Characterizing Precipitation Behaviors of H⁻ in the Martian Atmosphere

Solar wind protons can charge exchange with the extensive hydrogen corona of Mars, resulting in a significant flux of energetic neutral atoms (ENAs). As these solar wind hydrogen ENAs precipitate into the upper atmosphere, they can experience electron attachment or detachment, resulting in populations of H − and H + , respectively, with upstream velocity. We seek to characterize the behavior of H − in the ionosphere of Mars through a combination of in situ data analysis and mathematical models. Observations indicate that measurable H − precipitation in the ionosphere of Mars is rare, occurring during only 1.8% of available observations. These events occur primarily during high energy solar wind conditions near perihelion. We also compare H − fluxes to those of H + and find that H − fluxes are ∼4.5 times less than H + , indicating preferential conversion of hydrogen ENAs to H + . We develop a simple model describing the evolution of the charged and neutral fraction of ENAs and H − ions versus altitude. We find that 0.29%–0.78% of ENAs are converted to H − for solar wind energies 1–3 keV. We also predict that the effects of photodetachment on the H‐H − system are non‐negligible.