The Role of the Magnetic Field Orientation on the Variability of the Martian Topside Ionosphere Using MARSIS-Mars Express Data

Mars, without a global dipole magnetic field, has its atmosphere directly exposed to the impinging solar wind, producing a complex interaction due to the remnant crustal magnetic fields, which, especially in the southern hemisphere, have spatially varying strengths and inclinations. “Mini-magnetospheres” generated over the strong crustal field regions, along with the planet's rotation, increase the complexity of ionosphere dynamics. This study analyses 28 Mars Express orbits spanning 12 years of observations from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument to characterize the variability of topside electron density profiles over strong crustal field anomalies in the southern hemisphere and far away from them. Variations are evaluated by quantifying the amount by which the electron density and total electron content depart from model fits assuming a Chapman layer. Regions of enhanced electron density, or topside layers, predominantly form during low solar activity both over and far from crustal field regions, indicating a solar cycle dependence. We find that topside layers are mostly confined to regions where crustal fields have horizontal inclinations or undergo drastic changes in orientation from vertical to horizontal. Moreover, the most horizontally extensive transient topside layers, spanning up to ∼33° latitude, are observed over unmagnetized regions dominated by draped horizontal magnetic fields. It is found that the total electron content on the dayside is enhanced by 12.5% in regions with horizontal magnetic fields and decreased by 15.0% where the fields are near-vertical.