We present a comprehensive study of the nightside discrete electron aurora phenomenon on Mars, utilizing observations from EMUS onboard EMM. The oxygen emission at 130.4 nm is by far the brightest FUV auroral emission line observed at Mars. We identify auroral pixels in OI 130.4 nm disk observations, with higher sensitivity than previously possible. Our statistical analysis reveals regional, SZA, local time, and seasonal dependencies of auroral occurrence. Higher occurrence of aurora is observed in regions of open magnetic topology and vertical crustal magnetic fields. Aurora occurs more frequently closer to the terminator and is more likely on the dusk versus dawn sides 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 MGS data. The auroral spot is more frequent after midnight than before. Additionally, some regions on Mars are “aurora voids” where essentially no aurora occurs. The non-crustal field aurora exhibits a seasonal dependence, with major enhancements around Ls 235° (near perihelion) and Ls 30°. This is in line with the seasonal variability in ionospheric TEC observed by Mars Express, which is in turn related to the variability of solar irradiance and thermospheric density. Aurora occurrence also shows an increase with the rise of Solar Cycle 25. 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 heliospheric environment.

We evaluate for the first time the large-scale influence of the upstream interplanetary magnetic field (IMF) magnitude and orientation on Martian proton aurora. We identify a correlation between proton aurora activity and IMF magnitude that varies seasonally, with highest correlations around Mars southern summer solstice when proton aurora activity reaches an annual high. We identify preferentially increased proton aurora emission enhancements at IMF cone and clock angles consistent with the shape of the Parker spiral at Mars’s heliocentric distance. An increased proton aurora occurrence rate is observed for near-radial IMF orientations, particularly evident when the hydrogen corona and bow shock are annually decreased. Lastly, our results suggest that dayside magnetic reconnection may influence proton aurora, which are observed to have preferentially higher occurrence under strong ±Bz orientations. These findings enhance our understanding of the upstream IMF’s influence on Martian proton aurora as an important driver for auroral brightness and occurrence.

A document by Krishnaprasad Chirakkil. Click on the document to view its contents.

We compare observations of hydrogen (H) and protons associated with Martian proton aurora activity, co-evaluating remote sensing and in situ measurements during these events. Following the currently understood relationship between penetrating protons and H energetic neutral atoms (ENAs) in the formation of proton aurora, we observe an expected correlation between the H Lyman-alpha (Ly-α) emission enhancement (used herein as a proxy for H-ENAs) and penetrating proton flux. However, we observe a notable spread in the trend between these two datasets. We find that this spread is contemporaneous with one of two major impacting events: high dust activity or extreme solar activity. Proton aurora events exhibiting a relative excess in penetrating proton flux compared to Ly-α enhancement tend to correspond with periods of high dust activity. Conversely, proton aurora events exhibiting a relative deficit of penetrating proton flux compared to Ly-α enhancement are qualitatively associated with periods of extreme solar activity. Moreover, we find that the largest proton aurora events occur during concurrent dust storm and solar events, primarily due to the compounding intensified increase in H column density above the bow shock. Finally, we present a simplified empirical estimate for Ly-α emission enhancement during proton aurora events based on observed penetrating proton flux and a knowledge of local dust/solar activity at the time, providing a straightforward method for predicting auroral activity when direct observations are not available. The results of this study advance our understanding of the interconnected relationship between H and protons during Martian proton aurora activity.

Proton aurora are the most commonly observed yet least studied type of aurora at Mars. In order to better understand the physics and driving processes of Martian proton aurora, we undertake a multi-model comparison campaign. We compare results from four different proton/hydrogen precipitation models with unique abilities to represent Martian proton aurora: Jolitz model (3-D Monte Carlo), Kallio model (3-D Monte Carlo), Bisikalo/Shematovich et al. model (1-D kinetic Monte Carlo), and Gronoff et al. model (1-D kinetic). This campaign is divided into two steps: an inter-model comparison and a data-model comparison. The inter-model comparison entails modeling five different representative cases using similar constraints in order to better understand the capabilities and limitations of each of the models. Through this step we find that the two primary variables affecting proton aurora are the incident solar wind particle flux and velocity. In the data-model comparison, we assess the robustness of each model based on its ability to reproduce a MAVEN/IUVS proton aurora observation. All models are able to effectively simulate the data. Variations in modeled intensity and peak altitude can be attributed to differences in model capabilities/solving techniques and input assumptions (e.g., cross sections, 3-D versus 1-D solvers, and implementation of the relevant physics and processes). The good match between the observations and multiple models gives a measure of confidence that the appropriate physical processes and their associated parameters have been correctly identified, and provides insight into the key physics that should be incorporated in future models.

