Study of an isolated substorm with ISTP data

An isolated substorm of moderate intensity (AE index -500 nT), which occurred on February 9, 1995 after an extended interval of strong northward interplanetary magnetic field was examined with data from eleven spacecraft in space (Wind, IMP 8, Geotail, six geostationary satellites, one DMSP satellite, and Freja) and two networks of ground stations (Canopus and SuperDARN) covering both the northern and southern hemispheres. The extensive coverage of this event provides us with results (1) confirming some expected substorm phenomena, (2) bringing out some unusual characteristics possibly related to the isolated nature of the substorm, and (3) revealing some surprising features difficult to reconcile with the traditional substorm model. In the first category are several features in the substorm growth phase (equatorward movement of the auroral ovals and intensification of the large-scale field-aligned current and the cross-tail current systems) and in the substorm expansion phase (breakup arc located well within the closed field line region, dipolarization and particle injection, strong plasma flow, complex magnetic field structure, enhanced dawn-dusk electric field, and plasma sheet thinning in the mid-tail region). In the second category are the unusually long duration of the growth phase and the long time delay between substorm expansion onset and particle injection onset at the geostationary altitude. In the third category are the new evidence for multiple particle acceleration sites during substorm expansion, for sunward plasma flow during the late expansion phase of a substorm being not related to a single acceleration site (X-line) moving from the near-Earth tail to the more distant tail, and for possible identification of the optical signature of a bursty bulk flow event observed in the mid-tail region.