Response of the Mars ionosphere to solar flares: Analysis of MGS radio occultation data

Increased soft X-ray irradiance during solar flares produces increased electron densities in the lower ionosphere of Mars, and the relative changes in electron density during a flare are greater for lower altitudes and larger flares. However, this relationship has not been quantified. This has impeded the validation of simulations of the ionospheric response to flares, which are necessary for developing accurate descriptions of the physical processes governing ionospheric behavior under extreme conditions. Here we develop a response function, a mathematical expression for the change in electron density during a solar flare as a function of the change in solar flux and an optical depth proxy. This function is based on analysis of 20 Mars Global Surveyor (MGS) radio occultation electron density profiles measured during solar flares. We find that characterizing the response as a function of optical depth, rather than altitude, provides the best description of ionospheric variability during a flare. A separate response function, determined from analysis of a numerical simulation of the response to a solar flare, was found to be grossly similar to the observationally based response function, though with a weaker dependence on optical depth. We identify 15 MGS profiles with an apparent solar flare response, but no coincident detected solar flare. We suggest that the observed response function can be used to detect flares not visible from Earth and to give an approximation of their strength. Additionally, it can estimate ionospheric electron densities during a flare; however, precision is limited by a small number of observations.