Model calculations of electron precipitation induced ionization patches on the nightside of Mars

Using an electron transport model, we investigate the effect of electron precipitation on the electron density and total electron content in the nightside ionosphere of Mars. As input we use Mars Global Surveyor observations of a typical tail electron spectrum and an auroral-like electron spectrum. The accelerated electron spectrum increases the maximum number density and total electron content by a factor of 3 over that produced by the typical tail spectrum. Our calculations show a secondary electron density peak due to precipitation of several keV electrons not seen in previous modeling efforts. Regions of enhanced ionization are expected to be localized in space, corresponding to magnetic cusps formed by the interaction of crustal sources with the interplanetary magnetic field. Radio and radar measurements from both Mars Global Surveyor and Mars Express agree with this expectation. The horizontally inhomogeneous regions of ionization can affect signals used for subsurface sounding from orbit.