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Abstract: We report the detection of bright aurora spanning Mars' nightside during the space weather event occurring in September 2017. The phenomenon was similar to diffuse aurora detected previously at Mars, but 25 times brighter and detectable over the entire visible nightside. The observations were made with the Imaging UltraViolet Spectrograph, a remote sensing instrument on the Mars Atmosphere and Volatile EvolutioN spacecraft orbiting Mars. Images show that the emission was brightest around the limb of the planet, with a fairly uniform faint glow against the disk itself. Spectra identified four molecular emissions associated with aurora, and limb scans show the emission originated from an altitude of ~60 km in the atmosphere. Both are consistent with very high energy particle precipitation. The auroral brightening peaked around 13 September, when the flux of solar energetic electrons and protons both peaked. During the declining phase of the event, faint but statistically significant auroral emissions briefly appeared against the disk of the planet in the form of narrow wisps and small patches. These features are approximately aligned with predicted open field lines in the region of strong crustal magnetic fields in Mars' southern hemisphere. [ABSTRACT FROM AUTHOR]

Discrete aurora are sporadic emissions of light originating in Mars upper atmosphere. We report nadir imaging observations from MAVEN's Imaging UltraViolet Spectrograph which identify the conditions which trigger electron precipitation causing these events. Prior studies have shown that discrete aurora events in the strong crustal magnetic field region in the southern hemisphere are the brightest and most repeatable compared to events occurring outside the region. Our new data set offers a more complete and accurate characterization of aurora in this area. The region of strongest crustal fields is composed of two distinct magnetic regions, with magnetic fields in opposite directions; discrete aurora events trigger in one region after dusk and in the other before dawn. Magnetic reconnection in these two adjacent regions with the draped interplanetary field may open the crustal fields in these regions during opposing local times. Particle precipitation can then cause discrete aurora at the observed times and locations. Plain Language Summary: Mars has a surprising variety of types of aurora, all different from Earth's "northern lights." Mars lacks the familiar high‐latitude aurora because it no longer has the same kind of global magnetic field Earth does. This study examines "discrete aurora" events that occur in region in Mars southern hemisphere that has retained some of the ancient magnetic field. It takes the form of long arcades of magnetic loops that resemble a set of arches. As Mars rotates, these arcades are carried around the planet as the solar wind and its imbedded magnetic field are carried past the planet. We show that when conditions are favorable, the magnetic field locked in the solar wind can interact and "reconnect" with Mars magnetic loops, allowing energetic particles to spiral down the field lines into the atmosphere to cause discrete aurora. Key Points: New data confirm that Mars discrete aurora events occur most frequently near strong crustal fields and vary with local timeAuroral events in adjacent regions with opposite magnetic polarity occur before or after midnight depending on local magnetic field directionMagnetic reconnection between crustal fields and the draped interplanetary field appears to control regional and local time behavior [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Ion loss during solar events early in Mars history may have been the major cause to the long‐term evolution of the Mars atmosphere. Restricted by observation data, the relationship between ion loss and solar wind is still in debate. We have shown observational evidence for synchronized dayside proton aurora brightening and atmospheric ion loss intensifying on Mars, controlled by solar wind dynamic pressure using comprehensive observations by the NASA's MAVEN spacecraft. A power law model between ion escape rate and proton auroral emission enhancement is established for the first time, providing a new proxy of Mars' atmospheric loss with optical imaging that may be used remotely and with greater flexibility. The results presented in this study shed light on the direct control of dynamic pressure on interactions of the solar wind with Mars and other objects in solar system (e.g., Venus and Titan), and presumably stellar wind with exoplanets. Key Points: Synchronized proton aurora brightening and atmospheric ion loss intensifying on Mars, both controlled by solar wind dynamic pressureThe solar wind dynamic pressure possibly has a saturation effect on brightening proton auroraAn empirical power law model between ion escape rate and proton auroral emission enhancement is established [ABSTRACT FROM AUTHOR]

