Variability of Electron Densities in the Low-Altitude Martian Nightside Ionosphere Derived from the Intensity of Marsis AIS Surface Reflections

Abstract:

Sounding signals at frequencies higher than the ionospheric peak plasma frequency are not reflected by the ionosphere. Instead they make it to the ground where they are reflected by the planetary surface. We analyze the intensity of the surface reflections measured by the MARSIS ionospheric radar sounder on board the Mars Express spacecraft. Apart from the surface reflectivity, the intensity of the surface reflection is controlled primarily by the signal attenuation during ionospheric propagation. We focus on the nightside region, where the ionospheric densities in the main layer are too low to cause a significant attenuation and allow sampling of the surface reflection at frequencies down to 3 MHz. The attenuation is then expected to occur mainly at lower altitudes (<100 km), where electron-neutral collision frequency is a maximum. The intensity of surface reflections can thus serve as a proxy for the electron density at low altitudes not accessible by the direct MARSIS ionospheric radar sounding. We derive the intensity of surface reflections from all available MARSIS nightside ionograms, and we analyze its variability as a function of relevant controlling factors such as SZA, solar activity, magnetic field magnitude and inclination, and simultaneously observed electron density in the main ionospheric layer. The results obtained are discussed in terms of possible processes affecting the electron densities at low altitudes.