ULF Waves Propagating Through the Martian Magnetosheath into the Ionosphere: A Statistical Study Using Mars Express Observations

The Martian magnetosphere is induced, and the low-energy ion escape is related to the extreme ultraviolet (EUV) solar radiation and to the solar wind pressure variations. The ion reflection at the bow shock is a permanent source of ultra-low frequency (ULF) waves. These are convected through the magnetosheath and can penetrate into the upper ionosphere, which can cause additional turbulence at the ionopause which can then cause an enhanced erosion of the ionosphere. Knowing the important role of waves produced in the magnetosheath in the loss processes of the Martian atmosphere, the present study has the aim of making a statistical study of potential cases of wave propagation into the ionosphere of Mars using 14 years of Mars Express (MEX) observations. Data of electron density from the ASPERA-3 and the MARSIS instruments have been used. An enhanced intermittency of ULF waves in Mars magnetosphere was indicated by the kurtosis analysis, which values were higher then 3.0 for all events and on all scales that we investigated. The most energetic frequencies in the ionosphere are in the same range of the main frequencies observed in the magnetosheath. Moderate classification was observed for the most part of the correlations found between ASPERA-3 and MARSIS data. It seems that wave penetration at Mars tends to occur mainly in the descending/maximum solar cycle phase and on the dayside. It was also seen that these events are more frequent in the northern hemisphere Autumn Equinox and during conditions of moderate solar wind pressure.