Acceleration of oxygen ions in the dynamic magnetotail

The substorm-related acceleration and flux increases of energetic oxygen ions are studied on the basis of test particle orbits in the fields obtained from an MHD simulation of plasmoid formation and ejection and the collapse (dipolarization) of the inner tail. The simulated fluxes show large anisotropies and nongyrotropic effects, phase bunching, and spatially and temporally localized beams. The energy distribution of O+ in the region of an earthward beam in the near tail becomes significantly harder, more pronounced than for protons, in qualitative agreement with observations. The simulation also shows tailward beams of energetic O+ions closely associated with the passage of a plasmoid, both inside the plasma sheet boundary and inside the central plasma sheet, consistent with observations in the far tail. The acceleration at the near-Earth x-type neutral line produces a narrow duskward beam of energetic O+ in the duskward extension of the x-line, which was not found to be as pronounced in proton test particle simulations. Key words. Magnetospheric physics (energetic particles, trapped; magnetotail; storms and substorms