Reinvestigating the Nightside Ionosphere of Mars With 8 Years of Mars Express and MAVEN Data

Tailward ion escape in the nightside magnetotail is one of the dominant paths of ion escape from Mars. The Martian nightside ionosphere can serve as a major reservoir for this escape channel, but its global distributions and dependence on the upstream solar wind have not been fully investigated due to observational limitations. In this study, we analyzed 8 years of Mars Express's Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) observations and Mars Atmosphere and Volatile EvolutioN (MAVEN) upstream solar wind data from October 2014 to November 2022, thereby ensuring global coverage. From MARSIS data, we derived the peak electron density and detection rate of the nightside ionosphere and investigated their dependence on the MAVEN upstream solar wind conditions. We find that the observed dependence is different between weak and strong crustal magnetic field regions. In the weak crustal magnetic field regions, relatively low density () ionospheres were observed, and high solar wind dynamic pressure and the outward direction of the solar wind electric field are particularly favored for the detection of nightside ionospheric echoes. As for the strong crustal magnetic field regions, very high‐density () nightside ionospheres were observed. They tend to be observed under near anti‐parallel conditions between draped interplanetary magnetic fields (IMF) and crustal magnetic fields, suggesting a contribution of magnetic reconnection between them. These results align with Martian electron auroral observations and highlight the importance of electron impact ionization.