Joseph Scott Evans

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.

F _10.7 , Mg II, and F ₃₀ are widely used proxies for variability in solar Extreme UltraViolet (EUV, 5–120 nm) irradiance. An alternative proxy for solar EUV variability is Q _EUV , an indirect measure of total solar EUV irradiance, retrieved from observations of Earth's Far UltraViolet (FUV, 120–300 nm) airglow and from empirically modeled solar EUV spectra. Daily Q _EUV derived from FUV airglow is available from the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) Global Ultraviolet Imager (GUVI) and Global‐scale Observations of the Limb and Disk (GOLD) missions, as is Q _EUV from empirical solar EUV spectrum. Spectral measurements from GOLD are also used to retrieve lower‐middle thermospheric temperature, termed as T _disk . The availability of GOLD T _disk and Q _EUV provides a new opportunity to reassess the effectiveness of various solar EUV proxies for thermospheric use. This investigation showed that Q _EUV has the best correlation with GOLD T _disk compared to all the other EUV proxies. Therefore, it can be used as an alternate proxy for total solar EUV variability, on day‐to‐day to solar cycle time scales.

Introduction and GoalThe upper atmosphere of Mars, spanning the mesosphere (60–120 km) and the thermosphere up to the exobase (180–230 km), remains one of the least characterized regions of the planet [1]. Its structure is shaped by solar forcing, upward‑propagating waves, and dust‑driven variability originating in the lower atmosphere; however, the relative importance of these processes and their vertical coupling are not yet fully understood [2]. Two recent missions provide complementary datasets: NASA’s MAVEN mission, operational since 2014, focuses on upper‑atmosphere processes and atmospheric escape, while ESA–Roscosmos’ Trace Gas Orbiter (TGO), in routine operation since 2018, carries the NOMAD and ACS instruments, which retrieve atmospheric profiles from near‑surface layers to the upper thermosphere.In this work, we combine MAVEN and TGO observations with the primary objective of producing a comprehensive climatology of the thermal structure of the Martian upper atmosphere. This study is part of a broader andn recent project funded by the International Space Science Institute (ISSI) entitled “A multi-mission approach to close the gaps in understanding of the structure and variability in the Mars upper atmosphere”. Most TGO datasets concentrate below 100 km, whereas MAVEN primarily samples thermospheric altitudes. Because TGO data extend to lower altitudes, we actually present results spanning from the troposphere to the exosphere. The combined dataset integrates nine types of measurements from both missions, including in situ observations (NGIMS/MAVEN) and remote‑sensing measurements from NOMAD, ACS, EUVM, and IUVS (solar and stellar occultations, as well as dayglow). Together, these datasets cover multiple Martian years and an altitude range of 30–180 km, providing unprecedented vertical and temporal coverage. Exploiting this complementarity, particularly the vertical coupling between the lower and upper atmosphere, is also a major scientific goal of the present research.In this climatological study, we explicitly exclude a set of short‑lived and sporadic phenomena, such as dust storms, solar eruptions, and small‑scale atmospheric waves. Our focus is on the systematic variations associated with the following geophysical variables: altitude, latitude, season, local time, solar cycle, and longitude.Datasets, Synergies, and ChallengesThe datasets exhibit highly diverse characteristics, including field of view, spatial and temporal coverage, sensitivity, and noise levels, posing challenges for the synergetic analysis pursued here. To address coverage differences, we devoted substantial effort to designing appropriate binning strategies and averages across geophysical variables. Regarding sensitivity and retrieval performance, we used nominal uncertainties but filtered out the highest‑uncertainty percentile, and we developed a unified quality‑control framework applied to the nominal errors of each experiment. This framework aims to eliminate spurious values and identify potential biases, as revealed through cross‑comparisons within well‑defined geophysical bins. We also performed a variance‑partitioning analysis as a function of geophysical forcings, enabling us to distinguish intrinsic atmospheric variability from instrumental effects. Two vertical coordinates were used: altitude above the Martian aeroid and a modified‑CO₂ scale‑height coordinate, which is more suitable for upper‑atmosphere comparisons. Special attention was given to solar‑occultation datasets from both MAVEN and TGO (terminator measurements at dawn and dusk) owing to their synergistic local‑time coverage and excellent vertical resolution.Selected ResultsWe will present a representative sample of this extensive analyses across this broad parameter space, including global composite temperature profiles, deviations from mean distributions, multi‑year time series, and an assessment of the dominant sources of thermal variability. Our results reveal substantial atmospheric variability, highly non‑linear with respect to geophysical parameters, and a hierarchy of dominant processes that varies with altitude and which shapes the global thermal structure.In particular we found that thermospheric temperatures respond strongly to solar flux, with variations exceeding 50 K above 150 km and decreasing toward 100 km, being very small below this altitude. In contrast, mesospheric temperatures are dominated by global circulation, producing seasonal and latitudinal gradients of 20–40 K. We will show how the combined datasets also reveal interhemispheric asymmetries, the impact of dust storms at multiple altitudes, and the propagation of atmospheric waves.Comparisons with MarsPCM and MCD simulations demonstrate that the model reproduces very well the mean vertical structure as well the seasonal and latitudinal variations observed, but underestimates the variability by factors of 1.5–2 and exhibit a few biases in the mesopause and upper thermosphere. In particular, tidal amplitudes in the mesosphere are weaker in the models, the lower‑thermosphere thermal gradient occurs at lower altitudes than what is observed, and the coldest thermospheric temperatures predicted by the models are significantly colder than those reported by EUVM.The resulting MAVEN–TGO climatology provides a robust reference for future measurements, model validation, and studies of Martian atmospheric dynamics and evolution. References[1] Bougher et al., JGR-Planets, 2017 [2] López-Valverde et al., Space Sci. Rev., 2018 [3] Giuranna et al., Space Sci. Rev., 2025 [4] Heavens et al., Nature, 2018 [5] Yiğit et al., Nature Geoscience, 2023 [6] Jakosky et al., Science, 2015 [7] Korablev et al., Space Sci. Rev., 2018 [8] Vandaele et al., Space Sci. Rev., 2018

