Three-dimensional structure of the Martian nightside ionosphere: Predicted rates of impact ionization from Mars Global Surveyor magnetometer and electron reflectometer measurements of precipitating electrons

The nightside ionosphere of Mars is known to be highly variable: electron densities are below detection thresholds in certain regions and are almost comparable to the photoionization-produced dayside ionosphere in others. The factors controlling its structure include thermospheric densities, temperatures and winds, day-night plasma transport, plasma temperatures, current systems, meteoroid ablation, solar and galactic energetic particle events, and magnetic field geometry-topology and electron precipitation, none of which are adequately understood at present. Using a kinetic approach called Mars Monte Carlo Electron Transport, we model the dynamics of precipitating electrons on the nightside of Mars to study the impact of these last two listed factors (magnetic fields and electron precipitation) on ionospheric structure. As input, we use precipitating electron energy spectra and pitch angle distributions from the Mars Global Surveyor Magnetometer and Electron Reflectometer. We thus calculate ionization rate in three dimensions, both for specific observations and average cases. The very highest average rates are equivalent to photoionization rates on the dayside at high solar zenith angle. We predict complex geometrical patterns in the ionization and huge variability (similar to 4 orders of magnitude) in peak ionization rates, both on single orbits and between the averages for different geographic regions, and find a bimodal distribution of predicted ionization rates where the highest rates correlate with the most vertical magnetic fields. This model can be used as input to electrodynamic models of the Mars ionosphere, which can be compared with, and informed by, data from the upcoming 2013 Mars Atmosphere and Volatile Evolution Mission.