The role of magnetic topology and plasma pressure on the ionopause formation at Mars

At planets with no intrinsic dipolar magnetic field such as Mars, an ionopause is often formed, a boundary that separates the planetary plasma from the solar wind. The ionopause boundary and its formation drivers in the upper dayside ionosphere of Mars are yet to be fully characterised. In this study, we use observations from NASA's MAVEN mission to probe the Martian upper dayside ionosphere and describe the physics of the ionopause boundary as well as its variability drivers. We develop an automated method to identify the ionopause as the location of sharp gradients in electron density and temperature. We focus on the 8th MAVEN deep dip campaign (DD8) from October 2017 that consists of 50 consecutive orbits. The trajectories of the DD8 orbits are similar to each other and thus, the impact of the changing upstream conditions on the Martian ionosphere can be studied. We utilise and compare data from several instruments on board MAVEN, namely the LPW, SWEA, STATIC, SWIA and MAG instruments, in order to investigate in detail the factors controlling the ionopause. We find that the ionopause formation is correlated with the plasma pressure balance between the ionosphere and the magnetosheath, and with changes in magnetic topology. More specifically, an ionopause in most cases is formed where there is a change from closed to either open or draped magnetic field lines.