Martian Ionopause Boundary: Coincidence With Photoelectron Boundary and Response to Internal and External Drivers

The Martian ionopause boundary detected as steep gradients in the local electron density profiles from the Mars Advanced Radar for Subsurface and Ionospheric Sounder on Mars Express is studied individually and statistically and compared to the photoelectron boundary identified by the drop of photoelectron signature due to CO2 and O molecules. In ~90% of the cases where we have electron energy flux data, the ionopause coincides with the photoelectron boundary. The steep density gradients form at the border of the photoelectron region and above. The ionopause is observed in a wide range of latitude, longitude, and altitude. According to remote sounding investigations, the average thickness of the ionopause is ~30 km. The average altitude is between 500 and 700 km on the dayside. The altitude of the ionopause is inversely related to the solar wind dynamic pressure. Strong crustal magnetic fields increase the altitude of the boundary and they have a slight, negative effect on the occurrence only for high values. The ionopause occurs more frequently, and its altitude is higher during southern summer. The average altitude of the ionopause and solar irradiance are correlated with each other. The effect of the extreme ultraviolet flux on the occurrence rate is less noticeable. Key Points An ionopause boundary, defined as an altitudinal steep density gradient, has been detected in 13% of the cases in 12 years of MARSIS data In 89% of the cases, the ionopause coincides with the PEB The ionopause altitude is affected by solar wind dynamic pressure and crustal magnetic fields