The Response of the Mars Ionosphere to Solar Flares

During a flare, the increase in solar flux at X-ray and EUV wavelengths causes an enhancement in electron densities in planetary ionospheres. Here we identify and analyze a set of 12 Mars Global Surveyor (MGS) radio occultation electron density profiles which have been affected by solar flares. These profiles coincide with flares in GOES XL flux (0.1-0.8 nm), and exhibit electron densities which are significantly enhanced above the daily average (by >30% at 100 km), at altitudes below the M1 peak (~110 km), where X-ray flux is absorbed, (<10 nm). With this ensemble of flares and enhanced electron density profiles, we investigate the dependence of the ionospheric response on two important factors: the enhancement in X-ray solar flux during the flare, and on the optical depth of the atmosphere. We characterize the relationship between these quantities with a suitable function, and show that the flare-induced enhancement in electron density increases with increasing enhancement in solar flux and with increasing optical depth. We similarly analyze the results of a 1D photochemical model of the response of the Mars ionosphere to a solar flare. We show that the enhancement in model electron densities reproduces the trends observed in the flare-affected MGS profiles.