Total electron content depression in the nightside Martian ionosphere: statistical results from Mars Express MARSIS measurements and implications

Thermal electron depression and energetic electron depletion are two well-known features of the nightside Martian ionosphere. While the latter has been extensively studied in the past, there is no report of systematic and firm identification of the former and hence the correlation between them is unknown. Applying an automatic procedure to the Mars Express total electron content (TEC) measurements, we identify a large number of TEC depression features that amount to one-third of the available nightside measurements. Similar to the observation of energetic electron depletion, large amplitude TEC depression is more frequently observed near strong magnetic fields, as an expected result of the established solar-wind-driven scenario that the shielding of precipitating electrons by closed magnetic loops is responsible for the observed reduction in thermal electron density. However, small amplitude depression appears to be uniformly distributed across the Martian surface, implying an alternative origin yet to be determined. Contrary to our expectation, TEC depression and energetic electron depletion are not usually collocated, but this dilemma may simply reflect the complexity of the magnetic field topology with respect to the spacecraft trajectory.