Oxygen ion precipitation in the Martian atmosphere and its relation with the crustal magnetic fields

Abstract

Without a global intrinsic magnetic field, oxygen ions in the Martian atmospheric corona can be picked up by the solar wind directly. The majority of the pickup ions escape, while some can precipitate into the atmosphere, producing sputtering of atmospheric constituents, which may play a significant role in the loss of neutral atmosphere. With its widely distributed crustal magnetic fields, Mars is unique. While it has been shown that Mars's crustal fields can alter the global distribution of escaping ions, little is known about the influence of the crustal fields on the spatial distribution and energy deposition of precipitating pickup ions. In this paper, the global distribution and energy spectrum of the precipitating pickup oxygen ions are calculated using the test particle method in the electric and magnetic fields set up by MHD simulation. Cases of different crustal field conditions are compared to show the influence of the crustal fields on the oxygen ion precipitation. We find, using a test particle code, that the crustal fields likely change the spatial distribution of the precipitation, resulting in irregularly distributed low-energy ion precipitation parches and wide high-energy ion precipitation belts on both the dayside and nightside. The crustal fields increase O(+) ion precipitation substantially, enhancing energy deposition especially on the nightside. Our results imply that atmospheric sputtering, and accompanying neutral escape, might happen globally and may be enhanced by a factor of almost 2 with the presence of the crustal magnetic fields.