Nightside ionosphere of Mars studied with local electron densities: A general overview and electron density depressions

The radar sounder Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express spacecraft provides local electron densities from electron plasma oscillations. Here, we use the local electron densities to study the nightside ionosphere of Mars. In this study, in which local densities were sampled to a minimum altitude of about 275 km, we measured the maximum average densities as 1000 cm(-3) on the nightside. The electron density profiles on the nightside are highly variable. An inverse exponential relationship is observed between the electron density and the altitude. At low altitudes, the median electron density decreases with increasing solar zenith angle (SZA). However, at high altitudes no dependence on SZA is observed. Steep electron density gradients, similar to the ionopause at Venus, are also observed in 15% of the passes in the nightside ionosphere. A commonly encountered structure on the nightside is an ionospheric density depression, which is a deep trough in the electron density. Nightside density depressions are large features, with an average width of 950 km. In some cases, the depressions in MARSIS data are associated with ion flow features in the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) data. In other cases, the depressions correspond to density depletion regions. Half of the depressions are aligned with the edge of the dayside-generated photoelectrons. It is concluded that several different conditions can cause the electron density depressions.