Precipitating Solar Wind Hydrogen at Mars: Improved Calculations of the
Backscatter and Albedo with MAVEN Observations
Abstract
Outside the Martian bow shock, charge exchange between solar wind
protons and exospheric hydrogen produces energetic neutral atoms (ENAs)
that travel towards 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
Analzyer (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.