Two Types of Aurora on Mars as Observed by MAVEN's Imaging UltraViolet Spectrograph

The Imaging UltraViolet Spectrograph (IUVS) on the MAVEN spacecraft has detected two distinct types of auroral emission on Mars. First, we report the discovery of a low altitude, diffuse aurora spanning much of Mars' northern hemisphere coincident with a solar energetic particle outburst. IUVS observed northerly latitudes during late December 2014, detecting auroral emission in virtually all nightside observations for ~5 days spanning virtually all geographic longitudes. The vertical profile showed emission down to ~70 km altitude (1 microbar), deeper than confirmed at any other planet. The onset and duration of emission coincide with the observed arrival of solar energetic particles up to 200 keV precipitating directly and deeply into the atmosphere. Preliminary modeling of the precipitation, energy deposition and spectral line emission yields good matches to the observations. These observations represent a new class of planetary auroras produced in the Martian middle atmosphere. Given minimal magnetic fields over most of the planet, Mars is likely to exhibit aurora more globally than Earth. Second, we confirm the existence of small patches of discrete aurora near crustal magnetic fields in Mars' southern hemisphere, as observed previously by SPICAM on Mars Express (Bertaux et al., Nature, 435, 790-794 (2005)). IUVS observed southern latitudes in July and August 2015, detecting discrete auroral emission in ~1% of suitable observations. Limb scans resolved both vertically and along-slit indicate this type of auroral emission was patchy on the scale of ~40 km, and located at higher altitudes ~140 km. The higher altitudes imply a lower energy of precipitating particles. The mix of spectral emissions also differed signficiantly from the discrete aurora, indicating different excitation and quenching processes. We will discuss the observed properties of the aurora and associated charged particle precipitation, as well as the broader implications of this high-energy deposition into Mars' atmopshere.