Ion Reflection From the Martian Bow Shock

J. S. Halekas; D. L. Mitchell; C. X. Mazelle; J. R. Espley; J. Fruchtman

doi:10.1029/2025JA034317

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. The solar wind, a flow of hot ionized hydrogen and helium, flows outward from the Sun at supersonic speeds. When it encounters an obstacle, such as a planetary atmosphere or magnetic field, a shock wave forms to slow and deflect the flow around the obstacle. The shock converts a portion of the kinetic energy of the flow into other forms of energy, including thermal energy. In a collisionless medium such as the solar wind, the formation of the shock generally involves the reflection of a portion of the incident solar wind. These reflected ions follow complex trajectories, gyrating in the magnetic and electric fields upstream of the shock. Some of these ions gain energy in the upstream region and re‐impact the shock, where they contribute to the required increase in thermal energy in the downstream region. In this study, we utilize ion observations from the Mars Atmospheric and Volatile EvolutioN (MAVEN) mission to identify and study these reflected ions at Mars, in order to understand how they vary spatially and as a function of solar wind conditions and season. We utilize an analytical backtracing method to identify reflected ions upstream from the Martian bow shock and study their characteristics We measure ion reflection rates varying from a few percent to more than 10%, increasing with Mach number as expected We observe only minor seasonal variability, indicating that upstream solar wind conditions play a more important role than pickup ions

Ion Reflection From the Martian Bow Shock

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