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Precipitating Solar Wind Hydrogen at Mars: Improved Calculations of the Backscatter and Albedo With MAVEN Observations
Journal article   Open access   Peer reviewed

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

Z. Girazian and J. Halekas
Journal of geophysical research. Planets, Vol.126(2), e2020JE006666
02/2021
DOI: 10.1029/2020JE006666
url
https://doi.org/10.1029/2020JE006666View
Published (Version of record) Open Access

Abstract

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. Plain Language Summary Hydrogen atoms in the uppermost region of Mars’ tenuous atmosphere extend far into space, where they are directly exposed to the incoming solar wind. Protons in the solar wind collide with the hydrogen, producing high‐speed neutral atoms that travel into the atmosphere of Mars. While some of the high‐speed atoms deposit energy into the upper atmosphere, others undergo enough collisions with atmospheric gases to be reflected back to space. In this paper, we present a detailed study of the reflected particles using observations from the Mars Atmosphere and Volatile EvolutioN mission. We show that there is a larger fraction of reflected particles at high solar zenith angles and at low solar wind speeds. We also find that the energies of the reflected particles are consistent with predictions from models of the Mars‐solar wind interaction. Key Points We present new calculations of the hydrogen backscatter and albedo using low altitude proton measurements from MAVEN Our calculated penetrating and backscatter energy spectra are consistent with model predictions In agreement with expected trends, the albedo decreases with solar wind speed and increases with solar zenith angle
 Backscatter ENA Hydrogen Hydrogen albedo Mars MAVEN

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