Dayside ionosphere of Mars: Empirical model based on data from the MARSIS instrument

We present results of a systematic study of electron densities in the dayside Martian ionosphere measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on board the Mars Express spacecraft. There are two distinct regions controlled by different physical mechanisms. The first region is located at altitudes up to about 5 neutral scale heights above the altitude of peak electron density. Electron densities in this region are well described by the basic Chapman theory. The observed small deviations can be most probably explained by the neutral scale height and electron temperature increasing with altitude rather than being constant. The second region is located at altitudes higher than about 10 neutral scale heights above the altitude of peak electron density. It is controlled primarily by diffusion, and the observed electron densities decrease exponentially with increasing altitude. The corresponding diffusion scale height increases with increasing solar zenith angle, which can be probably explained by nearly horizontal magnetic fields in the ionosphere induced by interaction with the solar wind. The obtained dependencies can be used as a simple empirical model of the dayside Martian ionosphere.