Electron acceleration signatures in the magnetotail associated with substorms

Abstract

We present Cluster multisatellite observations of accelerated electrons in the near- Earth magnetotail associated with substorms. We found that the hardest electron energy spectra appear in the earliest stage of substorm expansion in the near-Earth tail region and that they gradually become softer during the events. Enhancement of the high-energy electron flux occurs generally associated with the bulk acceleration of ions (fast flow) and electrons. It is also shown that the high-energy electrons sometimes show preferential perpendicular acceleration associated with the temporal enhancement of the normal component of the magnetic field, and then the anisotropic distribution quickly becomes isotropic. During the dipolarization interval, in which no convection signature is observed, perpendicular flux drops to less than the initial value, and the parallel flux is more than the perpendicular flux. The results suggest that the electron acceleration mechanism is mostly consistent with adiabatic betatron acceleration, while Fermi acceleration is not clear in the high-energy part. The effect of the pitch angle scattering is also important. The dispersive signature of the high-energy electron flux indicates fast dawnward drift loss, namely, the three-dimensional effect of the limited plasma acceleration region.