Electron acceleration in the near-Earth magnetotail in substorms

We investigate substorm events in the near-Earth magnetotail in order to examine acceleration signatures of electrons using data from the Cluster satellites with separation larger than 1 RE. Thermal electrons detected by the PEACE instrument and the high-energy electron flux from the RAPID instrument are analyzed and compared with simultaneous magnetic field, electric field, and ion observations from FGM, EFW, and CIS instruments, respectively. It is found that electrons with energies up to a few hundreds keV exhibit the hardest spectra in the initial stage of the events. These electrons are associated with fast Earthward ion flows and the enhancement of the dipolar magnetic field and the electric field. Although most of the distributions are isotropic, electrons sometimes show the preferential increase of the perpendicular flux, suggesting the effect of betatron acceleration. These electron signatures last only for about one minute, and after that either the flux quickly decreases or a more isotropic flux is observed. The spectra gradually become softer in the course of substorms, and the spectra are softer than the initial state in some cases. The soft spectra are sometimes associated with the temporal drop of the perpendicular electrons. The larger flux is observed by the satellite closer to the Earth, while the satellite on the tailward side shows a faster response to magnetotail perturbations. We discuss possible acceleration mechanisms and the flux transport in the magnetotail.