Investigation of a substorm following an extended interval of northward interplanetary magnetic field

Abstract Strong northward interplanetary magnetic field was observed for an extended period by the Wind spacecraft at an upstream distance of ~200 R E from February 8–10, 1995. Within this period was a brief break of southward IMF on February 9 which led to a substorm of moderate internsity ( 500 nT) with its expansion onset at ~0431 UT. In this paper, this substorm is examined with data from eleven spacecraft in space and two networks of ground stations covering both the northern and southern hemispheres. Detailed analysis of this event shows (1) an unusually long duration of the magnetospheric reconfiguration prior to expansion onset for this isolated substorm (2) new evidence for multiple particle acceleration sites during substorm expansion, and (3) indications for sunward plasma flow in the plasma sheet during the late expansion phase of a substorm not related to a single acceleration site (e.g., an X-line) moving from the near-Earth tail to the more distant tail.