Spatial and temporal variations of the cold dense plasma sheet: Evidence for a low-latitude boundary layer source?

Abstract

We have surveyed 23 perigee passes by the Wind spacecraft through the near-Earth plasma sheet between X GSE = 0 and -24 R E to investigate the occurrence, location, flow properties, and IMF dependence of the cold dense plasma sheet. We find that the plasma sheet becomes colder and denser following an extended period of northward interplanetary magnetic field (IMF), with the best correlation observed when the IMF is averaged over many hours prior to the plasma sheet observations, similar to the findings of Terasawa et al. [1997]. The cold dense plasma sheet occur mainly at high latitudes, away from the neutral sheet (|B x | > 15 nT), and near the flanks (|Y GSM | > 7 R E ). To distinguish spatial variations from temporal dependencies we present a two-spacecraft conjunction study when the Wind and Geotail spacecraft were separated by more than 5 R E but observed the same transition from a hot and tenuous to a cold and dense plasma sheet simultaneously. The two-spacecraft study reveals that spatial gradients in the plasma sheet density and temperature are embedded in large-scale temporal variations correlated with the IMF direction. The observations are discussed in terms of existing models of plasma entry. Whether the LLBL is a source remains uncertain.