Greg Holsclaw

Far‐ultraviolet (FUV, ∼135–165 nm) dayglow emissions from N ₂ ‐rich Solar System Objects (SSOs) are dominated by the Lyman–Birge–Hopfield (LBH) band system. Consequently, observations from the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Titan and the Global‐scale Observations of the Limb and Disk (GOLD) mission at Earth use the LBH emission for upper atmospheric remote sensing at both SSOs. However, FUV dayglow‐based remote sensing analysis has been hampered by large uncertainties in existing LBH emission cross sections, particularly at low photoelectron energies. We present laboratory measurements of emission cross sections obtained at low electron energies (16, 20, and 30 eV). Our laboratory apparatus employs a high‐resolution, state‐of‐the‐art monoenergetic electron beam source and a large (1.5‐m diameter) chamber. With this apparatus, we produce single‐particle excitation of the optically forbidden upper state of N ₂ ( a ¹ Π _g ‐state), including cascade and collision induced excitation from nearby electronic states. The low energy electron impact cross sections from this study (at 16, 20, and 30 eV) are combined with our previous results to fit an analytic LBH cross section as a function of energy (16–200 eV). The “analytic” cross‐section is then used in a radiative transfer dayglow model to quantify LBH brightness without needing corrections for cascade contributions.

Far Ultraviolet (FUV) dayglow emissions from Titan and Earth have been observed by the Cassini/Ultraviolet Imaging Spectrograph (UVIS, 2005–2017, solar cycle 23–24) and the Global‐scale Observations of the Limb and Disk (GOLD; 2018 ‐present, solar cycle 25) missions, respectively. Both instruments operate over a nearly identical UV wavelength range (135–165 nm) and have observed remarkably similar ‐dominated FUV spectra despite differences in atmospheric environment. In both atmospheres, is the most abundant Thermosphere‐Ionosphere (T‐I) species and peak airglow emissions occur within ionospheric altitudes [near 900 km at Titan versus 120–150 km at Earth] with comparable thermodynamic conditions existing for atmospheric densities – ], although Titan and Earth have vastly different T‐I temperatures (150 K vs. ∼700 K) and the differing presence of different minor constituents on Titan versus O and on Earth]. We present a new model analysis of yearly averaged FUV dayglow observations of Titan's and Earth's atmosphere using updated UV optically thin electron‐impact fluorescence cross‐sections of the Lyman–Birge–Hopfield (LBH) band system derived from new laboratory measurements. The analysis shows that solar extreme ultraviolet (EUV) variability over the solar cycle is the dominant driver of dayglow brightness variations at both Titan and Earth. Despite large differences in atmospheric composition, thermospheric temperature, and solar distance, the observed FUV emissions display similar features on the day and night side and exhibit a common response to solar EUV forcing, indicating fundamentally similar upper atmospheric excitation processes in these two ‐dominated atmospheres.

Observations of ultraviolet dayglow provide valuable insights into the composition, energetics, and dynamics of planetary atmospheres. The Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM) uniquely captures extreme ultraviolet (EUV) and far ultraviolet (FUV) emissions from Mars’ upper atmosphere. Previous work reported irregular brightness enhancements referred to as “bright spots” in approximately one quarter of the OI1304 dayside disk images that are not linked to instrument noise or fluctuations in solar EUV flux. The origin of these enhancements is currently unknown. Here, we analyze a full Mars year of EMUS dayside disk observations to characterize the spectral, spatial, local time, and seasonal behavior of these bright spots. A clear preference for occurrence from early morning to afternoon local times is seen, with evidence of seasonal variability. Spectral analysis indicates that the cause of the bright spots is likely compositional and not auroral. These results provide evidence for intermittent, medium-scale compositional structure of unknown origin in the Martian thermosphere that is not explained by expected dayglow variations, providing new observational constraints for models.

We investigate Martian electron aurora utilizing 130.4 nm auroral observations from the Emirates Mars Ultraviolet Spectrograph and IMF clock angle measurements and estimates from MAVEN. Analysis reveals that more than 5 R aurora occurrence in a given region can vary by up to a factor of seven with IMF orientation, generally highest for southward IMF and lowest for westward IMF, with higher auroral occurrence post‐dusk, decreasing toward and past midnight before increasing again toward dawn. A broad diversity of IMF and local time dependence of aurora occurrence is observed across Mars' crustal magnetic fields. The polarity of a given crustal field determines whether its field lines preferentially magnetically connect to dayside ionospheric or nightside magnetotail electron sources in the post‐dusk or pre‐dawn sectors. We thus establish a “source preference metric”, that is, the excess likelihood of aurora when likely magnetically connected to the nightside versus dayside. A wide range of source preferences are observed across the crustal field features, with some showing higher occurrence for night sources, day sources, or neither, often differently when in the post‐dusk or pre‐dawn sectors. A generally stronger nightside source preference is found for radially inward crustal fields in the predawn sector, and for weaker outward crustal fields in the post‐dusk sector. These results highlight the complex and dynamic nature of Mars' magnetic topology in controlling auroral electron access. Future studies integrating in situ electron measurements with synoptic and limb auroral imaging will further constrain the relative contributions of dayside and magnetotail electron populations to Mars' auroral processes.

