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We present a comprehensive study of the nightside aurora phenomenon on Mars, utilizing observations from EMUS onboard Emirates Mars Mission. The oxygen emission at 130.4 nm is by far the brightest FUV auroral emission line observed at Mars. Our statistical analysis reveals geographic, solar zenith angle, local time, and seasonal dependencies of auroral occurrence. Higher occurrence of aurora is observed in regions of open magnetic topology, where crustal magnetic fields are either very weak or both strong and vertical. Aurora occurs more frequently closer to the terminator and is more likely on the dusk side than on the dawn side of the night hemisphere. A pronounced auroral feature appears close to midnight local times in the southern hemisphere, consistent with the spot of energetic electron fluxes previously identified in the Mars Global Surveyor data. This auroral spot is more frequent after midnight than before. Additionally, some regions on Mars are "aurora voids" where essentially no aurora occurs. Aurora exhibits a seasonal dependence, with a major enhancement near perihelion. Non–crustal field aurora additionally shows a secondary enhancement near Ls 30°. This seasonal variability is a combination of the variability in ionospheric photoelectrons and thermospheric atomic oxygen abundance. Auroral occurrence also shows an increase with the rise of Solar Cycle 25. The brightest auroral pixels are observed during space weather events such as Coronal Mass Ejections and Stream Interaction Regions. These observations not only shed light on where and when Martian aurora occurs, but also add to our understanding of Mars' magnetic environment and its interaction with the heliosphere. Plain Language Summary: In this study, we explore the phenomenon of aurora on the nightside of Mars, using observations from the highly sensitive Emirates Mars Ultraviolet Spectrometer (EMUS) on the Emirates Mars Mission. Our analysis reveals distinct patterns in auroral occurrence on the planet. For instance, there is a higher rate of auroral activity in regions where Mars' magnetic field lines are open (i.e., connected to the collisional atmosphere at one end). We also found that aurora is more common near the terminator (with the occurrence decreasing as the solar zenith angle increases), and particularly during the evening hours, as opposed to early morning. Interestingly, these auroral events also show a seasonal dependence, peaking around perihelion in a Martian year, when Mars is closest to the sun. This seasonal pattern corresponds with the variation of photoelectrons in Mars' dayside ionosphere and the atomic oxygen abundance in the thermosphere. Auroral occurrence increases with increasing solar activity. Also, the auroral brightness increases during space weather events. Our study not only gives us a clearer picture of where and when the aurora occurs on Mars but also hints at the underlying processes influencing them, offering insights into the planet's magnetic and charged particle environment. Key Points: Higher auroral occurrence is observed in regions of open magnetic topology, where crustal fields are either very weak or primarily verticalAurora occurs more frequently near the terminator compared to deep night, with higher occurrence at dusk than at dawnAurora occurs more frequently near perihelion, with the brightest auroral pixels observed during space weather events [ABSTRACT FROM AUTHOR]

We present the first observations of the dayside coronal oxygen emission in far ultraviolet (FUV) measured by the Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM). The high sensitivity of EMUS is providing an opportunity to observe the tenuous oxygen corona in FUV, which is otherwise difficult to observe. Oxygen resonance fluorescence emission at 130.4 nm provides a measurement of the upper atmospheric and exospheric oxygen. More than 500 oxygen corona profiles are constructed using the long–exposure time cross–exospheric mode (OS4) of EMUS observations. These profiles range from ∼200 km altitude up to several Mars radii (>6 RM) across all seasons and for two Mars years. Our analysis shows that OI 130.4 nm is highly correlated with solar irradiance (solar photoionizing and 130.4 nm illuminating irradiances) as well as changes in the Sun–Mars distance. The prominent short term periodicity in oxygen corona brightness is consistent with the solar rotation period (quasi–27–day). A comparison between the perihelion seasons of Mars Year (MY) 36 and MY 37 shows interannual variability with enhanced emission intensities during MY 37, due to the rise of Solar Cycle 25. These observations show a highly variable oxygen corona, which has significant implications on constraining the photochemical escape of atomic oxygen from Mars. Plain Language Summary: The highly sensitive Emirates Mars Ultraviolet Spectrometer (EMUS) onboard Emirates Mars Mission (EMM) is capable of observing ultraviolet emissions emanating from Mars. Oxygen in the Martian exosphere is hard to see because it's tenuous. In this study, the analysis of long exposure time EMUS optical observations show that the hot oxygen corona on Mars has a short term variability due to solar rotation. Hot oxygen corona also shows a long–term variability that depends on the Sun–Mars distance and the solar cycle progression. When comparing data from two Martian years, it is noticed that the oxygen corona became brighter when the Sun is more active. Key Points: Brighter O corona is observed during perihelion and dimmer during aphelion, indicating a strong relationship with the Sun‐Mars distanceThe variation in OI 130.4 nm brightness shows a linear correlation with solar EUV irradiance, with a short‐term solar rotation periodicityInterannual variability is observed from MY 36 to MY 37, showing an enhancement in O corona brightness with the rise of Solar Cycle 25 [ABSTRACT FROM AUTHOR]

