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

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 0-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 similar to 3 x 10(5) cm(-2) s(-1). The dayside magnetosheath produces H-ENA angular-differential fluxes of up to 3 x 10(5) cm(-2) s(-1) sr(-1), while the magnetosheath flanks populate the planet's nightside with H-ENA fluxes in the range of 10(4)-10(5) cm(-2) s(-1) sr(-1). The O-ENA environment is dominated by a near-isotropic similar to 10(5 )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 (10(3)-10(4) cm(-2) s(-1) sr(-1)) of energetic O-ENAs (greater than or similar to 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.

Automated summary

In this study, the distributions of H+, O+, and O-2(+) near Mars were determined using phase-space mapping and line integration of energetic neutral atom (ENA) production and loss rates. The results show that the H+ flux is mainly from the upstream solar wind, while the O-ENAs are dominated by a near-isotropic low-energy population originating in the top-side ionosphere. Synthetic ENA observations provide valuable insights into the near-Mars space environment, including the planet's plasma environment and exosphere.