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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]

Characterization, understanding, and prediction of the Martian radio environment are of increasing importance to the forthcoming human exploration of Mars. Here we investigate 3–5 MHz radio absorption in the nightside ionosphere of Mars caused by enhanced ionization at <100 km altitudes during solar energetic particle (SEP) events. We conduct a quantitative analysis of radio absorption and SEP flux data that have been accumulated by two spacecraft currently orbiting Mars, thereby demonstrating that radio absorption is clearly correlated with SEP fluxes. A comparison of the observations with radio absorption properties predicted by a numerical model indicates that the relative temporal changes, radio frequency dependence, and SEP energy dependence of the observed radio absorption are in agreement with the model prediction. Meanwhile, the model systematically overestimates the radio absorption in the ionosphere by a factor of 3.7. We explore several sources of uncertainty, including the electron‐neutral collision frequency, absolute sensitivity of the SEP instrument, and limited transport of SEPs to the atmosphere, but the ultimate cause of the systematic discrepancy between the measured and modeled radio absorption is yet to be identified. Further efforts should be put into the development of a comprehensive and observationally validated model of radio absorption in the Martian ionosphere to assist the future crew and spacecraft activities on the surface of Mars. Plain Language Summary: Radio waves are widely used for communication, positioning, and navigation in our society. As human exploration of Mars progresses in the coming decades, radio technology will be in use for crew and spacecraft activities on the surface of Mars. On Earth, high‐frequency radio communication is sometimes disrupted due to enhanced radio absorption in the lower ionosphere arising from solar flares. The ionosphere of Mars also causes strong radio absorption during space weather events as observed in the nightside ionosphere during solar energetic particle (SEP) precipitation. To assess the effects of ionospheric absorption on radio communication at Mars, we need to develop a reliable numerical model that is validated against observations. We initiate this effort by comparing observations with predictions from a state‐of‐the‐art numerical model. The model successfully reproduces important aspects of the observed radio absorption during SEP events, namely relative temporal changes, radio frequency dependence, and SEP energy dependence. However, the modeled and observed absorption levels systematically differ by a factor of 3.7, reasons for which have not been fully understood. Further research is required to improve and validate the model if we are to characterize the radio communication environment prior to the future arrival of humans to Mars. Key Points: Observations indicate a clear correlation between solar energetic particle fluxes and radio absorption in the nightside ionosphere of MarsThe observed properties of radio absorption are well reproduced by a numerical model except for a factor of 3.7 systematic discrepancyFurther model development and validation will pave the way for accurate prediction of the radio environment on the Martian surface [ABSTRACT FROM AUTHOR]

Since the discovery of atmospheric Mg+ on Mars in 2015 by the Mars Atmosphere and Volatile Evolution mission, there have been almost continuous observations of this meteoric ion layer in a variety of seasons, local times, and latitudes. Here, we present the most comprehensive set of observations of the persistent metal ion layer at Mars, constructing the first grand composite maps from pooled medians of subsamples of a metallic ion species. These maps demonstrate that Mg+ appears in almost all conditions when illuminated, with peak density values varying between 100 and 500 cm−3, dependent on season and local time. There exists significant latitudinal variation within a given season, indicating that Mg+ is not simply an inert tracer, but may instead be influenced by the meteoric input distribution and/or atmospheric dynamics and chemistry. Geographic maps of Mg+ density as a function of latitude and longitude indicate the influence of atmospheric tides, and there is no apparent correlation with remnant crustal magnetic fields. This work also presents counter‐intuitive results, such as a reduction of Mg+ ions in the northern hemisphere during Northern Winter in an apparent correlation with dust aerosols. Plain Language Summary: Metallic atoms in a planet's atmosphere are present when interplanetary dust particles burn up, releasing atomic species not typically found in the lower atmosphere. The discovery of a high altitude metallic layer on Mars in 2015 has led to continued monitoring in a variety of seasons across the entire planet. These results demonstrate that this magnesium ion (Mg+) layer appears throughout the year, with variations in peak abundances and layer heights, due to interactions with the background atmosphere. These variations track the dynamics of the middle atmosphere, providing insight into global climate patterns and may inform our understanding of seasonal deposition of interplanetary dust particles and their sources. This first‐order analysis supports future modeling efforts and provides model challenges to be understood, both of which can be explored in detail with time varying full planet climate modeling. Key Points: Eight Earth years of Mars Atmosphere and Volatile Evolution/Imaging Ultraviolet Spectrograph observations show that Mars' persistent meteoric metal ion layer is more dynamic than initially assumedMg+ layer peak altitude, abundance, and top and bottom side slopes vary significantly over the observed time periodThe relative absence of northern hemispheric Mg+ during southern summer is surprising and may be related to lower atmospheric dust loading [ABSTRACT FROM AUTHOR]

