Proton Aurora on Mars: A Dayside Phenomenon Pervasive in Southern Summer. Issue 12 (12th December 2019) - Hughes, Andréa
Abstract: We present observations of proton aurora at Mars made using the Imaging UltraViolet Spectrograph (IUVS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Martian proton aurora display a prominent intensity enhancement in the hydrogen Lyman‐alpha (121.6 nm) emission between ~110 and 150 km altitude. Using altitude‐intensity profiles from periapsis limb scan data spanning nearly two Martian years, we create a comprehensive database of proton aurora and characterize their phenomenology. Due to Mars' lack of a global dipole magnetic field, Martian proton aurora are expected to form on the dayside via electron stripping and charge exchange between solar wind protons and the neutral corona. We observe proton aurora in ~14% of dayside periapsis profiles (with notable seasonal variability), making proton aurora the most commonly observed type of aurora at Mars. We determine that the primary factors influencing proton aurora occurrence rates are solar zenith angle and season. The highest proton aurora occurrence rates are at low solar zenith angles on the Mars dayside, consistent with known formation processes. Proton aurora have highest emission enhancements, peak intensities, peak altitudes, and occurrence rates (nearing 100%) around southern summer solstice. This time period corresponds with the seasonal inflation of the neutral lower atmosphere, the onset of Martian dust storm season, seasonally increased coronal hydrogen column densities, and higher atmospheric temperature and solar wind flux following perihelion. The results of our study provide a new understanding of the primary factors influencing proton aurora, and the long‐term variability of these phenomena as observed over multiple Mars years. Plain Language Summary: We present the results of a multi‐year study of a new type of aurora recently identified at Mars. These "proton aurora" form when protons from the solar wind interact with hydrogen in the extended portions of the Martian atmosphere and travel to lower regions. Proton aurora are observed on the dayside of Mars in 14% of the data, which is far more often than initially expected and also more than any other type of aurora at Mars. Proton aurora occur most frequently (over 80% of the time, and in some cases almost 100% of the time) on the dayside side of the planet during the southern hemisphere's summer season (northern hemisphere winter). Around this time period, the lower atmosphere has previously been found to inflate, and dust storm season begins. During this time, hydrogen surrounding the planet also seasonally expands, allowing for more interactions between the solar wind and hydrogen in the upper atmosphere, and creating more proton aurora in this season. Through this study we hope to better understand the Sun‐Mars system and the variations in proton aurora as observed over many years. Key Points: Proton aurora form via interactions between the solar wind and H corona; as such, changes in the H corona strongly influence proton aurora Proton aurora occur in ~14% of dayside profiles in our data (varying with season), making them the most commonly observed aurora at Mars Proton aurora occurrence rates are highest in dayside southern summer observations, nearing 100% at low solar zenith angles at this time