Reduction of the cross-tail current during near-Earth dipolarization with multisatellite observations

We examine a near-Earth current disruption/dipolarization (CDD) event in which two THEMIS satellites were located at nearly identical equatorial projections but separated by the distance from the neutral sheet. One satellite was very close to the neutral sheet with |B-x|, |B-y| < 1.5 nT at CDD onset that coincided with ground signatures of substorm expansion onset. The cross-tail current density between the two satellites varied with a time scale of seconds during CDD and was reduced by similar to 40% eventually. If the current density profile could be represented well by the combination of a dipole field and a one-dimensional current sheet, then the current density integrated over the current sheet thickness was reduced by only similar to 12%. The difference can be attributed to plasma sheet expansion in association with CDD. There were occasional breakdowns of the frozen-in-field condition during CDD as well. The particle energization associated with CDD approached the satellite from the Earthward-dawnward direction near the neutral sheet based on the ion sounding technique, constituting compelling evidence that this near-Earth CDD arose from disturbances originating in the near-Earth region (X-gsm > -8.1 R-E) and was not due to magnetic flux pileup (requiring frozen-in condition) or arrival of a dipolarization front from mid-tail (X-gsm < -15 R-E) disturbances.