Energetic Neutral Atoms (ENA) at Mars: Properties of the Hydrogen Atoms Produced Upstream of the Martian Bow Shock and Implications for an ENA Sounding Technique

The solar wind upstream of the Martian bow shock is a region where a few percent of the solar wind protons charge exchange with the hydrogen exosphere and form energetic neutral hydrogen atoms, H-ENAs. Part of these solar wind H-ENAs hit the Martian upper atmosphere causing atmospheric effects and back scattered solar wind hydrogen atoms. We have studied the interaction of the fast solar wind hydrogen atoms with the Martian atmosphere by a three dimensional Monte Carlo simulation. Both H-ENAs scattered and non-scattered with the Martian atmosphere/exosphere were studied. The colliding H-ENAs were found to scatter both to the dayside and nightside. On the dayside they contribute to the so-called H-ENA albedo. On the nightside the heated and scattered hydrogen atoms were found also in the Martian wake. The density, the energy distribution function and the direction of the velocity of H-ENAs on the nightside are presented. Detailed study of the direction and the energy of the scattered and the non-scattered H-ENAs suggests that these particles provide information both of the upstream parameters and the Martian atmosphere. The results represent a novel new "ENA sounding" technique in which energetic neutral atoms are used to derive information of the properties of planetary exosphere and atmosphere in a similar manner that the solar wind photons are used to derive atmospheric densities by measuring the scattered UV light. The preliminary profiles in solar occultation experiments suggest that the ENA sounding is a method to study the interaction between the planetary atmosphere and solar wind, to monitor the atmospheric profiles, and likely to study magnetic field configurations in the upper atmosphere.