Response of Mars's Topside Ionosphere to Changing Solar Activity and Comparisons to Venus

Studying how planetary ionospheres respond to changes in solar irradiance can inform our understanding of how planetary atmospheres have reacted to our changing Sun over the course of solar system history. The response of Mars's main ionospheric peak to changing solar irradiance has been well-studied. Less well understood are the altitudes above the main peak. We use radio occultation observations by Mars Global Surveyor and ground-based solar radio flux measurements to quantify the response of Mars's electron densities to changing solar irradiance at these altitudes. This analysis shows that the electron densities respond modestly at and just above the main peak, with the response increasing above or below the peak. We use a one-dimensional photochemical equilibrium model to reproduce the observed electron densities and investigate the influence of minor ions, changing electron temperatures, and changing neutral densities and composition on the resultant electron density. We find that the neutral atmosphere is primarily responsible for the observed behavior. We also make comparisons to Venus, which shows a more dramatic increase in its ionospheric response as a function of altitude. We show that while the ionospheric responses of Mars and Venus as a function of altitude are markedly different, recasting the analysis in terms of number of scale heights above the main peak brings the two planets into closer agreement, especially at the main peak. The agreement is worse at higher altitudes and this is attributed to Venus's larger proportion of atomic oxygen.