Low-altitude acceleration of ionospheric ions at Mars



[1] The objective of this study is to understand the acceleration processes that lead to outflow and escape of ionospheric ions from Mars. Observations show that accelerated dayside and flank ionospheric ions move slowly antisunward along the direction of the external/magnetosheath flow. At high altitudes, in the central tail, ions are further accelerated, up to keV energies. However, the primary acceleration process gives velocities in the 5–15 km/s range. Two acceleration processes, capable of generating a tailward stream of low-energy ions are feasible: Mass-loaded ion pickup, and wave acceleration. We demonstrate that wave acceleration is quite adequate to generate the ion outflow characterized by density modulations in the ULF range (3–20 mHz). The waves, of magnetosheath origin, penetrate into the Martian magnetosphere, down to low (pericenter) altitudes. A close relationship is found between solar wind dynamic pressure, ULF wave activity, and mass-loaded wave acceleration of ionospheric ions. Species-dependent differences in outflow velocity are consistent with an altitude dependent mass-loaded ion acceleration process.