Precipitating Solar Wind Hydrogen at Mars: Improved Calculations of the Backscatter and Albedo With MAVEN Observations

Outside the Martian bow shock, charge exchange between solar wind protons and exospheric hydrogen produces energetic neutral atoms (ENAs) that travel toward Mars at the solar wind velocity. The penetrating ENAs deposit most of their energy near 150 km, but a fraction of them undergo enough collisions to be scattered back to space, resulting in a hydrogen albedo. Some of the penetrating ENAs are converted into protons upon reaching the collisional upper atmosphere. These protons can be measured by the Mars Atmosphere and Volatile EvolutioN’s Solar Wind Ion Analyzer (SWIA) during periapsis passes, providing information about the penetrating and backscatter populations. In this work, we perform the first detailed analysis of the backscatter and albedo using SWIA observations. We find that our calculated backscatter energy spectra are consistent with model predictions and that, as expected, the penetrating and backscatter particle fluxes increase with solar wind speed and decrease with solar zenith angle (SZA). We also find that the albedo, which has an average value of 0.20 ± 0.16, decreases with solar wind speed and increases at high SZAs near the terminator.