Discrete Aurora on Mars: Insights into reconnection?

Analysis of nightside nadir-viewing observations taken by MAVEN's Imaging Ultraviolet Spectrograph instrument has identified nearly 200 discrete aurora emissions. Discrete aurora are sporadic localized ultraviolet emissions originating in the upper Martian atmosphere that occur brightest and most frequently near regions of strong crustal magnetic field strength. The emission detections were verified and characterized by visual appearance across the disk and spectral analysis of Cameron band and ultraviolet doublet emissions. No geographic or magnetic field information was used to determine whether a suspected emission was real or an artifact in the data. Unlike limb observations, nadir observations have no line-of-sight ambiguity, allowing us to locate the emissions with high geographic accuracy. Nadir viewing also provides global coverage of the nightside disk, giving broad geographic and local time coverage. We find the same dependence on local time, crustal field strength and interplanetary magnetic field orientation seen in limb observations (Schneider et. al. 2021). A large fraction of the observed events occur in open field regions associated with the strongest radial magnetic fields. These events occur along approximately east-west lines at the footprints of two magnetic field arcades, one with a north-directed horizontal crustral field and one south-directed (see below). Observations show that these arcades become active in an auroral sense at opposite times of night, one pre-midnight and the other post-midnight. We will show that the geometry of draping of the interplanetary magnetic field over the crustal fields provides a natural explanation for the different local time auroral triggerings, with magnetic reconnection more likely in one arcade pre-midnight and the other post-midnight. Figure 1: Mars Crustal Magnetic Field Geometry