Solar wind erosion of the polar regions of the Mars ionosphere

Measurements conducted with the Analyzer of Space Plasmas and Energetic Atoms (ASPERA‐3) instrument of the Mars Express spacecraft provide data of plasma fluxes that stream away from the polar regions of the Mars ionosphere with energy spectra whose peak value increases with distance from the planetary surface. The observed energy distribution reveals a velocity boundary layer with ionospheric plasma that is eroded from the polar regions of the Mars ionosphere and that extends in the downstream direction within a geometry similar to that present along the polar flanks of the Venus ionosheath. The direction of motion of the ionospheric particles in those fluxes is close to that of the solar wind velocity and is not also oriented in a transverse direction as would have been expected if they were solely accelerated by the convective electric field of the solar wind. The ionospheric plasma eroded and deviated by the solar wind within the boundary layer forms a region whose shape is compatible with that of the asymmetric Mars plasma halo that was inferred from the X‐ray emission lines measured with the reflecting grating spectrometer of the XMM‐Newton telescope. The latter emission is interpreted as resulting from thermal dissipative processes associated with the transport of solar wind momentum to the polar upper ionosphere where both plasma populations interact with each other. Different conditions are applicable throughout most of the dayside hemisphere where the enhanced interplanetary magnetic field intensities that are observed within the ionosphere should make the interaction of the oncoming solar wind plasma with the ionospheric material less efficient.