Dawn/Dusk Asymmetry of the Martian UltraViolet Terminator Observed Through Suprathermal Electron Depletions

Suprathermal electron depletions are structures of the nightside ionosphere of Mars resulting from an equilibrium between electron loss and creation processes. Photoionization of oxygen and carbon dioxide by UV and EUV photons is the main ionization process of the Martian atmosphere. The observation of suprathermal electron depletions is strongly unexpected in the portion of the Martian environment where photoionization can occur. This region is delimited by the UltraViolet (UV) terminator, behind which no UV ionizing photons are detected. In this study suprathermal electron depletions are used to determine the position of the UV terminator thanks to MAVEN observations. The MAVEN spacecraft is now in its fourth year of data recording and has already covered more than one Martian year, a large range of latitude, local time, and solar zenith angle in the nightside down to 110-km altitude. This coverage enables us to determine the approximate position of the UV terminator over one Martian year. We then investigate the variation of its position on the dawnside and duskside and depending on seasons. Our results are compared with models of the Martian atmosphere and in situ data of the atmospheric composition which all highlight an asymmetry between the duskside and the dawnside at equinox. However, models show an inversion in the position of the dusk and the dawn UV terminator at perihelion and aphelion, which cannot yet be confirmed or disproved by the data.