Discrete aurorae have been observed at Mars by multiple spacecraft, including Mars Express, Mars Atmosphere and Volatile EvolutioN (MAVEN), and most recently the United Arab Emirates Hope spacecraft. Meanwhile, there have been studies on the source particles responsible for producing these detectable aurorae (termed "auroral electrons"). By utilizing empirical criteria to select auroral electrons established by Xu et al. (2022, https://doi.org/10.1029/2022GL097757), we conduct statistical analyses of the impact of upstream drivers on the occurrence rate and fluxes of auroral electrons. We find the occurrence rate increases with upstream dynamic pressure and weakly depends on the interplanetary magnetic field strength. Meanwhile, the integrated auroral electron flux somewhat decreases with increasing upstream drivers. Auroral electron precipitation also occurs more frequently and is more intense over regions of strong crustal fields compared to weak crustal fields. Aside from emissions, auroral electrons are expected to cause significant impact ionization and enhance the plasma density locally. In this study, we also quantify the nightside ionospheric impact of auroral electron precipitation, specifically the thermal ion (O+, O2+ ${O}_{2}^{+}$, and CO2+ $C{O}_{2}^{+}$) density enhancement, with MAVEN observations. Our results show that the ion density is increased by up to an order of magnitude at low altitudes. The crustal effects on ion density profiles for nominal electron and auroral electron precipitation are also discussed. Plain Language Summary: Localized auroral emissions have been observed at Mars by multiple spacecraft, including Mars Express, Mars Atmosphere and Volatile EvolutioN (MAVEN), and most recently the United Arab Emirates Hope spacecraft. Meanwhile, there have been studies on the source particles responsible for producing these detectable aurorae, termed "auroral electrons." By utilizing previously established criteria to select auroral electrons, we conduct statistical analyses of the impact of upstream drivers on the occurrence rate and also the intensity of auroral electrons. In the meantime, auroral emission is not the sole effect of electrons impacting the collisional atmosphere, but also an enhanced local ionization. Using both MAVEN observations, we show that the ion density is increased by up to an order of magnitude at low altitudes comparing auroral electron and nonauroral electron events. Key Points: The occurrence rates of auroral electrons increase but their integrated energy fluxes somewhat decrease with increasing upstream driversWe quantify the planetary ion density enhancement resulting from auroral electron precipitation to be up to an order of magnitudeAuroral electron precipitation occurs more frequently and is more intense over strong crustal fields than weak crustal fields [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]

Previous missions underlined the complex influence of the crustal magnetic fields on the Martian environment, including the plasma boundaries. Their influence on the bow shock is however poorly constrained, with most studies showing North/South differences attributed to the crustal fields, with various conclusions from little to strong variabilities. We analyze for the first time in detail the influence of crustal fields on the Martian shock location based on a multi‐mission analysis (MAVEN and MEX). We introduce the angular distance to the strongest crustal field region in the southern hemisphere that induces the largest influence (but not unique, with a minimum pressure threshold analyzed). Its impact is at large scale (>40–60° around), is modulated by the local time of the strongest source region (with no influence beyond terminator), and maximizes when the Interplanetary Magnetic field (IMF) is stable during the preceding hours. We introduce a technique, that is, partial correlations, to provide a coherent picture for both MAVEN/MEX due to existing cross correlations with Extreme UltraViolet (EUV). A composite parameter is proposed, that represents the combined influence of EUV, magnetosonic mach number (two major drivers) and crustal fields, the latter having an impact of hundreds of km. The influence of crustal fields on the shock appears seasonal and correlated with the Total Electronic Content, revealing a large scale coupling between the crustal fields, the ionosphere and the shock. The crustal field influence on the shock is thus significant and complex, with a coupling to both the ionosphere below and the IMF above. Plain Language Summary: The Mars Express (MEX) and Mars Atmosphere and Volatile Evolution (MAVEN) observations underlined the complex interaction between the solar wind plasma flowing out from the Sun and the martian environment. The supersonic flow creates a bow shock upstream of the planet, before it is deflected around the conductive ionized atmosphere of the planet that acts as an obstacle to the flow. Contrary to Earth, Mars lost its intrinsic global dynamo magnetic field that could protect efficiently the atmosphere from the incident solar wind sputtering, thus eroding the atmosphere with time. However, the planet possesses remnant crustal magnetic fields locked up in its crust, in particular in the southern hemisphere, that are known to play a major role in the martian interaction with the incident solar wind. We investigate here the influence of these crustal fields on the martian bow shock location, and show they are a significant driver according to MAVEN and MEX observations, despite being less important than the solar Extreme UltraViolet fluxes or magnetosonic mach number of the solar wind. We also show that that this influence varies with season, revealing a strong coupling between the crustal fields, the ionosphere and the bow shock location. Key Points: The influence of crustal magnetic fields on the Martian shock is significant based on the first multi‐mission studyThe strongest crustal field region has a major influence in a large angular range, when close to noon and when the interplanetary magnetic field is stableThe crustal field influence varies with season, showing a coupling between crustal fields, the ionosphere and the shock [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]

This paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora on the nightside of Mars. We run a 3‐D time‐dependent magnetohydrodynamic model to characterize the spatial and temporal dynamics of magnetic field and plasma distributions over the course of one planetary rotation. We perform eight simulation cases under solar minimum quiet‐solar‐wind conditions (four equinox/solstice seasons, each with two interplanetary magnetic field polarities) and in an actual interplanetary coronal mass ejection (ICME) case to assess quiet and space weather situations, respectively. The occurrence of detectable discrete aurora is subject to the combination of the probabilities that (a) the ionosphere is magnetically connected with high altitudes through open field lines and (b) precipitating energy fluxes of >30 eV electrons exceed 0.1 erg/cm2/s. Our results show that during quiet solar activity, discrete aurora occurs likely on small‐scale patches embedded inside strong crustal magnetic field regions (with a magnitude greater than 50 nT at 150 km), and the overall chance across the globe is ∼0.77%. The higher probability over strong crustal field regions is attributed to the stronger magnetic field convergence. Modeling shows the occurrence probability dramatically increases during the ICME event, particularly by more than an order of magnitude in weak crustal field regions. Our model results reasonably agree with NASA Mars Atmosphere and Volatile EvolutioN and Mars Express observations. Our study suggests that nightside discrete electron aurora is not caused by the direct entry of magnetosheath plasma in a cusp‐like process but due to the recycling of nightside magnetospheric electrons. Plain Language Summary: This paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora at Mars on a planetary scale. Discrete aurora is a patchy and sporadic phenomenon, posing a daunting challenge for understanding how and where auroral electron precipitation takes place in the ionosphere. To address this challenge, we apply a state‐of‐the‐art global model to characterize the spatial and temporal dynamics of near‐Mars plasma and magnetic field environments. We simulate the penetration of energetic electrons from high altitudes to the nightside ionosphere through open magnetic field lines, a process responsible for auroral emissions. Our results show that during quiet solar activity, the overall chance of observing a discrete auroral event on the nightside is only ∼0.77% across the globe, but the odds are significantly higher over strong crustal field regions. The occurrence probability is enhanced globally during an extreme space weather event, especially in weak crustal field regions. This study reveals a different source mechanism from the prevailing hypothesis that discrete aurora may be excited by the direct entry of magnetosheath plasma in a cusp‐like process. Instead, we propose that it is caused by the recycling of nightside magnetospheric electrons that return to the ionosphere. Key Points: Modeling shows that discrete aurora occurs more likely on small‐scale patches embedded inside strong crustal field regionsAuroral occurrence probability greatly increases during space weather events, particularly in weak crustal field regionsDiscrete aurora is not caused by the direct entry of magnetosheath plasma but due to the recycling of nightside magnetospheric electrons [ABSTRACT FROM AUTHOR]

Mars Atmosphere and Volatile Evolution (MAVEN)'s Imaging Ultraviolet Spectrograph has identified 278 occurrences of discrete aurora events on Mars, which are patchy, sporadic ultraviolet emissions emanating from the upper atmosphere. We confirm prior results 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. The 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. Plain Language Summary: Mars has an unusual magnetic field: it lacks a global field but retains the vestiges of an ancient field locked in regions of the crust. These fields are sufficient to focus charged particle precipitation into the atmosphere and cause aurora. The Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft carries an ultraviolet instrument capable of mapping aurora emissions and correlating them with conditions that favor aurora. We have detected hundreds of auroral events allowing the first statistical study of their behavior. 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. Astronauts, whenever they reach Mars, would have a good chance to see this natural wonder from above or below. Key Points: Mars Atmosphere and Volatile Evolution (MAVEN)/Imaging UltraViolet Spectrograph (IUVS) has detected hundreds of transient, patchy auroral emission events at Mars, mostly near strong crustal magnetic fieldsEvents are triggered in evening hours during favorable orientations of the interplanetary magnetic field, and may last for hoursVisible counterparts to the ultraviolet emissions could potentially be imaged by spacecraft and eventually seen by astronauts [ABSTRACT FROM AUTHOR]

