Spatio-temporal variability of topside layers in Mars' ionosphere using nearsimultaneous Mars Express and MAVEN conjunctions

This study investigates the spatio-temporal variability of topside ionospheric layers over the strong crustal magnetic fields and an unmagnetised region in the southern hemisphere of Mars with multi-point observations between Mars Express and Mars Atmosphere and Volatile EvolutioN (MAVEN) very close in time and space. We identify 79 events between 2014 and 2021 when both spacecraft trajectories nearly overlapped. Four of these events, all from 2015 and in the dayside ionosphere, are selected: three occur over crustal field regions, and one over an unmagnetized region. We analyse the ionosphere using radar sounding observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on Mars Express, and compare them to in-situ ionospheric measurements by the Langmuir Probe and Waves (LPW) instrument on MAVEN. From across the four events of this study, comparison of observations from both spacecraft shows the motion of topside ionospheric layers, which exhibited maximum displacements of in latitude (149.4 km), 5.4 in east longitude (337.6 km) and up to 28 km in altitude over a span of 8-43 minutes. Electron densities of some of these structures increase by up to a factor of 2.6 during the same period of time. In this study, we demonstrate small and rapid variability occurring in the topside ionosphere of Mars, which was only possible to observe thanks to multi-spacecraft and near-simultaneous observations.