Diffuse and Discrete Aurora on Mars: Insights into Mars' magnetospheric environment and solar drivers

Planetary auroras reveal the complex interplay between an atmosphere and the surrounding plasma environment. The Imaging Ultraviolet Spectrograph (IUVS) on the MAVEN spacecraft has revealed Mars to be a planet with astonishingly varied auroral activity. Contrary to expectations, the planet's lack of a global magnetic field allows additional types of auroral activity not possible on other worlds. This presentation describes two types of aurora, called diffuse and discrete aurora, while a companion presentation by Hughes et al. will describe proton aurora. The Imaging Ultraviolet Spectrograph (IUVS) on the MAVEN spacecraft has discovered a low-altitude, diffuse auroras spanning much of Mars' northern hemisphere, coincident with a solar energetic particle outburst. During multiple several-day periods, IUVS has detected auroral emission in nightside observations for up to ~5 days, spanning nearly all geographic longitudes. Emission extended down to ~60 kilometer (km) altitude (1 microbar), deeper than confirmed at any other planet. Solar energetic particles were observed up to 200 kilo-electron volts; these particles are capable of penetrating down to the 60 km altitude. Given minimal magnetic fields over most of the planet, Mars is likely to exhibit auroras more globally than Earth. MAVEN/IUVS has also identified 278 occurrences of discrete aurora events on Mars, which are patchy, sporadic ultraviolet emissions emanating from the upper atmosphere. We confirm prior results from Mars Express/SPICAM, finding that emissions are highly correlated with crustal magnetic fields results, with the brightest and most frequent occurrences located around strong crustal fields in the southern hemisphere. A six-year data set shows that events can also occur globally, in regions of weak or absent crustal fields. We find that events occur primarily in evening hours, especially during favorable orientations of the interplanetary magnetic field. Under these conditions, auroral events probably occur nightly and last for hours. Optical counterparts to these UV emissions would probably be detectable with present-day instrumentation, and would be visible to future astronauts. Taken together, the newly discovered and characterized forms of martian aurora are both a remarkable planetary phenomenon and an chance to learn about the martian magnetosphere and its interaction with the solar wind and solar storms. Many further opportunities await future missions instrumented to study aurora, at both ultraviolet and visible wavelengths.