MARSIS observations of the Martian nightside ionosphere dependence on solar wind conditions

Despite the absence of solar radiation on the Martian nightside, a weak, irregular, and variable ionosphere is produced there. The nightside ionosphere is thought to be maintained by two main sources: dayside-nightside plasma transport and electron precipitation. Observations by Mars Express (MEX) and Mars Global Surveyor (MGS) have shown that these plasma sources are either hindered or favored by the presence of strong crustal magnetic fields and that these effects are modulated by external parameters, such as the solar wind dynamic pressure and the orientation of the interplanetary magnetic field (IMF). These external drivers are expected to influence the supply of plasma to the nightside and thus the formation of the irregular nightside ionosphere. We here present a statistical study of the Martian ionosphere at solar zenith angle greater than 107 degrees from November 2005 to May 2006, using remote measurements of ionospheric echoes with the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) radar sounder onboard MEX and using MGS-based proxies for the solar wind dynamic pressure and the IMF clock angle. We find that the peak densities increase with the dynamic pressure and also that cases of very high peak density are almost always associated with Westward IMF orientation. We find that, using MEX/ASPERA-3 electron data, these cases often seem to be linked to accelerated electrons. Plasma transport is known to be important in the near nightside. On the other hand, electron precipitation prevails when the dynamic pressure is high enough to compress the ionosphere and in vertical field regions where the IMF orientation matters.