Observations of Energized Electrons in the Martian Magnetosheath

Abstract:

Electrons in Mars’s magnetosheath originate in the solar wind and are accelerated by an electric field when they cross the bow shock. The magnitude of this acceleration can be explained by classic bow shock theory. Assuming that this acceleration is localized to the shock, the field-aligned electron distributions in the sheath are expected to be highly asymmetric since the opposite-flowing electrons on the same field line crossed the bowshock at two different locations. However, in a statistical study of the box shock acceleration the expected statistical asymmetry was not as significant as expected. Based on the analysis here, it is suggested that an additional parallel acceleration takes place downstream of the Martian bow shock. This additional acceleration suppresses the expected asymmetry of the electron distribution. Consequently along a flux tube in the magnetosheath, that is tied on both ends to the bow shock, the difference in energization between parallel and anti-parallel electrons is typically less than 20 eV. We suggest that the relative isotropy of the electron distributions in the sheath may be explained by the presence of a (nearly potential) ambipolar electric field in this region.