The Ionosphere of Mars After 20 Years of Mars Express Contributions

Abstract

The Martian ionosphere originates from the ionization of the planetary neutral atmosphere by solar radiation. This conductive layer is embedded within the thermosphere and exosphere of Mars where it forms a highly variable interaction region with the solar wind. The Martian ionosphere has been continuously observed by the three plasma instruments MaRS, MARSIS and ASPERA-3 on Mars Express for the last 20 years ( >10 Martian years). Those long-term observations laid a solid foundation for what we know today about the Martian ionosphere, and provided numerous opportunities for collaboration and coordinated observations with other missions. This review describes the most significant achievements of Mars Express for the ionosphere, such as the dynamics and structures of both day and nightside, its variability and couplings with the lower atmosphere, as well as the improvement of atmospheric and ionosphere modelling. Mars Express has also provided a better characterization of the role of several external and internal drivers in controlling the ionosphere, such as the Martian crustal magnetic fields, solar activity, seasons, dust lifting from the surface, and even the direct interaction of the Martian ionosphere with the coma of an Oort-cloud comet (C/2013 A1, Siding Spring).

EMM EMUS Observations of FUV Aurora on Mars: Dependence on Magnetic Topology, Local Time, and Season

“…Sánchez-Cano et al ( 2018) attributed the variability in solar EUV irradiance (peaking near Mars perihelion) as the cause for the primary TEC enhancement, while the secondary TEC peak is not related to the solar irradiance variation but instead coincides with an increase in the thermospheric atomic oxygen density, with the Mars Climate Database (MCD) predicting an increase in atomic oxygen column density between 100 and 200 km during this period of the Martian year (Sánchez-Cano et al, 2018). The thermospheric atomic O density increase is possibly linked to the compositional variation during non-dusty season, produced by atmospheric cycles such as the dust, CO 2 , and water cycles (Peter et al, 2024;Sánchez-Cano et al, 2018). This suggests that a combination of ionospheric photoelectron content and atomic oxygen abundance in the thermosphere is responsible for the seasonal variability of nightside FUV aurora.…”