Discrete Aurora on the Nightside of Mars: Occurrence Location and Probability

We report the first attempt to predict the occurrence location and probability of discrete electron aurora on the nightside of Mars. We run a 3-D time-dependent MHD model to characterize the spatial and temporal dynamics of magnetic field and plasma distributions over the course of one planetary rotation. We perform 8 simulation cases under solar minimum quiet-solar-wind conditions (4 equinox/solstice seasons, each with 2 IMF polarities) and in an actual interplanetary coronal mass ejection (ICME) case toassess quiet and space weather situations, respectively. The occurrence of detectable discrete aurora is subject to the combination of the probabilities that 1) the ionosphere is magnetically connected with high altitudes through open field lines, and 2) precipitating energy fluxes of >30 eV electrons exceed 0.1 erg/cm2/s. Our results show that during quiet solar activity, discrete aurora occurs likely on small-scale patches embedded inside strong crustal magnetic field regions (with a magnitude greater than 50 nT at 150 km), and the overall chance across the globe is 0.77%. The higher probability over strong crustal field regions is attributed to the stronger magnetic field convergence. The occurrence probability dramatically increases during the ICME event, particularly by morethan an order of magnitude in weak crustal field regions. Our model results reasonably agree with MAVEN and MEX observations. Our study suggests that nightside discrete electron aurora is not caused by the direct entry of magnetosheath plasma in a cusp-like process but due to the recycling of nightside magnetospheric electrons.