A denser and elevated Martian nightside ionosphere during Mars season Ls 180–360°

Using the Martian ionospheric electron density data obtained by the LPW instrument onboard MAVEN spacecraft from Oct. 2014 to Oct. 2018, we analyzed the seasonal behaviors of the Martian nightside ionosphere. It is found that from Mars season Ls 0–180° to Mas season Ls 180–360°, 1) at 200–300 km, the nightside ionospheric electron densities uniformly increased; 2) below 200 km, the electron density profiles rose in altitude, and peak electron densities increased. It is also found that the seasonal behaviors of the Martian nightside ionosphere are related to the crustal magnetic field topology. Larger seasonal variations of nightside ionospheric electron density can be observed in the regions where the crustal magnetic field is weak or quasi-vertical. We found that our current understandings of the Martian nightside ionosphere can only explain part of its seasonal behaviors, while further observations and simulations are needed to explain the remaining part.