Electron densities in the ionosphere of Mars: A comparison of MARSIS and radio occultation measurements

Radio occultation electron densities measurements from the Mariner 9 and Viking spacecraft, which orbited Mars in the 1970s, have recently become available in a digital format. These data are highly complementary to the radio occultation electron density profiles from Mars Global Surveyor, which were restricted in solar zenith angle and altitude. Wehave compiled data from the Mariner 9, Viking, and Mars Global Surveyor radio occultation experiments for comparison to electron density measurements made by Mars AdvancedRadar for SubsurfaceandIonosphere Sounding (MARSIS), the topside radar sounder on Mars Express, and MARSIS-based empirical density models. Wefind that the electron densities measured by radio occultation are in generally good agreement with the MARSIS data and model, especially near the altitude of the peak electron density but that the MARSIS data and model display a larger plasma scale height than the radio occultation profiles at altitudes between the peak density and 200 km. Consequently, the MARSIS-measured and model electron densities are consistently larger than radio occultation densities at altitudes similar to 200-300 km. Finally, we have analyzed transitions in the topside ionosphere, at the boundary between the photochemically controlled and transport-controlled regions, and identified the average transition altitude, or altitude at which a change in scale height occurs. The average transition altitude is similar to 200 km in the Mariner 9 and Viking radio occultation profiles and in profiles of the median MARSIS radar sounding electron densities.