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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 similar to 50% of the nightside ionosphere below similar to 160km and similar to 25% above similar to 220km (on average).

Plain Language Summary Mars' dayside ionosphere is produced by ionization of the neutral atmosphere by solar extreme ultraviolet and X-ray photons. This photoionization source is absent at night, so the nightside ionosphere must be supplied by plasma transport from day to night or created in situ by impact ionization caused by the precipitation of energetic electrons onto the neutral atmosphere. In this study, we use measurements from the Mars Atmospheric and Volatile EvolutioN spacecraft to identify these sources and estimate their contributions to the nightside ionosphere. At altitudes below similar to 160km, electron impact ionization accounts for half of the nightside ionosphere, with the other half supplied by transport from the dayside via collisional coupling with neutral winds. At higher altitudes, the nightside plasma density is found to be higher when it is directly supplied by transport from the dayside ionosphere.