Acceleration of Oxygen Ions in Dipolarization Events: 1. CPS Distributions

摘要

Using an MHD simulation of near tail reconnection associated with a flow burst and the collapse (dipolarization) of the inner tail in combination with test particle tracing we study the acceleration and flux increases of energetic oxygen ions (O+). The characteristic orbits, distributions, and acceleration mechanisms are governed by the dimensionless parameter sigma = omega(ci)t(n), where omega(ci) is the ion gyro frequency and t(n) a characteristic Alfven time of the MHD simulation. For sigma 1) the O+ distributions resemble more closely those of protons, consisting of two counter-streaming field-aligned beams and an, albeit more tenuous and irregular, ring population perpendicular to the magnetic field. The existence of the beams, however, depends on suitable earthward moving source populations in the plasma sheet boundary layer or the adjacent lobes. The acceleration to higher energies is found to indicate a charge dependence, consistent with a dominance of more highly charged ions at energies of a few hundred keV. As in earlier simulations, the simulated fluxes show large anisotropies and nongyrotropic effects, phase bunching, and spatially and temporally localized beams.