The Ionopause at Mars and Its Correlation With Magnetic Topology and Plasma Pressure

We utilize Mars Atmosphere and Volatile Evolution (MAVEN) observations to investigate the ionopause boundary at Mars, the formation process of which is not yet well described. We focus on the eighth deep dip campaign (DD8), which consists of 50 consecutive orbits, and we develop an automated routine to identify ionopause boundaries in electron density and temperature data. We find ionopause boundaries in 54 out of 100 ionospheric crossings and an average ionopause altitude of 368 km. Having detected the ionopause boundaries, we then examine in detail all the DD8 orbits using complementary observations from several MAVEN instruments. We show examples of two orbits, illustrating how the shapes of the topside ionosphere and ionopause can differ among ionospheric crossings and how the ionopause formation is correlated with changes in magnetic topology and the plasma pressure balance between the ionosphere and the magnetic pile-up region (MPR). We find that 70% of the detected ionopauses are formed where there are changes in magnetic topology, particularly from closed to either open or draped magnetic field lines, and 80% of the boundaries are also formed where the ionospheric plasma pressure becomes equal to the plasma pressure of the MPR. Finally, we confirm that the ionopause boundary is more likely to be formed under high solar wind dynamic pressure conditions.