Precipitation of Solar Wind and Planetary Hydrogen Atoms to the Upper Atmosphere of Mars

Abstract:

Energetic neutral atoms (ENA) at Mars are produced by charge exchange (CX) reactions of the solar wind and planetary ions, mainly protons, with the neutral exosphere. ENAs can be used as a diagnostic tool to remote-sense the original plasmas, and they also serve as a channel for the impinging of mass, momentum and energy from the solar wind to Martian upper atmosphere. The effects of precipitating ENAs have been observed: the scattered hydrogen ENAs from the dayside hemisphere, the solar-wind-spectrumed protons below the exobase, the dayside proton aurora in the ionosphere. In this study we investigate the spatial, angular and spectral distributions of precipitating hydrogen ENAs using numerical simulations.

We use a quasi-neutral hybrid model (particle ions and fluid electrons) to simulate the plasmas in the solar wind interaction and a Mars exosphere model under the solar minimum condition to model the neutralizing charge exchange reactions. We run the model with a typical magnetic field in the solar wind, but three different density-velocity combinations for different solar wind dynamic pressures (P_dyn). The magnetic field is in the “equatorial” plane. The ENAs in the outputs are projected along their velocity vectors to a spherical surface at 200 km altitude above Mars surface, mimicking the exobase, where the precipitation is defined.

ENAs generated in different regions have different characteristics. The upstream solar wind ENAs has a flux of ~1% of the upstream solar wind proton flux. The spectrum of this population is identical to the solar wind protons. The magnetosheath ENAs has a comparable precipitating rate. The spatial distribution shows a maximum at ~15° away from the subsolar point along the convective electric field (Ec). This population has a spectrum of Maxwellian distribution with ~ 200 eV equivalent temperature. The dependence of magnetosheath ENA flux on P_dyn is weak. Magnetosheath ENAs can also reach deep night exosphere (SZA>150°). The planetary hydrogen ENAs are neutralized protons originating from the hydrogen exosphere of Mars. Their precipitation strongly concentrates in the +Ec hemisphere during low P_dyn, while strong precipitation moves towards the subsolar region as P_dyn increases. Planetary ENAs have power-law like spectrum.