We present an analysis of hundreds of middle ultraviolet auroral spectra collected at the limb with the Imaging UltraViolet Spectrograph (IUVS) instrument on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. While the companion paper by Schneider et al. (2021), https://doi.org/10.1029/2021JA029428 focuses on the detection, location, and occurrence frequency of discrete auroral events, this study addresses the spectral properties and vertical profiles of the auroral emissions. Our independent selection of events is based on a combination of automatic and manual detection methods with adequate signal‐to‐noise ratio of both the CO Cameron bands and the CO2+ ultraviolet doublet (UVD) at 190–270 and 288–289 nm, respectively. We find that the ratio of these two features remains quasi‐constant for UVD intensities exceeding ∼200 rayleighs (R), but the CO Cameron/CO2+ UVD ratio may become increasingly large for low UVD intensities. Three weak N2 Vegard‐Kaplan bands are identified in the Martian aurora for the first time. Limb profiles of the [OI] line at 297.2 nm indicate that the visible oxygen green line brightness may reach a few kilorayleighs. The distribution of the altitude of the emission peaks in the aurora is identical in and out of the region of crustal magnetic field located in the southern hemisphere. Comparisons of in situ measurements of electron energy spectra and ultraviolet auroral detections have been made for five optical detections. They generally show temporal coincidence but not necessarily quantitative agreement with the altitude and brightness expected from the characteristics of the measured electron energy spectra. Plain Language Summary: The MAVEN spacecraft has been orbiting Mars since September 2014 to study its upper atmosphere, ionosphere, and interaction with the solar wind. We focus on the discrete aurorae, previously discovered from the Mars Express orbiter. Discrete aurorae are observed on the nightside and are localized both in space and time. We analyze the ultraviolet spectra observed at the limb with the Imaging Ultraviolet Spectrograph to determine the main characteristics of these aurorae. We show that the emission peaks occur at ∼130 km. They are composed of several emissions, such as CO, CO2+, N2, and atomic oxygen. We find that they occur more often in the presence of strong crustal magnetic field. We also deduce that the altitude of the aurora is independent of many parameters except for seasons. We analyze the intensity ratio of these emissions and show that anomalies occur in the ratio of the CO Cameron bands to the CO2+ UV doublet at low intensity. Finally, we compare the detection of auroral events with the presence of strong electron precipitation measured on board MAVEN. A temporal correlation is generally observed but association of the electron flux and specific UV auroral detection is not always clear because measurements are not colocated. Key Points: N2 Vegard‐Kaplan bands have been detected in the ultraviolet Martian auroraThe presence of crustal magnetic field does not influence the altitude of the auroral emission peakGreen line intensities up to a few kilorayleighs are expected in regions of crustal magnetic field and may be visible with the naked eye [ABSTRACT FROM AUTHOR]

The effects of solar flares on the upper atmosphere of Mars are large but poorly constrained by observations. These effects are an important aspect of the response of Mars to space weather events and may also influence the escape of volatiles from Mars, particularly in the solar system's early history. Here we report the effects of the X8.2 flare on 10 September 2017 on the density and composition of the thermosphere of Mars. This analysis uses neutral number densities of He, O, N2, CO, Ar, and CO2 from the MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS). From these observations, we investigate how the enhanced solar irradiance during the flare produced changes in the neutral upper atmosphere of Mars due to atmospheric heating and photochemistry. Flare‐produced photochemical changes in the neutral thermosphere of Mars have been previously implied but not quantified. We find that at fixed altitudes, the number densities of all species barring He increase and the O/CO2 ratio decreases by ∼15–45%, indicating thermal expansion. However, when viewed at fixed total number densities, the densities of CO2 and Ar decrease, and the O/CO2 ratio increases by up to a factor of ∼3. The photodissociation of CO2 is one photochemical process that produces changes resembling those identified. The quantified changes will help to constrain the shifting chemical makeup of the upper atmosphere due to these impactful flare events and may aid modeling of flare behavior and the impact of flares on the evolution of the Martian climate. Key Points: Mars thermospheric densities and composition are affected by this solar flareThermospheric responses at fixed altitude (heating‐based changes) and fixed number density (chemistry‐based changes) differThe O/CO2 ratio nearly triples at low number densities during this flare [ABSTRACT FROM AUTHOR]

