On near-tail bubble penetration into geosynchronous altitude

Abstract

Dipolarization of the magnetic field at the near-Earth tail is usually associated with the local reduction of pV5/3 compared to that of the background, where p is the plasma pressure and V is the volume of the unit magnetic flux tube. This can be interpreted as a bubble, which can propagate earthward by the interchange process. How deep such a bubble can penetrate earthward, and what is the critical factor are critical questions that need to be answered. In this paper, we examine these issues by comparing near-tail observations by inner probes of THEMIS with geosynchronous magnetic observations by GOES. We identified a number of bubble events associated with near-tail dipolarization, which we call "tail bubble," and checked geosynchronous disturbances. We find a statistical trend that geosynchronous disturbance is more likely to occur when associated with (or when hit by) an earthward moving tail bubble with a more-depleted pV5/3. We estimated the background pV5/3 profile statistically and used it to determine expected equilibrium (or stop) positions for earthward moving bubbles where the bubble's pV5/3 is equal to that of the background. Statistically, we find that the equilibrium position is more inward for tail bubbles with a lower pV5/3, for which the probability of causing geosynchronous disturbance is higher. For example, the probability of a tail bubble being associated with geosynchronous disturbance is 75% if the bubble's equilibrium position is <8 RE. However, for all the events studied here, the bubble equilibrium positions are still outside the geosynchronous altitude. Although this result may be subject to change due to the uncertainty in estimating p V5/3 and the limited number of the events identified near geosynchronous altitude, we suggest that an overshooting of the penetrating bubbles beyond equilibrium positions is a possible explanation. © 2012. American Geophysical Union.