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Using Magnetic Topology to Probe the Sources of Mars' Nightside Ionosphere

We combine thermal electron densities in Mars' ionosphere with magnetic topology information to investigate the sources of the nightside ionosphere. Thermal electron density is measured in situ by the Langmuir Probe and Waves experiment onboard Mars Atmospheric and Volatile EvolutioN, while magnetic topology is simultaneously inferred from suprathermal electron energy‐pitch angle distributions measured by the Solar Wind Electron Analyzer and the Magnetometer. Topologically closed regions inhibit electron impact ionization, allowing us to isolate the effects of plasma transport from the dayside, which exhibits a dawn‐dusk asymmetry. Pressure gradient forces on open magnetic field lines connected to the dayside ionosphere source the high‐altitude nightside ionosphere, resulting in higher densities. Regions that are topologically open to the nightside ionosphere allow us to assess in situ production by electron impact ionization, which is responsible for ~50% of the nightside ionosphere below ~160 km and ~25% above ~220 km (on average).