We present the first measurements of Mars discrete aurora in the extreme ultraviolet (<110 nm) and the first synoptic aurora images in the far ultraviolet (110–180 nm). Auroral emission is detected in >75% of nightside images, with patterns shifting visibly over 15–20 min. Aurora is observed most frequently in regions of open magnetic topology (where crustal magnetic fields are very weak and/or vertical), with the brightest aurora where crustal fields are strongest. We present the first disk‐averaged spectrum of discrete aurora, with several O, C, and CO features as expected for electron impact primarily on CO2. We categorize discrete auroral morphology into three types: crustal field aurora, non‐crustal field patchy aurora, and a new type we call "sinuous" aurora, an elongated serpentine structure that stretches thousands of kilometers into the nightside from near midnight in the northern hemisphere. These observations point to a highly dynamic environment in Mars' magnetotail. Plain Language Summary: In this study, we present near‐global images of localized aurora on Mars and the first measurements of these aurora at very short ultraviolet wavelengths (<110 nm). They are caused by energetic electrons from the solar wind smashing into Mars' upper atmosphere. We find auroras in >75% of images, with their patterns shifting visibly over 15–20 min. They are observed most frequently in regions where magnetic fields are very weak or both strong and vertical, with the brightest aurora where magnetic fields are strongest. We present the first disk‐averaged spectrum of these auroras, showing features expected for electrons striking CO2 (the most abundant gas in Mars' atmosphere). We categorize discrete auroral patterns into three types: those near strong vertical crustal magnetic field, patchy aurora near very weak crustal fields, and a new type we call "sinuous," an elongated serpentine structure that stretches thousands of kilometers into the nightside from near midnight in the northern hemisphere. These observations point to a highly dynamic environment in Mars' nightside space environment. Key Points: We present the first disk measurements of Mars discrete aurora in the EUV end FUV, with the oxygen feature at 130.4 nm being the brightestAuroras are detected in ∼75% of nightside images and are more likely where crustal fields are either very weak or both strong and verticalAn elongated, sinuous discrete aurora is discovered, extending far into the nightside. It may be related to the magnetotail current sheet [ABSTRACT FROM AUTHOR]

Discrete aurorae have been observed at magnetized planets such as Earth and Jupiter, triggered by accelerated electrons. Similar aurorae have also been observed at Mars with only localized strong crustal magnetisms. However, our understanding of this phenomenon at Mars is still limited. In particular, direct and quantitative comparisons of the auroral and its source electron events are lacking as these two types of observations are usually made at different times and/or locations. In this study, we establish empirical criteria to select electron events ("auroral electrons") that could trigger detectable auroral emissions with Mars Atmosphere and Volatile EvolutioN measurements, thereby enabling a direct statistical comparison. We find auroral electrons share similar statistical characteristics to those previously reported for discrete auroral events. This study bridges the gap between electron observations and auroral detections and enables collaborations across different Mars missions, as well as comparative planetary studies of discrete aurora. Plain Language Summary: Aurorae have been observed in the polar regions at magnetized planets such as Earth and Jupiter, triggered by accelerated electrons. Similarly, localized auroral emissions have also been reported at Mars with no global dipole field but localized strong crustal magnetisms. However, our understanding of aurorae at Mars is still very limited in terms of their characteristics and the source electrons. In particular, direct and quantitative comparisons of the auroral and electron events are lacking as these two types of observations are usually made at different times and/or locations because of the limitation of single‐spacecraft observations. This study establishes empirical criteria to select auroral electrons and statistically compare these two types of observations. This study bridges the gap between the electron observations and auroral detections and enables collaborations across different Mars missions, as well as studies of auroral phenomena at different planets. Key Points: We establish empirical criteria to select electron events that could trigger detectable auroral observationsWe find similar statistical behaviors in selected electron events as discrete auroral events and explore the cause of statistical trendsThis study bridges the gap of these two types of observations and motivates collaborations across different Mars missions [ABSTRACT FROM AUTHOR]

Whistler mode waves have been proposed as a crucial mechanism in determining the velocity‐space distribution of electrons on the dayside crustal magnetic fields of Mars. A superthermal electron transport model has been unable to reproduce the observed pitch angle distributions on a crustal field line. The two key differences are that the observed pitch angle distributions are much more isotropic and the observed high energy pitch angle distributions have a flux peak at perpendicular pitch angles. We solve the bounce‐averaged quasi‐linear diffusion equation to calculate the steady‐state pitch angle distribution of electrons along a crustal field line when in resonance with whistler mode waves. We perform two simulations, changing the background ionosphere, which affects what energies are in resonance with the whistler mode wave. The wave parameters are chosen based on previous observations of whistlers at Mars. Our results reconcile both qualitative differences between the previous data‐model comparisons. Plain Language Summary: An understanding of how electrons move through space environments is important for a multitude of reasons. It tells us where electrons will transfer their energy to the neutral atmosphere and it can indirectly inform us of where the magnetic field lines are connected to (the planet or solar wind). If the physical processes that control electron transport are unknown, then incorrect assumptions may be made. At Mars, our satellite observations and numerical simulations have not agreed, indicating that we do not include all the relevant physics in our models. Whistler mode waves are extremely low frequency radio waves that interact with electrons and can change the direction they are moving and increase their velocity. In this study, we simulate the effect of whistler mode waves on electrons at Mars. We find that our simulation results agree quite well with the data and reconciles the two key qualitative differences between previous data‐model comparisons. Key Points: We have solved the bounce‐averaged quasi‐linear diffusion equation along a Mars crustal field line to study wave‐particle interactionsSteady‐state due to photoelectron resonance with whistler mode waves was reached on the order of minutesBoth previous data‐model discrepancies are resolved: a high energy perpendicular flux peak and increased isotropy [ABSTRACT FROM AUTHOR]