Using NASA's Global‐scale Observations of the Limb and Disk (GOLD)'s Dark Limb Mode (DLM) observations, we provide a multi‐year characterization of nighttime Equatorial Ionization Anomaly (EIA) morphology and variability. From geostationary orbit, GOLD measures OI 135.6 nm limb radiance at ∼33°E and ∼128°W longitudes, yielding routine, time‐evolving altitude–latitude cross sections of the nighttime ionosphere (∼100–430 km) and enabling investigation of EIA morphology and temporal variability. Forward‐modeled line‐of‐sight integrations using WACCM‐X electron densities and GLOW‐derived volume emission rates indicate that GOLD limb radiance peaks at tangent altitudes ∼40 km lower than the contributing F‐region electron density peak at the limb longitude. Multi‐year (2019–2025) observations reveal limb‐to‐limb differences, pronounced day‐to‐day variability, and strong solar‐flux dependence in the EIA crest. This new DLM data set provides a distinct observational capability for ionosphere–thermosphere coupling studies and space‐weather applications.

NASA’s Mars Atmosphere and Volatile EvolutioN spacecraft carries an extensive suite of instruments for characterizing the Mars upper atmosphere. Of these, the NGIMS, IUVS, and EUVM instruments produce datasets for CO ₂ density and neutral atmosphere temperature. The different instruments and retrieval methods utilized provide an expansive view of the Mars upper atmosphere. To make full use of the geophysical coverage offered by these datasets, we undertake a systematic comparison of the datasets to understand where they are different and how any biases between datasets can be removed. We conduct pairwise comparisons between datasets, binning the data by geophysical and forcing parameters, to develop adjustment factors that can be used to adjust the measured CO ₂ density and neutral temperature of one dataset to nominal agreement with another. The determined adjustment factors are reported for use by the wider Mars aeronomy community.

We report the signature of solar forcing in the thermosphere using daytime temperature profiles from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument on NASA's Ionospheric Connection (ICON) Explorer mission. We compare temperatures from 90 to 133 km altitude with solar extreme ultraviolet flux and also with results from a Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM‐ICON) forced with MIGHTI wind and temperature observations at the bottom boundary near 97 km. For moderate solar activity in late 2021 to early 2022, MIGHTI temperature oscillations correlate with the 27‐day solar rotation period ( r = 0.84) at 133 km with an average amplitude of 38 K, more than twice the GCM amplitude for the same time period. At 110 km, average amplitudes are 8 K whereas the GCM shows insignificant correlation below 115 km (<0.3). These results reveal that solar forcing in the lower thermosphere is stronger than predicted.

F10.7, Mg II, and F30 are the current, standard measures of variability in solar Extreme UltraViolet (EUV) irradiance. An alternative proxy of solar variability is QEUV, an indirect measure of total solar EUV irradiance, which has been retrieved using observations of Earth’s Far UltraViolet (FUV) airglow as well as from EUV spectra modeled empirically by using solar EUV observations. Daily QEUV derived from FUV airglow is available from the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) Global Ultraviolet Imager (GUVI) and Global-scale Observations of the Limb and Disk (GOLD) missions, as is QEUV from solar EUV observations. Spectral measurements from GOLD are also used to retrieve thermospheric temperature, termed as Tdisk. The availability of GOLD Tdisk and QEUV provided an opportunity to reassess the effectiveness of various solar EUV proxies for thermospheric use. This investigation demonstrates that QEUV provides a more accurate representation of total solar EUV variability, on day-to-day to solar cycle time scales, than the other proxies.

Atmospheric gravity waves (GWs) are believed to transport energy and momentum between different regions of the atmosphere. Historically, observations of these waves from both ground and space have been relatively abundant at altitudes up to the lower thermosphere, and somewhat less abundant in the upper thermosphere and F‐region ionosphere altitudes. Much of what is known of the typical properties and occurrence of these waves at thermospheric altitudes has been inferred from their impacts on the ionospheric density and motion, as direct observations of the neutral atmosphere have been less prevalent. Gravity waves in the middle thermosphere, from ∼120–200 km altitude, have rarely been observed directly and as such, their properties at these altitudes are less well documented. NASA's Global‐Scale Observations of the Limb and Disk (GOLD) mission makes observation of the middle thermosphere during daytime. During dedicated campaigns, GOLD has been able to observe GWs in this region. This study leverages 22 such campaigns during quiet geomagnetic conditions and low to moderate solar activity levels. Waves were observed with typical periods ∼2–4 hr. Leveraging ground‐based observations, the wavelengths were identified to be between ∼1,500–5,000 km, with phase speeds ∼150–600 m/s. The waves observed were seen to propagate primarily meridionally, in agreement with prior daytime mid‐latitude observations. Using observations of the background wind, the energy and momentum fluxes carried by these waves were found. During the quiet conditions observed, the waves were seen to transport energy flux over a wide range of latitudes.