Pickup oxygen ion velocity space and spatial distribution around Mars

Abstract

We report a newly created highly parallelized global test particle model for resolving the pickup oxygen ion distribution around Mars. The background magnetic and convection electric fields are calculated using a three-dimensional multispecies magnetohydrodynamic model, which includes the effect of the Martian crustal magnetic field. In addition to photo-ionization, charge exchange collisions and solar wind electron impact ionization are included for the pickup ion generation. The most novel feature of our model is that more than one billion test particles are launched in the simulation domain in total. This corresponds to a profound enhancement by at least 3 orders of magnitude in the total number, compared to all existing test particle models. This substantial improvement enables an unprecedented examination of the pickup ion flux distribution in velocity space, which is not achievable in previous simulation studies due to the insufficient statistics arising from the limited number of test particles. Using the velocity space distribution of pickup O+ ions as a tool, the Mars-solar wind interaction can be investigated in a unique way. It is shown that the velocity space distribution is highly non-Maxwellian, exhibiting non-gyrotropic and non-symmetric distributions, including many beam-like features. In the tail region, pickup ions have a prominent outflowing component in the whole energy range. The energy examination of particles traveling across the tail region shows that the acceleration highly depends on the source region where the particles originate. The strong convection electric field in the magnetosheath region is favorable to the pickup ion acceleration. Copyright 2008 by the American Geophysical Union.