Density and Temperature of the Upper Mesosphere and Lower Thermosphere of Mars Retrieved From the OI 557.7 nm Dayglow Measured by TGO/NOMAD

Abstract

The upper mesosphere and lower thermosphere of Mars (70–150 km) is of high interest because it is a region affected by climatological/meteorological events in the lower atmosphere and external solar forcing. However, only a few measurements are available at this altitude range. OI 557.7 nm dayglow emission has been detected at these altitudes by the limb observations with Nadir and Occultation for Mars Discovery (NOMAD) aboard the ExoMars Trace Gas Orbiter (TGO). We develop an inversion method to retrieve density and temperature at these altitudes from the OI 557.7 nm dayglow limb profiles. We demonstrate that the atmospheric density around 90 and 140 km and temperature around 80 km during the daytime can be retrieved from the TGO/NOMAD limb measurements. The retrieved densities show a large seasonal variation both around 90 and 140 km and reach maximum values around perihelion period. This can be explained by temperature variation in the lower atmosphere driven by the dust content and Sun‐Mars distance. Temperature around 80 km is higher than predicted by general circulation models, which is tentatively consistent with the warm atmospheric layer recently discovered in nighttime. The temperature retrieval relies on the temperature dependence of the quenching coefficient of 1S oxygen by CO2. Further validation of this coefficient in the range of the Mars upper atmosphere is needed for the verification of the retrieved high temperature.

The Mars Oxygen Visible Dayglow: A Martian Year of NOMAD/UVIS Observations

“…The main features are well reproduced, including the presence of the two branches in Figures 9d and 10d. We thus confirm that the PAM model is highly reliable for local simulations as was shown by Gkouvelis et al (2018Gkouvelis et al ( , 2020a, Gérard et al (2020Gérard et al ( , 2021, and Aoki et al (2022). This implies that the photochemistry seems to be well understood and that the MCD values of the Mars atmosphere density are also adequate to predict the green line behavior.…”

“…(2020, 2021), and Aoki et al. (2022). This implies that the photochemistry seems to be well understood and that the MCD values of the Mars atmosphere density are also adequate to predict the green line behavior.…”

“…Figure 8a represents the altitude of 557.7 nm emission peak, while its brightness is shown in Figure 8c. Note that a similar figure is shown in Figure 3c of Aoki et al (2022) but Figure 8a is generated here with binned profiles to make comparison with those of the red line. The colors of the dots clearly show that the altitude of the green emission peak is higher during southern summer (86.3 km in average) than during northern summer (78.0 km).…”

“…In contrast, model simulations indicate that the brightness of the lower peak depends on the Ly-α solar flux reaching the top of the Martian atmosphere. Here, instead of considering all the available spectra, we only use the intensity and altitude of the lower peak obtained with the 105 NOMAD/UVIS inertial limb profiles with a SZA less than 70° determined in Aoki et al (2022). We compare it to the solar flux measured by the Extreme Ultraviolet Monitor onboard the Mars Atmosphere and Volatile Evolution (MAVEN) orbiter (Eparvier et al, 2015).…”

“…The entire database thus includes 137 orbits and 25,816 spectra. Aoki et al (2022) focused on the retrievals of atmospheric density and temperature from the green oxygen dayglow measured by the inertial limb mode of UVIS. In this paper, we further investigate the variations of the green and red oxygen dayglow by using both inertial and limb tracking modes.…”