Tailward propagating cross‐tail current disruption and dynamics of near‐Earth Tail: A multi‐point measurement analysis

Plasma and magnetic field data taken simultaneously in the near‐Earth plasma sheet (6.6–13 RE) aboard three satellites, GEOS‐2, ISEE‐1, and ISEE‐2, are used to infer directly the location and tailward propagation velocity of the partial cross‐tail current disruption associated with a well‐defined substorm. This study shows that the disruption starts at 6–9 RE and propagates down the tail with a velocity of the order of 150 to 250 km/s over tens of earth radii during the substorm expansion phase. Moreover, magnetic variations measured aboard GEOS‐2 suggest that the partial current disruption also propagates longitudinally. After a time delay of about 2 minutes after the disruption onset, an injection front of accelerated particles is measured at geostationary orbit. This time delay is interpreted as indicating that the front propagates earthward with a velocity of the order of 20 to 160 km/s and also eastward. The overall post‐onset magnetic signatures measured at geostationary orbit are shown to be mainly due to an expansion (thickening) of the current sheet in the close vicinity of GEOS‐2 during the expansion phase.