Discrete Aurora on Mars: Spectral Properties, Vertical Profiles, and Electron Energies

We present an analysis of hundreds of middle ultraviolet auroral spectra collected at the limb with the Imaging UltraViolet Spectrograph (IUVS) instrument on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. While the companion paper by Schneider et al. (2021), focuses on the detection, location, and occurrence frequency of discrete auroral events, this study addresses the spectral properties and vertical profiles of the auroral emissions. Our independent selection of events is based on a combination of automatic and manual detection methods with adequate signal-to-noise ratio of both the CO Cameron bands and the CO2+ ultraviolet doublet (UVD) at 190-270 and 288-289 nm, respectively. We find that the ratio of these two features remains quasi-constant for UVD intensities exceeding similar to 200 rayleighs (R), but the CO Cameron/CO2+ UVD ratio may become increasingly large for low UVD intensities. Three weak N-2 Vegard-Kaplan bands are identified in the Martian aurora for the first time. Limb profiles of the [OI] line at 297.2 nm indicate that the visible oxygen green line brightness may reach a few kilorayleighs. The distribution of the altitude of the emission peaks in the aurora is identical in and out of the region of crustal magnetic field located in the southern hemisphere. Comparisons of in situ measurements of electron energy spectra and ultraviolet auroral detections have been made for five optical detections. They generally show temporal coincidence but not necessarily quantitative agreement with the altitude and brightness expected from the characteristics of the measured electron energy spectra.

Automated summary

This study presents an analysis of middle ultraviolet auroral spectra collected at the limb on Mars using the IUVS instrument on board the MAVEN spacecraft. It focuses on the spectral properties and vertical profiles of the auroral emissions, identifying new features and providing insights into the Martian aurora. Comparisons with in situ measurements of electron energy spectra show temporal coincidence but not necessarily quantitative agreement with the characteristics of the auroral emissions.