We report results from a study of two consecutive Martian years of imaging observations of nitric oxide ultraviolet nightglow by the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile Evolution (MAVEN) mission spacecraft. The emission arises from recombination of N and O atoms in Mars' nightside mesosphere. The brightness traces the reaction rate as opposed to the abundance of constituents, revealing where circulation patterns concentrate N and O and enhance recombination. Emissions are brightest around the winter poles, with equatorial regions brightening around the equinoxes. These changes offer clear evidence of circulation patterns transitioning from a single cross‐equatorial cell operating during solstice periods to more symmetric equator‐to‐poles circulation around the equinoxes. Prominent atmospheric tides intensify the emissions at different longitudes, latitude ranges, and seasons. We find a strong eastward‐propagating diurnal tide (DE2) near the equator during the equinoxes, with a remarkably bright spot narrowly confined near (0°, 0°). Wave features at the opposite winter poles are dissimilar, reflecting different circulation patterns at perihelion versus aphelion. LMD‐MGCM simulations agree with the patterns of most observed phenomena, confirming that the model captures the dominant physical processes. At the south winter pole, however, the model fails to match a strong wave‐1 spiral feature. Observed brightnesses exceed model predictions by a factor of 1.9 globally, probably due to an underestimation of the dayside production of N and O atoms. Further study of discrepancies between the model and observations offers opportunities to improve our understanding of chemical and transport processes controlling the emission. Plain Language Summary: The MAVEN spacecraft orbiting Mars has obtained a new type of imaging data which reveals the effects of global‐scale winds and waves in the upper atmosphere. Ultraviolet emissions confirm the expected change of wind patterns with season and pinpoint an unusually bright spot on Mars' nightside. Computer model runs successfully match many of the observed features and allow an examination of the underlying physical causes. Differences between the model and observations offer insights on how to improve the model. Key Points: The MAVEN spacecraft orbiting Mars has obtained a new type of nightside imaging data which reveals the effects of global‐scale winds and waves in the upper atmosphereUltraviolet nightglow emissions confirm the expected change of circulation patterns with season and pinpoint an unusually bright spot on Mars' nightsideA Mars general circulation model successfully matches many of the observed features and allows an examination of the underlying physical causes [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]

The persistent two‐peaked vertical structure of the Martian ionosphere is created by extreme and far ultraviolet radiation whose energies, respectively, determine their ionization altitude. A third low‐altitude transient layer (previously referred to as M3 or Mm) has been observed by radio occultation techniques and attributed to meteor ablation. However, recent remote sensing and in situ observations disfavor a meteoric origin. Here we propose an alternative hypothesis for these apparent layers associated with impact ionization from penetrating solar wind ions, previously observed as proton aurora. Localized ionization, occurring nonglobally at a given altitude range, breaks the symmetry assumed by the radio occultation technique, and creates electron layers apparently lower in the ionosphere than their true altitude. This may occur when the upstream bow shock is altered by a radial interplanetary magnetic field configuration, which allows the solar wind to penetrate directly into the thermosphere. This localized ionization hypothesis provides an explanation for apparent layers' wide variation in heights and their transient behavior. Moreover, this hypothesis is testable with new observations by the Mars Atmospheric and Volatile EvolutioN Radio Occultation Science Experiment in future Mars years. This hypothesis has implications for the ionospheres of Venus and Titan, where similar transient layers have been observed. Plain Language Summary: The ionosphere of a planet couples the neutral atmosphere to the exosphere and solar wind. Created by solar ionizing radiation, the ionosphere on the dayside of planet is expected to be hemispherically symmetric. When Mars Express discovered transient low‐altitude ionospheric layers in 2005, they were initially predicted to be due to meteor ablation. However, recent observations indicate that they are due to a geometric observational effect related to local, not hemispheric, ionization. Observations from three instruments on Mars Atmospheric and Volatile EvolutioN are used concurrently to understand this phenomenon. These transient layers have been observed at Venus and Titan as well, and this hypothesis indicates novel physics for those ionospheric systems. Key Points: Low‐altitude transient ionospheric layers may be explained by a geometric observational effect from localized, not hemispheric, ionizationA radio occultation of a transient ionospheric layer was obtained with observations of variable proton aurora and penetrating solar windTransient layers observed at Mars, Venus, and Titan are consistent with the presence of solar wind impact ionization at those planets [ABSTRACT FROM AUTHOR]

