TAILWARD PROPAGATING CROSS-TAIL CURRENT DISRUPTION AND DYNAMICS OF NEAR-EARTH TAIL - A MULTIPOINT 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.