Three-dimensional structure of the Martian nightside Ionosphere

The night side ionosphere of Mars is known to be highly variable: essentially nonexistent in certain regions, while occasionally nearly as strong as the photoionization-produced dayside ionosphere in others. Using a kinetic approach called MarMCET (Mars Monte Carlo Electron Transport) [3], we model the dynamics of precipitating electrons on the nightside of Mars to study the effects of magnetic field geometry and electron precipitation on ionospheric structure over the geographic region with the strongest crustal magnetic field (140°-220°E, 20°-70°S). As input, we use nightside precipitating electron energy spectra and pitch angle distributions from the Mars Global Surveyor (MGS) Magnetometer & Electron Reflectometer (MAG/ER), as well as a vector sum of a typical external tail-lobe magnetic field and crustal magnetic field model. We thus calculate ionization rate in 3 dimensions, both for specific observations and average cases. We find that magnetic topology (via pitch angle distributions) plays an important part in determining peak ionization rates. Also, we see large geographic differences in average precipitating electron spectra, likely due to the dynamical coupling of the rotating planet-fixed crustal field with the draped interplanetary magnetic field. This leads to differences in peak ionization rates of more than 4 orders of magnitude across this region of the Martian nightside (see figure 1). We also see a strong dependence of peak ionization rate on magnetic elevation angle measured at MGS mapping orbit altitude of 400 km, as precipitating fluxes are generally lower in regions of closed magnetic topology where magnetic field is generally horizontal at this altitude.