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It is well established that a transverse (y) component in the interplanetary magnetic field (IMF) induces a By component in the closed magnetosphere through asymmetric loading and/or redistribution of magnetic flux. Simultaneous images of the aurora in the two hemispheres have revealed that conjugate auroral features are displaced longitudinally during such conditions. Although the direction and magnitude of this displacement show correlations with IMF clock angle and dipole tilt, single events show large temporal and spatial variability of this displacement. For instance, we know little about how the displacement changes during a substorm. A previous case study demonstrated that displaced auroral forms, associated with the prevailing IMF orientation, returned to a more symmetric configuration during the expansion phase of two substorms. Using the far ultraviolet cameras on board the Imager for Magnetopause‐to‐Aurora Global Exploration and Polar satellites, we have identified multiple events where conjugate auroral images are available during periods with substorm activity and IMF By≠0. We identify conjugate auroral features and investigate how the asymmetry evolves during the expansion phase. We find that the system returns to a more symmetric state in the events with a clear increase in the nightside reconnection rate and that the displacement remains unchanged in the events with little or no net closure of open magnetic flux. The return to a more symmetric state can therefore be interpreted as the result of increased reconnection rate in the magnetotail during the expansion phase, which reduces the asymmetric lobe pressure. Key Points: Longitudinally displaced auroral features become more symmetric during substorm expansion phaseThere is a relationship between the change in asymmetry and the nightside reconnection rateThe observed reduction in asymmetry is consistent with By being introduced into the closed magnetosphere by asymmetric pressure gradients [ABSTRACT FROM AUTHOR]

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). 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 ~160 km, 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. Key Points: At low altitudes ion motion collisionally couples to neutral winds, sourcing the nightside ionosphere asymmetrically from dawn and duskOn open field lines connected to the dayside, the nightside ionosphere is sourced by dayside plasma via pressure gradient forcesOn average, electron impact ionization produces ~50% of Mars' nightside ionospheric plasma below ~160 km and ~25% above 220 km [ABSTRACT FROM AUTHOR]