Nightside ionosphere of Mars: Radar soundings by the Mars Express spacecraft

We present results of a survey of the nightside ionosphere of Mars as observed byAdvanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board the Mars Express spacecraft. The occurrence rate of the nightside ionosphere is studied as a function of solar zenith angle (SZA), magnetic field magnitude, and magnetic field inclination. It is shown that at locations with weak crustal magnetic fields the occurrence rate of the nightside ionosphere decreases with increasing SZA up to about 125 degrees, suggesting that plasma transport from the dayside plays a crucial role in its formation. However, at locations with strong crustal magnetic fields, the dependence on SZA is no longer apparent and the inclination of magnetic field becomes a crucial parameter: the occurrence rate of the nightside ionosphere is more than 4 times larger at locations with nearly vertical magnetic fields as compared to the locations with nearly horizontal magnetic fields. This indicates that impact ionization by precipitating electrons is the main ionization source at these locations. Observed peak electron densities are less than 2 x 10(4) cm(-3) in the vast majority of cases. Lower estimates of altitudes of peak electron densities are mostly between 100 and 150 km.