Dayside Temperature Maps of the Upper Mesosphere and Lower Thermosphere of Mars Retrieved From MAVEN IUVS Observations of O I 297.2 nm Emission. Issue 2 (2nd February 2023) - Evans, J. S.
Abstract: We present temperature maps derived from number density retrievals of carbon dioxide (CO2 ) for the upper mesosphere and lower thermosphere of Mars using limb observations from the Imaging Ultraviolet Spectrograph (IUVS) aboard NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We retrieve CO2 densities using O( 1 S) metastable atoms that radiatively relax by emitting photons at 297.2 nm, producing a double‐peaked emission profile detectable by IUVS. Retrieved CO2 densities are used to derive altitude profiles of temperature as a function of latitude, longitude, local time, season, dust activity, and solar activity. CO2 density and temperature profiles retrieved using the O I 297.2 nm emission feature presented herein extend previous IUVS retrievals from 130–170 km down to 80 km. We validate retrieved CO2 densities and derived temperatures using coincident measurements and corresponding data products produced by MAVEN IUVS, as available. Analysis of this comprehensive data set, which spans Mars years 32–36, shows (a) a consistently well‐defined mesopause at approximately 120 km, (b) warming at high pressures (typically below ∼100 km) for a variety of geophysical conditions, (c) asymmetry in temperatures at dawn and dusk with respect to latitude during different seasons (warmer temperatures at dawn during northern hemisphere autumn/winter and cooler temperatures at dusk during spring/summer), and (d) longitudinal waves with a dominant wave‐3 component in both the upper mesosphere and lower thermosphere, with lower (80–90 km) and upper (135–145 km) atmospheric waves about 65° out of phase. Plain Language Summary: We show the first O I 297.2 nm emission‐derived temperature profiles extending from 80 to 150 km in the atmosphere of Mars obtained from observations by the Imaging Ultraviolet Spectrograph onboard NASA's Mars Atmosphere and Volatile EvolutioN spacecraft. This new analysis enables us to bridge the gap and improve our understanding of the coupling that occurs between the middle and upper atmospheres of Mars. Our analysis has revealed thermal variability in the Martian atmosphere with respect to latitude, longitude, local time, season, dust activity, and solar activity. As suggested by previous studies, waves generated in the lower atmosphere propagate to the upper atmosphere, coupling the atmospheric layers, and produce fluctuations in density (up to 40% locally) that can affect the background atmospheric temperature, which is a key parameter driving thermal escape of atomic hydrogen. The density and temperature data presented here provide an important but heretofore missing source of information that is needed to directly link weather and waves in the lower atmosphere to perturbations in composition and temperature in the upper atmosphere. Key Points: Our analysis reveals a consistently well‐defined mesopause at approximately 120 km We observe an asymmetry in temperatures at dawn and dusk with respect to latitude during different seasons Longitudinal waves with dominant wave‐3 component are characterized in both the upper mesosphere and lower thermosphere