The Emirates Mars Ultraviolet Spectrometer (EMUS) aboard the Emirates Mars Mission (EMM) has been studying backscattered interplanetary hydrogen (interplanetary hydrogen (IPH)) Lyman emissions with dedicated observation strategies viz., U‐OS3b and U‐OS4b. These observation techniques involve looking away from Mars from the EMM's orbit. U‐OS3b provides broader coverage of the Martian sky, while U‐OS4b is designed to achieve high signal‐to‐noise measurements. Here we present analysis of the interplanetary hydrogen emission distribution across the sky, particularly at the Lyman‐ and emission wavelengths. EMUS observes a well‐known reduction in the brightness of interplanetary hydrogen as the observation angle increases across the sky from the hydrogen bulk flow upwind direction. Our modeled emission intensities indicate that when Mars is around aphelion, the observed emissions are dominated by IPH rather than Martian exospheric hydrogen. Our modeled intensities for H Ly‐ emission are smaller by a factor of 1.6 compared to the observations, and they are consistent with the observations for H Ly‐ emission. The H Ly‐ intensities measured by the Imaging Ultraviolet Spectrograph (IUVS) on board the Mars Atmosphere and Volatile EvolutioN mission were found to be 45% lower than those from EMUS measurements, likely owing to differences in the instruments' calibration factors. During the observation period, differences in the measured intensities between the Solar Wind ANisotropy (SWAN) instrument aboard the Solar and Heliospheric Observatory and EMUS were also found to vary. These discrepancies can be attributed not only to instrumental factors but also to the relative positions of Earth and Mars.

This paper presents far-ultraviolet through mid-infrared (0.12–20 μm) reflectance spectra of 27 fine-particulate (μm) terrestrial mineral samples, providing continuous spectra that cover an unusually broad spectral range and are of unusually fine particle size relative to most existing spectral libraries. These spectra of common geologic materials are useful for future applications that study the dust on various planetary bodies. Reflectance spectra were acquired of the samples at multiple laboratories at multiple wavelengths. All of the spectra were compared to one another to observe the general, common spectral characteristics (e.g., slope, band shape, and band depth), and the best segments of the spectra representing the mineral reflectance were scaled and spliced together to form a “Frankenspectrum” for each mineral that best represents the full wavelength range of far-ultraviolet, visible, near-infrared, and middle-infrared wavelengths. These scaled and spliced Frankenspectra, as well as the entire set of individual “original” reflectance spectra from each laboratory, are available in the Planetary Data System Geosciences Node.

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 L_s 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.

The Emirates Mars Ultraviolet Spectrometer (EMUS), aboard the Emirates Mars Mission (EMM), has been conducting observations of ultraviolet emissions within the Martian exosphere. Taking advantage of the distinctive orbit of the EMM around Mars, EMUS utilizes a dedicated strafe observation strategy to scan the illuminated Martian exosphere at tangential altitudes ranging from 130 to over 20,000 km. To distinguish between emissions of Martian origin and those from the interplanetary background, EMUS conducts specialized background observations by looking away from the planet. This approach has allowed us to investigate the radial and seasonal variations in Martian coronal emission features at H Lyman‐α, β and γ wavelengths. Our analysis supports the previous studies indicating that Martian exospheric hydrogen Lyman emission brightness attains its highest levels around the southern summer solstice and reaches its lowest levels when Mars is near aphelion. Additionally, a secondary peak emission at all altitudes is observed after perihelion during Martian Year (MY) 36, which can be attributed to a Class C dust storm. Our study establishes a strong correlation between solar flux and coronal brightness for these emissions, highlighting the impact of solar activity on the visibility of Martian corona. In addition, we have examined interannual variability and found that emission intensities in MY 37 surpassed those in MY 36, primarily due to increased solar activity. These observations help to understand potential seasonal patterns of exospheric hydrogen, which is driven by underlying mechanisms in the lower atmosphere and solar activity, eventually suggesting an impact on water loss in the Martian atmosphere.

We present a comprehensive study of the nightside 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. 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 MGS 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 CMEs and SIRs. 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.

We examine the newly discovered phenomena of sinuous aurora on the nightside of Mars, using images of 130.4 and 135.6 nm oxygen emission measured by the Emirates Mars Mission EMUS ultraviolet spectrograph, and upstream measurements from the MAVEN and Mars Express spacecraft. They are detected in ∼3% of observations, totaling 73 clear detections. These emissions are narrow, elongated (1,000–6,000 km), cross Mars' UV terminator, and are oriented generally toward the anti‐solar point, clustering into north, south, east, and west‐oriented groups. Diverse morphologies are observed, though some spatial features, such as broad curves, may in some cases be due to temporal aliasing of aurora motion as each image is built up over 15–20 min. Sinuous aurora form away from Mars' strongest crustal magnetic fields and can be interrupted by moderate crustal fields. Sinuous aurora occurrence increases strongly with solar wind pressure, though brightness shows only a weak positive dependence on pressure. Interplanetary magnetic field (IMF) clock angle affects their occurrence and orientation: sinuous aurora show a broad range of orientations centered on the solar wind convection electric field (E_conv) direction and forming in the +E_conv hemisphere, although with moderate clockwise and counterclockwise average “twists” for westward and eastward IMF, respectively. From these features we infer a link between sinuous aurora and electron energization in Mars' magnetotail current sheet, where field geometry on the +E_conv side of the sheet is more organized and symmetric. Determination of specific triggering conditions for sinuous aurora requires further investigation.