Characterizing Average Electron Densities in the Martian Dayside Upper Ionosphere

We use more than 10 years of the Martian topside ionospheric data measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding radar sounder on board the Mars Express spacecraft to derive an empirical model of electron densities from the peak altitude up to 325 km. Altogether, 16,044 electron density profiles obtained at spacecraft altitudes lower than 425 km and at solar zenith angles lower than 80° are included in the analysis. Each of the measured electron density profiles is accurately characterized by the peak electron density, peak altitude, and three additional parameters describing the profile shape above the peak: (i) steepness at high altitudes, (ii) main layer thickness, and (iii) transition altitude. The dependence of these parameters on relevant controlling factors (solar zenith angle, solar irradiance, crustal magnetic field magnitude, and Sun‐Mars distance) is evaluated, allowing for a formulation of a simple empirical model. Mars Atmosphere and Volatile EvolutioN Extreme Ultraviolet monitor data are used to show that the solar ionizing flux can be accurately approximated by the F10.7 index when taking into account the solar rotation. Electron densities predicted by the resulting empirical model are compared with electron densities locally evaluated based on the Mars Advanced Radar for Subsurface and Ionosphere Sounding measurements, with the Langmuir Probe and Waves electron density measurements on board the Mars Atmosphere and Volatile EvolutioN spacecraft, and with electron densities obtained by radio occultation measurements. Although the electron densities measured by the Langmuir Probe and Waves instrument are systematically somewhat lower than the model electron densities, consistent with former findings, the model performs reasonably well. The ionosphere of Mars is the ionized part of its atmosphere, on the dayside ultimately controlled by the solar irradiation. Information about the electron density in there can be, among others, obtained by the radar sounding from a spacecraft orbiting the planet. Such measurements have been performed since 2005 by the Mars Advanced Radar for Subsurface and Ionosphere Sounding on board the Mars Express spacecraft, and they provide us with electron density profiles from the spacecraft altitude down to the altitude of the peak electron density. We use more than 10 years of such measurements to develop an empirical model of typical ionospheric electron densities. The obtained results are compared with electron densities measured in situ by the Langmuir Probe and Waves instrument on board the MAVEN spacecraft available since 2014. A reasonable agreement between the model predictions and these independent observations is found. Finally, the analysis of solar radiation measured by Extreme Ultraviolet monitor on board the MAVEN spacecraft is used to show that, when the solar rotation is properly accounted for, the solar ionizing flux at Mars can be surprisingly well approximated by the solar radio flux measured at Earth. Each electron density profile is described by five parameters, allowing for a simple empirical modelSolar ionizing flux is well expressed by F10.7 index when taking into account the solar rotationModel densities agree with radio occultation and Mars Express MARSIS and MAVEN LPW local electron density measurements