MAVEN Observations of H ⁻ Ions in the Martian Atmosphere

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.