Seasonal and Geographical Variability of the Martian Ionosphere From Mars Express Observations

We study the seasonal and geographical variability of the peak electron density and the altitude of the main ionospheric peak at Mars. For this purpose, we use the data obtained by the ESA Mars Express mission, namely by the radar MARSIS and the radio occultation experiment MaRS. The accumulation of data during the long lifetime of Mars Express provides for the first time an almost complete seasonal and geographical coverage. We first remove the dominant variability factors affecting the main ionospheric peak, namely the effect of changes in the solar zenith angle (SZA), and the changes in the solar ultraviolet radiation output at the Sun. When averaging results obtained at all latitudes, we find that the seasonal variation of both the peak density and the peak altitude can be well reproduced by sinusoidal functions with amplitudes about 8%-9% of the annually averaged peak density, and between 8 and 9.5 km for the peak altitude. We also find elevated peak electron densities in the region of strong crustal fields and latitudinal asymmetries in both the peak density and altitude. Comparing the seasonal evolution of the peak altitude during Mars Year 28, a year with a global dust storm, and the rest of the years, we find that the global dust storm raised the altitude of the ionospheric peak by about 10-15 km.