Response of peak electron densities in the martian ionosphere to day-to-day changes in solar flux due to solar rotation

Photochemical Chapman theory predicts that the square of peak electron density, Nm, in the dayside ionosphere of Mars is proportional to the cosine of solar zenith angle. We use Mars Global Surveyor Radio Science profiles of electron density to demonstrate that this relationship is generally satisfied and that positive or negative residuals between observed and predicted values of Nm are caused by periods of relatively high or low solar flux, respectively. Understanding the response of the martian ionosphere to changes in solar flux requires simultaneous observations of the martian ionosphere and of solar flux at Mars, but solar flux measurements are only available at Earth. Since the Sun’s output varies both in time and with solar latitude and longitude, solar flux at Mars is not simply related to solar flux at Earth by an inverse-square law. We hypothesize that, when corrected for differing distances from the Sun, solar fluxes at Mars and Earth are identical when shifted in time by the interval necessary for the Sun to rotate through the Earth–Sun–Mars angle. We perform four case studies that quantitatively compare time series of Nm at Mars to time series of solar flux at Earth and find that our hypothesis is satisfied in the three of them that used ionospheric data from the northern hemisphere. We define a solar flux proxy at Mars based upon the E10:7 proxy for solar flux at Earth and use our best case study to derive an equation that relates Nm to this proxy. We discuss how the ionosphere of Mars can be used to infer the presence of solar active regions not facing the Earth. Our fourth case study uses ionospheric observations from the southern hemisphere at latitudes where there are strong crustal magnetic anomalies. These profiles do not have Chapman-like shapes, unlike those of the other three case studies. We split this set of measurements into two subsets, corresponding to whether or not they were made at longitudes with strong crustal magnetic anomalies. Neither subset shows Nm responding to changes in solar flux in the manner that we observe in the three other case studies. We find many similarities in ionospheric responses to short-term and long-term changes in solar flux for Venus, Earth, and Mars. We consider the implications of our results for different parametric equations that have been published describing this response. r 2005 Elsevier Ltd. All rights reserved.