Energetic Neutral Atoms near Mars: Predicted Distributions Based on MAVEN Measurements

Abstract

Abstract Through phase-space mapping 6 yr of H+, O+, and O 2 + distributions measured in situ by the Mars Atmosphere and Volatiles EvolutioN (MAVEN) orbiter, we derive semiempirical average energetic neutral atom (ENA) distributions near Mars. Differential fluxes of H-ENAs and O-ENAs are estimated by line-integrating ENA production and loss rates in the average phase-space ion distributions. By repeating the procedure for a systematic series of vantage points, we produce synthetic ENA observations for virtual circular orbits with radii twice that of Mars, revealing the angular dependence of the observer’s position. Accounting for known ENA production and loss terms, we find that H+ in the upstream solar wind yields total antisunward H-ENA fluxes of ∼3 × 105 cm−2 s−1. The dayside magnetosheath produces H-ENA angular-differential fluxes of up to 3 × 105 cm−2 s−1 sr−1, while the magnetosheath flanks populate the planet’s nightside with H-ENA fluxes in the range of 104–105 cm−2 s−1 sr−1. The O-ENA environment is dominated by a near-isotropic ∼105 cm−2 s−1 sr−1 relatively low-energy (tens of eV) population originating in the top-side ionosphere, particularly on the dayside. Lower fluxes (103–104 cm−2 s−1 sr−1) of energetic O-ENAs (≳100 eV) are mainly found on the nightside and above the electric polar regions of the induced magnetosphere. Asymmetries in the ENA flow are largely limited to the energetic O-ENA populations, while the H-ENA distribution is mostly symmetric around the Sun‒Mars line. We discuss how synthetic ENA observations can provide insight into the near-Mars space environment, including the planet’s plasma environment and exosphere.