Total electron content in the Mars ionosphere: Temporal studies and dependence on solar EUV flux

Total electron content (TEC) derived from radar signal distortions is a useful tool in probing the ionosphere of Mars. We consider 26 months of data from the subsurface mode of the Mars Express MARSIS instrument and confirm that the TEC dependence on solar zenith angle (SZA) approximately matches Chapman theory. After detrending this dependence, we find no clear trend with Martian season or dust activity but find that disturbed solar and space weather conditions can produce prolonged higher TEC values and that isolated solar energetic particle events are coincident with short-lived increases in TEC of similar to 10(15) m(-2) at all SZAs. We present the first comparison between TEC and directly measured solar EUV flux in the 30.4 nm He-II line. We find that the relationship between TEC and both He-II line irradiance and F-10.7 solar radio flux (a long-used EUV proxy) can be expressed as power laws with exponents of 0.54 and 0.44, respectively, in approximate agreement with Chapman theory.