The upper atmosphere of Mars is directly affected by solar activity and the resulting solar irradiance impinging upon it. Variations in solar forcing can affect the rate at which atmospheric species escape from the planetary system. Remotely sensed observations of the upper atmosphere of Mars have been made during solar activity extrema of Solar Cycles 22 and 24. These observations were made of D and H Lyman-a emissions using the Mars Atmosphere and Volatile Evolution (MAVEN) mission and the Hubble Space Telescope (HST) high resolution spectrographs. Data obtained from the two missions are analyzed and used to derive densities and escape rates of D and H from the martian upper atmosphere. The results show that the properties of these two water-spawned atoms vary with solar cycle, and display significant inter-annual variability, mainly due to variations in atmospheric temperature. The findings suggest that cooler atmospheric temperatures due to reduced solar EUV flux may enhance the abundance of H atoms in the upper atmosphere of Mars, yet this does not increase their escape rates.

Mars’ ultraviolet airglow has been used to study its upper atmosphere for over four decades. Identifying variations in emission features has provided information on composition, density and temperature. The Emirates Mars Ultraviolet Spectrometer onboard the Emirates Mars Mission observes Mars’ airglow at Far and Extreme UV wavelengths. Variations in disk emission features are studied, with a focus on O I 1304 Å, CO Fourth Positive Group and C I 1561 Å. All show variations with local time and emission angle as expected. Dawn-dusk asymmetry observed is attributed to local time differences in advection. Variations in the brightness of several dayglow features, including 1304 Å, with irregular shapes are noted in around 25% of the disk images. These display some local time and hemispheric asymmetry in their occurrence rates. Examination of their spatial structure, occurrence, and spectra suggests these are associated with variations in composition and photoelectron flux.

Discrete aurora at Mars, characterized by their small spatial scale and tendency to form near strong crustal magnetic fields, are emissions produced by particle precipitation into the Martian upper atmosphere. Since 2014, Mars Atmosphere and Volatile EvolutioN’s (MAVEN’s) Imaging Ultraviolet Spectrograph (IUVS) has obtained a large collection of nightside UV discrete aurora observations. Initial analysis of these observations has shown that, near the strong crustal field region (SCFR) in the southern hemisphere, the aurora detection frequency is highly sensitive to the interplanetary magnetic field (IMF) clock angle. However, the role of other solar wind properties in controlling the aurora detection frequency has not yet been determined. In this work, we use IUVS discrete aurora observations, and MAVEN solar wind observations, to determine how the discrete aurora detection frequency varies with solar wind dynamic pressure, IMF strength, and IMF cone angle. We find that, outside of the SCFR, the detection frequency is relatively insensitive to the IMF orientation, but significantly increases with solar wind dynamic pressure and moderately increases with IMF strength. Interestingly, the auroral emission brightness outside the SCFR is insensitive to the dynamic pressure. Inside the SCFR, the detection frequency is moderately dependent on the dynamic pressure and is much more sensitive to the IMF clock and cone angles. In the SCFR, aurora are unlikely to occur when the IMF points near the radial or anti-radial directions. Together, these results provide the first comprehensive characterization of how upstream solar wind conditions affect the formation of discrete aurora at Mars.

We report six years of observations of dayside temperatures of the middle and upper atmosphere of Mars made by the Imaging Ultraviolet Spectrograph instrument aboard the MAVEN spacecraft. Thermospheric temperatures show strong long-term variability associated with Martian season and solar cycle. Temperatures from both the Martian thermosphere and mesosphere show strong short-term variability indicating coupling from the lower atmosphere. The observed local time effect is strong in both upper and middle atmosphere temperatures. The thermosphere tends to be colder in the morning compared to the evening when temperatures are higher. Middle atmospheric temperatures show cooling during the dawn and dusk hours. Our analysis shows strong tidal activity during aphelion, whereas non-migrating tides are suppressed during perihelion, possibly due to increased dust activity. Observations during the deep minimum of solar cycle 24 reveal that thermospheric temperatures are highly variable with respect to local time if solar forcing, Mars-Sun distance, and spatial effects are removed. We will discuss these results in the context of coupling between the lower and upper atmosphere of Mars.

The Mars Atmosphere and Volatile Evolution (MAVEN) mission instrument suite includes an ultraviolet echelle spectrograph with high-spectral resolution designed to resolve D and H Lyman-α emissions. The high-spectral resolution mode was previously characterized in the lab and in the cruise phase to Mars and had been calibrated using observations and models of interplanetary hydrogen Lyman-α emissions. This work presents improved characterizations of the high-spectral resolution mode using in-orbit observations that allow for more robust detections of the faint D Lyman-α emission line. Additionally, the instrument was re-calibrated using simultaneous and comparable observations made with the Hubble Space Telescope high-spectral resolution instrument. Comparisons to Lyman-α observations made with the low-resolution UV channel on the spectrometer, that had been calibrated with stars, showed consistency in the brightness values for measurements obtained at similar observational conditions. The combined upgrades to the faint-emission fitting and new calibration techniques of the MAVEN echelle channel have resulted in an improved data-reduction pipeline with favorable implications for the science utility of D and H Lyman-α emissions.