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.

Proton aurora at Mars were discovered using Mars Atmosphere and Volatile EvolutioN (MAVEN) mission Imaging UltraViolet Spectrograph (IUVS) limb scan observations (Deighan et al., 2018). The aurora results from the collision of H Energetic Neutral Atoms (ENAs) and protons with the bulk atmosphere, with every collision potentially resulting in an electronically excited ENA that promptly emits H spectrum photons. These H ENAs are produced upstream of the Martian bow shock when solar wind protons charge exchange with Mars coronal H, resulting in neutrals with the solar wind velocity that are not deflected around the bow shock with the rest of the solar wind (Ramstad et al., 2022). Ritter et al. (2018) showed that proton aurora can be triggered by coronal mass ejections and/or corotating interaction regions, consistent with this formation mechanism. In addition to proton aurora, this process results in thermospheric penetrating protons (Halekas et al., 2015) and H − ions (Jones et al., 2022), as well as coronal H pickup ions (Rahmati et al., 2018) and proton cyclotron waves (Romeo Abstract Proton aurora at Mars are thought to form indirectly, as a result of solar wind proton charge exchange with planetary coronal hydrogen upstream of the bow shock. This charge exchange produces beamed energetic neutral atoms that bypass the induced magnetosphere and cause spatially uniform auroral emission when they collide with the thermosphere. Here we report multiple definitive observations of spatially localized "patchy" proton aurora at Mars using the Emirates Mars Ultraviolet Spectrometer on the Emirates Mars Mission, and characterize the plasma environment during these events using contemporaneous Mars Atmosphere and Volatile EvolutioN mission measurements. Multiple mechanisms are required to explain these observations, including at times the direct deposition of solar wind plasma into the thermosphere, particularly during radial interplanetary magnetic field conditions. Much future work will be needed to assess these mechanisms and understand the impact of these auroral events on Mars atmospheric evolution.Plain Language Summary Even though Mars does not have a global magnetic field like the Earth, it still possesses multiple kinds of aurora. One of these is proton aurora, which is thought to form mainly by an indirect process that allows a small fraction of the solar wind to rain down on the planet uniformly across the dayside. We present observations of patchy proton aurora at Mars that require a different explanation. By examining multiple Emirates Mars Mission observations of patchy aurora that have different shapes and locations, and combining these images with plasma measurements made by NASA's Mars Atmosphere and Volatile EvolutioN mission, we conclude that a number of processes can produce patchy aurora. This patchy aurora is mostly the result of plasma turbulence, which under some circumstances leads to direct deposition of the solar wind across the entire Martian dayside, with a potentially large impact on long term ...

The Martian proton aurora is a distinct aurora phenomenon resulting from the direct deposition of solar wind energy into Mars' dayside atmosphere. What solar wind parameters influence the aurora activity in the short term is yet unknown, as are the associated repercussions in the Martian atmospheric ion loss. Here we present observational evidence of synchronized proton aurora brightening and atmospheric ion loss intensifying on Mars, controlled by solar wind dynamic pressure, using observations by the Mars Atmosphere and Volatile Evolution spacecraft. The solar wind dynamic pressure possibly has a saturation effect on brightening proton aurora. Significant erosion of the Martian ionosphere during periods of high dynamic pressure indicates at least five‐to‐tenfold increase in atmospheric ion loss. An empirical relationship between ion escape rate and auroral emission enhancement is established, providing a new proxy of Mars' atmospheric ion loss with optical imaging that may be used remotely and with greater flexibility.

Io is a priority destination for solar system exploration, as it is the best natural laboratory to study the intertwined processes of tidal heating, extreme volcanism, and atmospheremagnetosphere interactions. Io exploration is relevant to understanding terrestrial planets and moons (including the early Earth), ocean worlds, and exoplanets across the cosmos.1. Io is a priority destination for future exploration. Jupiter's innermost large moon, Io, is the most geologically active world in the solar system (Fig. 1). Io's surface is marked by hundreds of active volcanoes, erupting lava fountains, evolving sulfurous ices, enormous mountains, and deposits from towering volcanic plumes that pollute the Jovian system and feed its enormous magnetosphere. This unparalleled activity is powered by rampant tidal heating, where the gravitational interactions between Io and its neighboring moons result in time-varying tides from Jupiter that deform and heat Io's interior. Io is the best natural laboratory to study these intertwined processes, and it is a vitally important destination for addressing high priority, cross-cutting science investigations relevant to broad swaths of planetary science-from the Hadean Earth-Moon system when life emerged, to present-day potentially habitable ocean worlds, and distant exoplanets where conditions are even more extreme. Characterization of Io will guide future observations of both ocean worlds and exoplanetary targets. In a sense, Io is the uninhabitable world that teaches us how habitable worlds form and work.