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 (∼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 (∼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 .