Directionality and variability of energetic neutral hydrogen fluxes observed by Mars Express

Abstract

Using data collected by the neutral particle detector of the analyzer of space plasmas and energetic atoms on board Mars Express, we establish the global morphology of hydrogen energetic neutral atom (ENA) fluxes. The observations were conducted in March–July 2004 and reveal that low flux ENAs (<5×105cm−2s−1sr−1) are emitted roughly isotropically from the dayside of the planet. These ENA emissions originate from the backscattering of the hydrogen ENAs produced by the charge exchange of the upstream solar wind in the extended Martian exosphere. ENAs of higher fluxes are emitted preferentially in the directions perpendicular to the undisturbed solar wind velocity. This population is due to the charge exchange of the magnetosheath plasma. At a great distance, these ENA emissions resemble a thick layer or a wall. The emissions are enhanced in the direction opposite to the convective electric field (Ec). This enhancement reflects the asymmetric shape of the induced magnetic boundary that is close to the planet hemisphere where the Ec points toward the Sun‐Mars line (−Ec hemisphere). The ENA flux shows a positive correlation with the solar wind dynamic pressure. The asymmetry in the ENA emissions is modest during quiet solar wind conditions but becomes more pronounced when the solar wind dynamic pressure increases.