Abstract The Emirates Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM) Hope probe images Mars at wavelengths extending from approximately 100 to 170 nm. EMUS observations began in February 2021 and cover over a full Mars year. We report the first limb scan observations at Mars of ultraviolet emissions Ar I 106.6 nm, N I 120 nm, and carbon monoxide (CO) Fourth Positive Group ( A − X ) band system excited by electron impact on CO. We use EMUS limb scan observations to retrieve number density profiles of argon, molecular nitrogen, atomic oxygen, and CO in the upper atmosphere of Mars from 130 to 160 km. CO is a sensitive tracer of the thermal profile and winds in Mars' middle atmosphere and the chemistry that balances CO 2 in the atmosphere of Mars. EMUS insertion orbit special observations demonstrate that far ultraviolet limb measurements of the Martian thermosphere can be spectroscopically analyzed with a robust retrieval algorithm to further quantify variations of CO composition in the Martian upper atmosphere.
Abstract Global‐scale waves have been seen throughout the Martian atmosphere and can achieve significant amplitude at higher altitudes. Previous observations of the upper atmosphere have also revealed wavenumber‐3 signatures with significant amplitudes that have been interpreted as signatures of diurnal and semidiurnal tides or stationary planetary waves. This study focuses on two intervals during which concurrent observations of the upper thermosphere made in situ, the middle thermosphere made remotely, and the middle atmosphere made remotely provide observations of the atmospheric tides between 50‐ and 200‐km altitude. Focusing on high latitudes, these observations are able to identify strong wavenumber‐3 signatures in the thermosphere that propagate eastward with increasing local time, consistent with an eastward propagating tide. A complementary analysis of the data from the middle atmosphere reveals wavenumber‐3 signatures that move eastward and upward. During the first interval, these analyses reveal a combination of the diurnal tide DE2 and the semidiurnal tide SE1 to be present, while during the second only the semidiurnal SE1 is seen. During both intervals, the observations provide a consistent picture of these waves being present from the middle atmosphere to the upper thermosphere, consistent with the theory that such waves are generated in the lower atmosphere and propagate upward throughout the entire atmosphere.
Abstract Using the Mars Atmospheric and Volatile EvolutioN mission (MAVEN) Imaging Ultraviolet Spectrograph (IUVS), we found periodic longitudinal variations in CO 2 density in the Martian atmosphere. These density variations are derived from observations of the ( ) emission from limb scans in the 100–190 km altitude range. The variations exhibit significant structure with longitudinal wave numbers 1, 2, and 3 in an effectively constant local solar time frame, and we attribute this structure to nonmigrating tides. The wave‐2 component is dominated by the diurnal eastward moving DE1 tide at the equator and the semidiurnal stationary S0 tide at the midlatitudes. Wave‐3 is dominated by the diurnal eastward moving DE2 tide, with possibly the semidiurnal eastward moving SE1 tide causing an amplitude increase at the midlatitudes. Structure in the wave‐1 component can be explained by the semidiurnal westward moving SW1 tide.
Abstract 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 R M ) 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.
Abstract E‐region models have traditionally underestimated the ionospheric electron density. We believe that this deficiency can be remedied by using high‐resolution photoabsorption and photoionization cross sections in the models. Deep dips in the cross sections allow solar radiation to penetrate deeper into the E‐region producing additional ionization. To validate our concept, we perform a study of model electron density profiles (EDPs) calculated using the Atmospheric Ultraviolet Radiance Integrated Code (AURIC; D. Strickland et al., 1999, https://doi.org/10.1016/s0022-4073(98)00098-3 ) in the E‐region of the terrestrial ionosphere. We compare AURIC model outputs using new high‐resolution photoionization and photoabsorption cross sections, and solar spectral irradiances during low solar activity with incoherent scatter radar (ISR) measurements from the Arecibo and Millstone Hills observatories, Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC‐1) observations, and outputs from empirical models (IRI‐2016 and FIRI‐2018). AURIC results utilizing the new high‐resolution cross sections reveal a significant difference to model outputs calculated with the low‐resolution cross sections currently used. Analysis of AURIC EDPs using the new high‐resolution data indicate fair agreement with ISR measurements obtained at various times at Arecibo but very good agreement with Millstone Hills ISR observations from ∼96–140 km. However, discrepancies in the altitude of the E‐region peak persist. High‐resolution AURIC calculations are in agreement with COSMIC‐1 observations and IRI‐2016 model outputs between ∼105 and 140 km while FIRI‐2018 outputs underestimate the EDP in this region. Overall, AURIC modeling shows increased E‐region electron densities when utilizing high‐resolution cross sections and high‐resolution solar irradiances, and are likely to be the key to resolving the long standing data‐model discrepancies.
