Variability in Martian Proton Aurora as Observed by MAVEN/IUVS

We present observations of proton aurora at Mars using the Imaging UltraViolet Spectrograph (IUVS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Proton aurora are a third type of aurora identified at Mars in addition to discrete and diffuse solar energetic charged particle aurora (Deighan et al., 2018). While terrestrial proton aurora are strongly influenced by Earth's global magnetic field, Martian proton aurora have inherently different formational processes and constraints due to Mars' lack of a planetary-wide magnetic field. We present the results of a detailed study of Martian proton aurora, characterizing these phenomena and their driving mechanisms, and creating a comprehensive database of proton aurora events. Using the hydrogen Lyman-alpha spectral emission at 121.6 nm wavelength during IUVS periapsis, we construct altitude-intensity profiles of IUVS limb scan data. Due to the optically thick nature of Lyman-alpha, the typical altitude profile of hydrogen is relatively flat. Martian proton aurora profiles, however, display a prominent emission enhancement peak between 120-150 km altitude. We observe this characteristic proton aurora signature as an increased brightness-altitude peak in many IUVS limb scans across the mission. Further, we observe variations in the shape, intensity, and altitude of the emission peaks corresponding with changing season, solar zenith angle, and solar flux. These observations provide a deeper understanding of the characteristics and driving processes of Martian proton aurora, as well as the Sun's interaction with the Martian upper atmosphere. By understanding proton aurora at Mars we can better understand the connection between the solar wind and Mars' hydrogen corona, which in turn informs our understanding of other planetary bodies that exhibit a neutral hydrogen corona and lack a global magnetic field.