Proton Aurora on Mars: A Dayside Phenomenon Pervasive in Southern Summer

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 SummaryWe 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.

Martian Oxygen and Hydrogen Upper Atmospheres Responding to Solar and Dust Storm Drivers: Hisaki Space Telescope Observations

“…The disk brightness in dusty periods (i.e., L S > 180°) was ∼2–5 times larger than that in periods 1 and 2 which correspond to the nondusty season (i.e., L S < 180°). This increase rate is similar to or somewhat smaller than those seen in other features in the hydrogen upper atmosphere from the nondusty season to the southern summer solstice (i.e., L S = 270°), such as hydrogen escape rates (10 times) (Chaffin, Deighan, Schneider, & Stewart, 2017; Rahmati et al., 2018), hydrogen column density upstream the BS (∼10 times) (Halekas, 2017), the deuterium Ly‐α limb brightness (∼10 times) (Mayyasi et al., 2017), the occurrence rate of the proton aurora (∼4–5 times) (Hughes et al., 2019). This indicates that the effect of the optical thickness becomes important and the disk brightness does not simply reflect the column density in the dusty season, meaning that the relative change in the Ly‐β brightness should be a lower limit of the relative change in the column density.…”

“…However, the residual from the linear fit was large (+58%) in this period and thus other effects might have contributed to the large disk brightness. One possible candidate is the proton aurora (Deighan et al., 2018; Hughes et al., 2019). The solar wind variation associated with the ICME could increase the proton aurora and change the disk brightness.…”

Precipitating Solar Wind Hydrogen at Mars: Improved Calculations of the Backscatter and Albedo With MAVEN Observations

“…Traveling at the solar wind speed, and unhindered by electromagnetic fields, the hydrogen ENAs precipitate into the upper atmosphere. There, the hydrogen ENAs interact with the atmosphere through a variety of collisional processes that result in energy deposition, excitation, and ionization of atmospheric species (Kallio & Barabash, 2001), proton aurora (Deighan et al., 2018; Hughes et al., 2019; Ritter et al., 2018), and ENA electron attachment and stripping (Bisikalo et al., 2017; Shematovich et al., 2011). Through these collisional processes, the ENAs are scattered, and a small fraction of them undergo enough collisions to be reflected back to space, resulting in a population of backscattered ENAs (Kallio & Barabash, 2001; Wang et al., 2018).…”

Kinetic Calculations of the Charge Exchange Efficiency for Solar Wind Protons in the Extended Martian Hydrogen Corona

“…In contrast to electron polar aurorae, the proton aurorae are observed only on the day side of the Mars, and these demonstrate an enhanced brightness of the hydrogen Lyα (121.6 nm) radiation revealed in limb observations within the altitude range of 120-150 km [4,6]. In the peak of radiation, the brightness increases by 50% (compared with the optically thick emission background observed throughout the day time) and continues to several hours maximum.…”

“…In the first approximation, we need to know only the number and spectrum of ENA-H atoms crossing the IM boundary for a correct interpretation of the observed proton aurorae. However, the flux of ENA-H atoms strongly depends on variations occurring in the density of the hydrogen corona and in the SW parameters [4,6]; thus, for the interpretation of observed proton aurorae, we must estimate the flux of ENA-H atoms considering parameters of the real medium. For the first time, the charge exchange efficiency has been calculated in [1,2], but the recent discovery of proton aurorae requires further investigations of the proton penetration into upper layers of the Martian atmosphere.…”