On near-tail bubble penetration into geosynchronous altitude

摘要

Dipolarization of the magnetic field at the near-Earth tail is usually associated with the local reduction of p ~(V5/3)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 p ~(V5/3). We estimated the background p ~(V5/3)profile statistically and used it to determine expected equilibrium(or stop)positions for earthward moving bubbles where the bubble's p ~(V5/3)is equal to that of the background. Statistically, we find that the equilibrium position is more inward for tail bubbles with a lower p ~(V5/3), 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.