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MAVEN Observations of H− Ions in the Martian Atmosphere
Journal article   Peer reviewed

MAVEN Observations of H− Ions in the Martian Atmosphere

N. Jones, J. Halekas, Z. Girazian, D. Mitchell and C. Mazelle
Journal of geophysical research. Planets, Vol.127(6), e2021JE006999
06/2022
DOI: 10.1029/2021JE006999

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Abstract

At Mars, charge exchange between solar wind protons and neutral exospheric hydrogen produces energetic neutral atoms (ENAs) that can penetrate into the collisional atmosphere, where they can be converted through collisions into H+ and H−. The Mars Atmosphere and Volatile EvolutioN mission observed a population of negatively charged particles at low altitudes, whose energies, angular distribution, and dependence on the upstream solar wind were consistent with H− produced by solar wind hydrogen ENAs. The highest fluxes of H− were observed near perihelion and the southern summer solstice. We calculated an average ratio of ∼4% between H− density and H+ density, implying a slightly smaller relative abundance than reported previously (∼10%). We found that the fraction of H ENAs converted to H− increases with the solar wind energy, in agreement with laboratory measurements of the H–CO2 electron capture cross section. Plain Language Summary At Mars, interactions between solar wind protons and neutral hydrogen in the outer atmosphere produces energetic neutral atoms that travel into the inner atmosphere, where collisions with atmospheric gas can produce H+ ions and H− ions. The Mars Atmosphere and Volatile EvolutioN mission observed H− ions in the inner atmosphere, with energies and velocities that matched the solar wind. The highest fluxes of H− ions were seen when Mars was closest to the Sun, during the southern summer. We found that the relative amount of H− ions to H+ ions was smaller than previous studies had found. We also found that the amount of H− ions produced depended on the solar wind energy, a result that agrees with laboratory experiments. Key Points We observed H− ions at low altitudes in the Martian atmosphere whose energies and angular distribution matched those of the upstream solar wind Fluxes of H− varied seasonally, with a maximum near perihelion and the southern summer solstice We found a ratio of about four percent between H− density and H+ density, lower than previously reported ratios
 charge exchange H− ions Mars Atmosphere

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