Electron acceleration in the dynamic magnetotail: Test particle orbits in three-dimensional magnetohydrodynamic simulation fields

Electron acceleration and energetic electron flux increases in the inner tail are investigated on the basis of test-particle orbits in the dynamic fields of a three-dimensional magnetohydrodynamic simulation of neutral line formation and dipolarization in the magnetotail. Past models have mostly considered equatorial orbits, using the gyrocenter drift approximation. In this paper, the investigation is extended to include nonequatorial drifts and full orbit integrations in regions where the drift approximation breaks down. Typical acceleration mechanisms consist of betatron acceleration at large pitch angles and Fermi acceleration at small pitch angles, resulting from the dipolarization and shortening of field lines moving earthward from the neutral line. In comparison, acceleration at the near-Earth neutral line plays a negligible role in flux increases observed in the near tail. Energetic electron fluxes appear preferentially enhanced around 90degrees pitch angle, so that restriction to those pitch angles might lead to an overestimate of flux increases. Otherwise, the results explain the observed limitation of the range of flux enhancements between a few keV and a few hundred keV. (C) 2004 American Institute of Physics.