The effect of the induced magnetic field on the electron density vertical profile of the Mars׳ ionosphere: A Mars Express MARSIS radar data analysis and interpretation, a case study

We report the indirect detection of an induced magnetic field in the ionosphere of Mars and its effects on the electron density behaviour. The observations were made by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) aboard Mars Express, in its Active Ionospheric Sounding mode. During several orbits on June 2006, the ionosphere showed an unusual behaviour, characterised by a compression of the plasma above the main ionospheric peak as observed by the topside total electron content, the plasma scale height, and the local plasma in the Mars Express surroundings. The compression was most likely due to an induced magnetic field originating from the solar wind and measured by the MARSIS antennas, which was able to penetrate into the ionosphere. In particular, for several profiles, the density distribution can be clearly defined by two different plasma scale heights, which indicates a transition region between both of them. From the balance of magnetic and thermal plasma pressures and from a comparison with a numerical model of the Martian ionosphere, the hypothesis of a penetrating induced magnetic field down to a transition altitude around 150 km is confirmed. This compressed ionosphere has also been compared with data from other orbits in the same location and at the same time period, i.e. 18.5 days of difference between first and last orbits, where there is no measured induced magnetic field, and the orbits show a clearly different behaviour.