We report a new set of stellar occultation measurements for nightside temperature profiles made by the Mars Atmosphere and Volatile EvolutioN/Imaging Ultraviolet Spectrograph that provide evidence for a recurring layer of warm air between 70 and 90 km altitudes in the nightside mesosphere of Mars during Ls = 0–180° in Martian Year 33–34. The nightside profiles reveal a recurring peak of atmospheric temperature around 80 km over the equator to the middle latitudes in the northern hemisphere. The predictions of the Mars Climate Database have a warm layer with much smaller amplitudes. The observed peak amplitudes are larger than those predicted by the model by up to 90 K. Wavenumber‐3 structures are seen in the warm layer that are potentially signatures of thermal tides or stationary planetary waves, with amplitudes two times larger than predicted. Plain Language Summary: The Mars middle atmosphere is an intermediate region with rich and complex dynamics influenced by the underlying lower atmosphere and the overlying upper atmosphere. We report a new set of stellar occultation measurements made by the MAVEN/IUVS that provide evidence for a warm layer between 70 and 90 km altitude in the nightside mesosphere of Mars during Ls = 0–180° in Marian Year 33–34. Key Points: MAVEN/IUVS revealed a warm layer between 70 and 90 km altitude in the nightside on Mars during Ls = 0–180° in Martian Year 33–34The observed peak temperature amplitudes of nightside profiles are higher than those predicted by the model by up to 90 KLongitudinal wavenumber‐3 structures are seen in the warm layer, two times larger amplitudes than predicted [ABSTRACT FROM AUTHOR]

We provide the first detailed characterization and analysis of the C I 156.1 nm ( 3 D o → 3 P) and 165.7 nm ( 3 P o → 3 P) dayglow in the Martian atmosphere. Using MAVEN/IUVS periapse limb observations from two time periods corresponding to the aphelion and perihelion seasons, we determine through photochemical and radiative transfer modeling that both emissions are driven by electron impact dissociation (EID) of CO 2 and resonant scattering of solar photons by atomic C. EID of CO 2 is dominant at the lower latitudes, while resonant scattering by C is dominant at the higher altitudes, with the cross-over point occurring at ∼ 155 km. The high-altitude C I 156.1 nm dayglow allows us to make the first direct measurement of C densities in the upper Martian atmosphere: at aphelion, (4. 0 ± 0. 3) × 1 0 5 cm−3 at ∼ 165 km altitude, and at perihelion, (4. 4 ± 0. 5) × 1 0 5 cm−3 at ∼ 175 km altitude. These high C densities, while providing support for the addition of CO 2 photodissociation as a major C production channel, suggest that C production rates in photochemical models still have to be higher. Constraints from the dayglow observations also indicate that actual CO 2 + e rates are as much as a factor of 2 compared to photochemical modeling, with implications on slightly higher overall C photochemical escape rates. Contrary to model predictions, we found the altitude of the emission peak at perihelion to be higher than it is at aphelion, and higher for the C I 156.1 nm emission than the C I 165.7 nm emission at the same season. The former seasonal variation can be attributed to low modeled atmospheric temperatures at perihelion. • We analyze the C I 156.1 nm and 165.7 nm dayglow in the Martian atmosphere. • This dayglow is driven by C resonant scattering and CO 2 electron impact dissociation. • We make the first direct measurement of C density in the Martian upper atmosphere. • Observed C densities are significantly higher than modeled. • Observed CO 2 electron impact dissociation rates are also higher than modeled. [ABSTRACT FROM AUTHOR]