Polar cap deflation during magnetospheric substorms

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Abstract
The expanding/contracting polar cap model has been used to simulate DE 2 ion drift data during substorms as determined using the AL index. Of the 39 cases modeled, 57% required the opening of a nightside gap which maps to where reconnection occurs in the tail; 75% of the 16 recovery phase cases required a nightside gap while only 29% of the 17 expansion phase cases required a nightside gap. On the basis of this result, we conclude the following. If a nightside gap implies tail reconnection, then reconnection probably occurs after expansion phase onset and continues throughout most of the recovery phase of a substorm.