LOCATION AND PROPAGATION OF THE MAGNETOTAIL CURRENT DISRUPTION DURING SUBSTORM EXPANSION - ANALYSIS AND SIMULATION OF AN ISEE MULTI-ONSET EVENT

Magnetic field variations measured onboard the ISEE-1 spacecraft in the tail northern lobe (approximately 22 R(E) geocentric distance) during a substorm with multiple onsets are analyzed and simulated using a model current sheet. This remote sensing study of the cross-tail current sheet shows that substorm onsets are associated with a partial disruption of the cross-tail current that begins in the near-earth plasma sheet (approximately 7 R(E)) and that expands tailward during substorm expansion with a velocity of the order of 300 km/s. The two consecutive substorm onsets, which occur during a very active period (AE > 500 nT), correspond to a similar partial disruption of the near-earth current sheet that expands tailwards over a distance of the order of 80 R(E).