ionosphere and their associated motions have been observed by a variety of ground-based, rocket-borne, and satellite instruments. Various theories, based primarily on the Rayleigh-Taylor instability mechanism, have been proposed for the formation and motion of these bubbles. With the realization that bubbles are actually depleted magnetic flux tubes, the vertical E multiplied by B motion of these depleted regions are investigated. The resulting calculations show that the vertical bubble velocity as a function of time critically depends on the background ionospheric electric field and that this dependence extends to much greater heights than was previously thought. Bubbles which are initiated at 350-km altitude (1900 LT) with a 5% depletion in electron content attain an upward velocity of 200 m/s at 1920 LT when the background electric field is 0. 6 mV/m. Bubble altitude at this time is 447 km with an 88% depletion in electron content. In the absence of an ambient electric field, 1 hour is required for the vertical bubble velocity to reach 200 m/s.