Abstract The global‐scale observations of the limb and disk (GOLD) Mission images middle thermosphere temperature and the vertical column density ratio of oxygen to molecular nitrogen ( O/N 2 ) using its far ultraviolet imaging spectrographs in geostationary orbit. Since GOLD only measures these quantities during daylight, and only over the ∼140° of longitude visible from geostationary orbit, previously developed tidal analysis techniques cannot be applied to the GOLD data set. This paper presents a novel approach that deduces two specified non‐migrating diurnal tides using simultaneous measurements of temperature and O/N 2 . DE3 (diurnal eastward propagating wave 3) and DE2 (diurnal eastward propagating wave 2) during October 2018 and January 2020 are the focus of this paper. Sensitivity analyses using TIE‐GCM simulations reveal that our approach reliably retrieves the true phases, whereas a combination of residual contributions from secondary tides, the restriction in longitude, and random uncertainty can lead to ∼50% error in the retrieved amplitudes. Application of our approach to GOLD data during these time periods provides the first observations of non‐migrating diurnal tides in measurements taken from geostationary orbit. We identify discrepancies between GOLD observations and TIE‐GCM modeling. Retrieved tidal amplitudes from GOLD observations exceed their respective TIE‐GCM amplitudes by a factor of two in some cases.
Numerous instruments for UV-visible optical measurements of terrestrial backgrounds have recently flown or are scheduled for launch in the near future. In order to maximize the scientific return from such flight opportunities, simulations of data acquired by imaging and spectrographic imaging instruments spanning wide wavelength ranges are required to support experiment planning and post-launch data analysis/fusion activities. We are currently developing comprehensive capabilities for modeling these types of remote sensing data suitable for a number of mission-support applications, with specific focus on data acquired by the UVISI instruments on the Midcourse Space Experiment satellite. These capabilities are described in this presentation. The core modeling capabilities reside in a suite of well-tested first principles and empirical modeling codes for atmospheric radiances arising from a variety of physical processes (e.g., photoelectron impact excitation, Rayleigh and aerosol scattering, solar resonance and resonant fluorescence scattering, chemistry). Image generation and LOS spectral radiance evaluation techniques permitting continual change in observer location and viewing geometry without incurring large computational burdens have been set up to ingest the radiance modeling results to create high fidelity synthetic satellite data. Illustrative examples are presented.
Abstract The ionospheric O + number density can be measured remotely during the day by observing its optically thick 83.4 nm radiance. Some ambiguity is present in the process of retrieving the density due to uncertainties in the initial excitation rate. This can be removed by observing a companion optically thin emission at 61.7 nm originating from the O + (3s 2 P) state, providing that the ratio of the initial excitation rates is known. Analyses of ICON EUV data using an 83.4/61.7 emission ratio of order 10 result in O + densities lower by ∼2 than other measurements. Key to relating the two emissions is accurate knowledge of the partial photoionization cross sections and the spectroscopy of O + —the topic of this paper. Up to now, no independent evaluation of the ratio of the 83.4/61.6 emission ratio exists. The recent availability of state‐of‐the‐art calculations of O partial photoionization cross sections into a variety of O + states presents an opportunity to evaluate the O + (2p 4 4 P)/O + (3s 2 P) ionization rate ratio. We calculate excitation of these parent states of the emissions including both direct and cascade excitation from higher lying O + energy states. The resulting theoretical prediction gives ratios that range from 13.5 to 12 from solar minimum to maximum, larger than the value of 10 used by the ICON 83.4 and 61.7 nm algorithm. The higher theoretical values for the ratio reconcile the ∼2 discrepancy between simultaneous ICON and other electron density measurements.
