Overlapping ionospheric and surface echoes observed by the Mars Express radar sounder near the Martian terminator

Abstract

Radar soundings from the Mars Express spacecraft occasionally show ionospheric and surface echoes that overlap in frequency. For specular reflection from a horizontally stratified ionosphere such a frequency overlap is not possible, since ionospheric and surface reflections can only occur at frequencies below and above the maximum ionospheric plasma frequency, respectively. In this paper we show that such overlapping echoes are only observed near the terminator where strong horizontal gradients are present in the ionosphere. Using a simple ionospheric propagation model we show that the observed frequency overlaps are consistent with current estimates of the horizontal gradients near the terminator.