Mars Discrete Aurora: A Comprehensive Survey for Detection & Characterization by MAVEN/IUVS

We present the results of a comprehensive search for discrete aurora emissions on Mars from six years of observations by
MAVEN's Imaging UltraViolet Spectrograph. Discrete aurora is a localized and transient form of aurora apparently unique to
Mars, owing to its lack of a global magnetic field. The auroral emissions originate from precipitating electrons accelerated by the
reconfiguration of Mars' crustal magnetic fields as the planet rotates relative to the external magnetic field carried by the solar
wind. This process is distinct from other more widespread diffuse and proton aurora also seen at Mars.
Discrete aurora was discovered in regions of strong crustal magnetic fields by the SPICAM instrument on Mars Express using limb
scanning [Bertaux et al., 2005]. The emission appeared in patches ~tens of km across at altitudes ~130 km. Further analysis
revealed a total of 20 instances of auroral patches during 10 years of intermittent SPICAM observations [Gérard et al., 2015].
Auroral excitation was attributed to the precipitation of electrons, typically ~100 eV - 1 keV. MAVEN/IUVS obtained the first
images of the phenomenon (Schneider et al. 2018).
We have examined MAVEN's mission-long dataset of nightside limb scans spanning more than 10,000 orbits over nearly 6 Earth
years. Events were identified by significant emission in the CO Cameron bands (190-270 nm) and were individually confirmed to
be free of stray light and cosmic ray interference. More than 500 discrete aurora events were detected, increasing the number of
known events by more than an order of magnitude. The figure shows a remarkable string of distinct events seen during a single 20-
minute passage of Mars' crustal field region. The observed events show a strong concentration near crustal fields in the south, but
also exhibit a substantial distribution spread more uniformly over the entire planet. Some events are seen at the tangent altitude
expected for electron precipitation, but many appear at lower projected altitudes. We infer these are small patches of emission in
front of (or behind) the limb itself, and in some cases the spacecraft was probably imbedded in the emission.