Substorm expansion phase: observations from Geotail, Polar and IMAGE network

[1] We describe the signature of a substorm detected in the midtail while Geotail was located close to the midnight meridian. At the same time, the UVI imager on board Polar identifies a bulge which develops at low latitude and rapidly expands towards the north, east, and west, corresponding to the expansion phase. The magnetograms of the IMAGE network are consistent with these observations; during the expansion phase they give evidence for a northward expansion of the magnetic perturbation. Mapping indicates that the Geotail footprint is located north of the initial bulge and south of the high-latitude oval. During the expansion phase, Geotail is located in the center of the neutral sheet and detects an ion flow velocity, perpendicular to Bo and directed tailward while Bz changes from positive to negative. During the recovery phase, Geotail, which is not at the center of the sheet anymore, detects an ion velocity directed earthward but essentially field aligned, while Bz is positive and the high-latitude auroral structure is located north of Geotail footprint. The radial component of the velocity is always dominant. We interpret these observations as evidence for a tailward moving dipolarization front that first destroys the inner part of the thin current sheet (TCS) formed during the growth phase. This dipolarization/current disruption starts in the near-Earth plasma sheet and expands tailward. This erosion'' produces a negative Bz component at the earthward edge of the TCS. For large enough distances this contribution can eventually be dominant, thereby producing a negative Bz. In this interpretation the formation of an X-point/X-line is the consequence of the erosion of the currents in near-Earth tail. Present observations give evidence for an association between increases in the ion velocity and small-scale Alfvenic fluctuations. The plasma sheet electrons are heated via interaction with the waves. This heating is preferentially along the direction of the magnetic field. Large ion flow velocities coincide with wave observations.