Electron Pitch-angle Distributions at Dipolarization Fronts: THEMIS Multi-point Observations

We report on multi-event studies of electron pitch angle distributions (PAD) obtained by five THEMIS probes in the vicinity of dipolarization fronts in the near-Earth plasma sheet. In the selected events, the THEMIS probes were separated by 2 to 3 RE along X, approximately 1 RE along YGSM, and approximately 1 RE along ZGSM. Such configurations provide the unprecedented opportunity to study signatures in PAD and energy spectra observed at various locations around the front. The event studies have shown that the energetic electron flux increases at the central part of the front and decreases on the dusk side of the front. The decrease is due to the ambient plasma's vortical motion near the front. The type of electron PAD depends on the observer's position with respect to the neutral sheet. Specifically, the energetic electron PADs are mainly pancake-type (with a maximum at 90 degrees pitch angle) near the neutral sheet and mainly cigar-type (with maxima at 0 and 180 degrees) away from the neutral sheet. A short-time dropout in high-energy electron fluxes right ahead of the fronts were found to be associated with a dip in the northward magnetic field component ahead of the fronts.