Data-model comparisons of the nightside ionosphere of Mars: joint constraints on neutral densities and electron temperatures

While progress has recently been made in understanding the dayside ionosphere of Mars, our knowledge of the nightside is substantially incomplete. Observations of localized patches of ionization in the upper atmosphere and downgoing accelerated electron distributions suggest that electron precipitation can form a spatially inhomogeneous ionosphere. We use precipitating electron measurements from ASPERA-3 Electron Spectrometer (ELS) as input to the MarMCET Monte Carlo model of electron precipitation, with simplified ion chemistry, to compute a range of predicted electron density profiles. These computer profiles are compared/fitted to simultaneous topside ionospheric profiles and total electron content (TEC) measurements from the Mars Advanced Radar for Subsurface and Ionosphere Sounder (MARSIS). The precipitation model is qualitatively validated and the fitting results are used to probe the 3-dimensional structure of the nightside ionosphere and provide joint observational constraints on neutral densities and (for the first time) electron temperatures on the nightside of Mars over the approximate altitude range 150-230 km. Knowledge of such temperatures forms an important constraint on models of the solar wind interaction with Mars, and hence forms an important piece of the 'puzzle' of atmospheric escape. Illustration of the creation of a nightside ionosphere within magnetic cusps. ASPERA-3 ELS measures the electrons precipitating along open magnetic field lines. MARSIS measures electron density profiles and TEC below the spacecraft position in an effective cone of ~1° width (exaggerated in figure).