Multipoint study of a substorm on February 9, 1995

Abstract

Abstract. An extended interval of strong northward interplanetary magnetic field (IMF) was observed by the Wind spacecraft located at an upstream distance of-193 Roe from February 8-10, 1995, with a brief break of southward IMF from 0200 to 0400 UT on February 9. This brief interval of southward IMF led to an isolated substorm of moderate intensity (-500 nT) with expansion phase starting at -0431 UT. This substorm may be triggered by the northward turning of the IMF since its onset time matched well with the time expected for the arrival of the northward turning of the IMF at Earth. The substorm activities were monitored by 11 spacecraft in space (Wind, IMP 8, Geotail, six geosynchronous 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) showing some unusual characteristics possibly related to the isolated nature of the substorm and (2) revealing some surprising features difficult to reconcile with the traditional substorm model. In the first category is unusually long duration of the growth phase and the long time delay between substorm expansion onset and particle injection onset at the geosynchronous orbit. In the second category is new evidence for multiple particle acceleration sites during substorm expansion and for sunward flow during the late expansion phase of a substorm being unrelated to a single acceleration site (X line) moving from the near-Earth tail to the more distant tail. We also present observations which show the possible optical signature on the ground of bursty bulk flows in the magnetotail.

Two distinct substorm onsets

“…Alternative models have suggested diverse mechanisms for the principal cause of substorms including current disruption (CD) with a "cross-tail current partial disruption" [Rothwell et al, 1988;Lui, 1996;Lui et al 1998;Kan, 1998 is not excluded as the result of the CD. The two approaches (CD and NENL) also differ in the physics of the local plasma instabilities that are responsible for the active phase onset.…”

On the use of photometer data to map dynamics of the magnetotail current sheet during substorm growth phase

“…This substorm has been extensively studied [Lui et al, 1998]. Owing to an unusually long northward interplanetary magnetic field, the magnetosphere was as close to a low energy or "quiet" state as is likely possible ß before a substorm.…”

Observation of an inner magnetosphere electric field associated with a BBF-like flow and PBIs

“…A further connection with a localized increase in the nightside reconnection rate was also found. Lui et al (1998) observed a possible relation between BBFs and optical auroral signatures moving equatorward, while Lyons et al (1999) found that BBFs were more frequent and stronger during periods of PBIs, compared to periods without PBIs. Although a oneto-one correlation between PBIs and BBFs could not be confirmed, Zesta et al (2006) could associate all of their observed PBIs with fast flow channels in the tail, occurring in the same local time sector.…”