From February to March 2019, the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft repeatedly observed aurora near periapsis over Mars' southern strong crustal fields. During these orbits, the Solar Wind Electron Analyzer observed accelerated electrons at similar locations to where the auroras were observed, resembling the inverted‐V structure observed near Earth's auroral region. In this study, we present a case study of such an acceleration event, where we estimate a field‐aligned electrostatic potential drop of ∼440 V. We determine the field‐aligned current from the observed magnetic perturbation reaches 1.1 μA/m2, agreeing reasonably well with the estimated net electron current carried by acceleration electrons with a maximum of 2.5 μA/m2. Similar to Earth, the potential drop develops when the ambient plasma cannot sustain the imposed field‐aligned current. We also estimate the potential layer to be located above 750‐ to 850‐km altitude and the associated electric field to be ∼0.6 V/m. Key Points: We estimate a field‐aligned potential drop of 440 V during an inverted‐V electron acceleration event concurring with auroral observationsThe current density estimated from the observed magnetic perturbation reaches 1.1 μA/m2, agreeing with estimated net electron currentSimilar to Earth, this potential drop develops when the ambient plasma is unable to support the necessary field‐aligned current [ABSTRACT FROM AUTHOR]

Abstract: On 10 September 2017, some of the strongest solar activity occurred in association with active region 12673 (AR2673), including an X‐class solar flare and a fast coronal mass ejection. Although AR2673 was not centrally facing Mars, the activity impacted the local space weather conditions at Mars. We give an overview of observations obtained from the Mars Atmosphere and Volatile EvolutioN, Mars Science Laboratory, and Mars Express missions. Numerical results from the Wang‐Sheeley‐Arge (WSA)‐Enlil‐cone model together with Earth/L1 and STEREO‐A observations are also presented to provide some heliospheric context. We discuss the initial results on the space weather impacts at Mars, which include heating of the upper atmosphere by solar flare emissions, flare‐related enhancements of ion and neutral densities, solar energetic particles impacting the atmosphere and surface, bright emissions of a diffuse (global) aurora, deeply penetrating interplanetary magnetic fields over the Martian dayside, and enhanced atmospheric escape rates. [ABSTRACT FROM AUTHOR]

We report a new water‐ice cloud feature observed during the Mars year 34 global dust storm: twilight cloud bands that routinely formed just past the evening terminator. We use images taken by the MAVEN/IUVS instrument. These bands were often latitudinally continuous, spanning over 6,000 km and were present between 18:00 and 19:00 local time. They were present for nearly the entire time IUVS imaged the evening terminator and often reached altitudes of at least 40 to 50 km during the mature phase of the storm. We compare these observations to LMD global climate model simulations. The simulations generally contain the temporal and spatial extents of the bands seen in IUVS data throughout the storm, but there are some discrepancies. We infer that these clouds formed as a result of semidiurnal thermal tides. Plain Language Summary: Water‐ice clouds and dust are among the most common particles in the Martian atmosphere, but the effect of global dust storms on cloud formation is largely unknown. We observed cloud bands that repeatedly formed near dusk during the Mars year 34 global dust storm. We saw these bands throughout the storm at locations all across the planet. We investigate this feature by using a global climate model, which predicted the formation of these cloud bands as a result of rapidly changing temperatures. The simulations of the bands contain several features seen in our observations, but not all. Key Points: We observed a new cloud formation phenomenon just after sunset during the Mars year 34 global dust storm in MAVEN/IUVS imagesGlobal climate model simulations are able to reproduce several but not all aspects of our observationsThese twilight clouds formed due to semidiurnal thermal tides [ABSTRACT FROM AUTHOR]