Ion Reflection From the Martian Bow Shock

Abstract We utilize Mars Atmospheric and Volatile EvolutioN (MAVEN) observations to study reflected ions upstream from the Martian bow shock. We conduct back‐tracing of trajectories to separate ions in velocity space and quantify the reflected ion characteristics. Reflected ion densities vary from a few percent to more than 10% of the solar wind density, with reflection increasing with Mach number. Reflected ions follow asymmetric trajectories with respect to the motional electric field, but we find no significant asymmetry in reflection efficiency or extent of reflected ions. The extent of reflected ions scales with the shock foot thickness predicted by a specular reflection model, but a significant fraction of observed trajectories prove inconsistent with specular reflection. For small predicted foot thicknesses, the extent of reflected ions exceeds the specular prediction by a factor of two, possibly due to non‐ideal behavior at smaller scales. Reflected ions observed at Mars have several unexpected characteristics. We observed greater and more extended ion reflection in the flanks than in the subsolar region, possibly due to the small size of the Martian bow shock. The reflection rate and extent anticorrelate with solar wind dynamic pressure, likely due to the correlation between dynamic pressure and IMF magnitude. The reflection rate does not greatly vary between low‐hydrogen aphelion times and high‐hydrogen perihelion times, and actually slightly increases at aphelion. This may result from a seasonal Mach number dependence, or may indicate preconditioning of the flow upstream of the shock by the incorporation of pickup ions during high‐hydrogen times.