Effect of the lateral exospheric transport on the horizontal hydrogen distribution near the exobase of Mars

Abstract

We simulate the hydrogen density near the exobase of Mars, using the 3-D Martian Global Circulation Model of Laboratoire de Météorologie Dynamique, coupled to an exospheric ballistic model to compute the downward ballistic flux. The simulated hydrogen distribution near the exobase obtained at two different seasons—Ls = 180° and Ls = 270°—is close to Zero Net Ballistic Flux equilibrium. In other words, the hydrogen density near the exobase adjusts to have a balance between the local upward ballistic and the downward ballistic flux due to a short lateral migration time in the exosphere compared to the vertical diffusion time. This equilibrium leads to a hydrogen density n near the exobase directly controlled by the exospheric temperature T by the relation nT5/2 = constant. This relation could be used to extend 1-D hydrogen exospheric model of Mars used to derive the hydrogen density and escape flux at Mars from Lyman-α observations to 3-D model based on observed or modeled exospheric temperature near the exobase, without increasing the number of free parameters. ©2018. American Geophysical Union. All Rights Reserved. ; This project was partly funded by the Programme National de Planetologie and Programme National Soleil Terre and by the Centre National d'Etudes Spatiales. This work has been partially funded by the European Union Horizon 2020 Programme (H2020 Compet-082014) under grant agreement UPWARDS-633127. ; Peer reviewed