We report the first remote observation of nitric oxide (NO) densities on Mars. The Imaging Ultraviolet Spectrograph (IUVS) on NASA's Mars Atmosphere and Volatile Evolution (MAVEN) satellite observes NO γ band solar resonance fluorescence between 213.0 and 225.5 nm. We invert an average dayglow limb radiance profile to retrieve a number density profile between 80 and 130 km. The retrieved IUVS NO number density at 117 km is 5 times smaller than those measured by Viking mass spectrometers over 40 years ago but consistent with photochemical model results within the IUVS statistical uncertainty. These observations may therefore help to reconcile a longstanding problem in our understanding of NO photochemistry in the Martian upper atmosphere. We also report the first detection of the CO+ First Negative bands in the Martian dayglow near 219 nm. Plain Language Summary: Nitric oxide has been an important diagnostic for the effects of solar activity since its detection in the Earth's upper atmosphere over 50 years ago. We report the first remote observation of nitric oxide densities on Mars. The densities are five times smaller than those measured by Viking mass spectrometers upon entry into the atmosphere over 40 years ago. These new nitric oxide observations may help to reconcile a longstanding problem in our understanding of NO chemistry in the Martian upper atmosphere. Key Points: First remote observations of nitric oxide densities on MarsNitric oxide densities are a factor of five smaller than those reported by Viking over 40 years agoPhotochemical model results show much better agreement with the new remote observations than with Viking measurements [ABSTRACT FROM AUTHOR]

Magnetic topology is important for understanding the Martian plasma environment, including particle precipitation, energy transport, cold ion escape, and wave‐particle interaction. In this study, we combine two independent but complementary methods in order to determine magnetic topology based on superthermal electron energy and pitch angle distributions. This approach removes ambiguities that result from using either energy or pitch angle alone, providing a more accurate and comprehensive determination of magnetic topology than previous studies. By applying this combined technique, we are able to identify seven magnetic topologies, including four types of closed field lines, two types of open field lines, and draped. All seven topologies are present in the Mars environment and are mapped in longitude, latitude, solar zenith angle, and altitude with the combined technique near the terminator. We find that closed field lines with double‐sided loss cones are frequently present over stronger crustal field regions at higher altitudes. We also show that the cross‐terminator closed field lines are more spatially confined over strong crustal regions, likely connecting nearby magnetic crustal patches. In contrast, cross‐terminator closed loops over weak crustal regions have more distantly separated foot points, most likely connecting distant crustal patches. Key Points: A technique combining e‐ pitch angle and energy distribution is developed to most accurately infer up to seven magnetic topologies at MarsClosed magnetic loops with trapped electrons occur frequently over stronger crustal field regions at higher altitudesCross‐terminator closed loops are more spatially confined over strong crustal regions and distantly separated over weak crustal regions [ABSTRACT FROM AUTHOR]

Understanding how the Mars plasma environment responds to space weather events provides insights into the early Sun‐Mars interaction and helps constrain the extrapolation of atmospheric loss back in time. The 2003 Halloween interplanetary coronal mass ejection (ICME) event is one of the most extreme space weather events encountered by Mars during the last two decades. Mars Global Surveyor's circular orbit at ∼400 km allows us to observationally study the magnetic topology response to this extreme event globally for the first time. We analyze the suprathermal electron data and magnetic field measurements from Mars Global Surveyor to infer magnetic topology before, during, and after this ICME event and quantify the variation of the topology over both weak and strong crustal regions and correlate the variation with the upstream dynamic pressure. Over weak crustal regions, more draped field lines and fewer closed field lines are observed under high dynamic pressures, implying a deeper interplanetary magnetic field penetration during the ICME event. Over dayside strong crustal regions, the dominant magnetic topology switches from closed field lines during quiet periods to open field lines during disturbed periods, suggesting more reconnection occurring between the crustal magnetic fields and interplanetary magnetic field. This mixed magnetic topological response is consistent with predictions from magnetohydrodynamic simulations for ICME events in previous studies. Key Points: We analyze Martian magnetic topology response to the 2003 Halloween ICME event and correlate the variation with upstream driversA deeper IMF penetration is found over weak crustal regions but more open field lines over strong crustal regions during the eventThis mixed magnetic topological response confirms previous predictions from MHD simulations for ICME events [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]