Cluster observations of magnetospheric substorm behavior in the near- and mid-tail region

Abstract

The Cluster constellation of spacecraft has returned substantial new data on particle and field variations in the near- and mid-magnetotail regions of Earth’s magnetosphere. Using the Research with Adaptive Particle Imaging Detectors (RAPID) system onboard the four Cluster vehicles, we have identified substorm-related energetic ( E > 20 keV) electron enhancement events during the period March 2001 through October 2001 in the geocentric radial range of 4–19 Earth radii. We have used concurrent data from other Cluster instruments as well as from the POLAR, IMAGE, FAST, GPS, and geostationary orbit spacecraft in order to understand particle injection and transport phenomena throughout this key region of the magnetotail. Electron enhancements in the plasma sheet at intermediate radial distances have been studied in a global substorm context. A particularly well-observed substorm case occurred on August 27, 2001 when Cluster was almost exactly in the midnight meridian. We find evidence that Cluster was very near the near-Earth substorm neutral line and that magnetic reconnection began some seven minutes prior to the substorm auroral brightening of the expansive phase onset. We also study in some detail the recovery phase of the substorm and the associated expansion of the plasma sheet over the 4-satellite Cluster constellation.