Thermal Structure of the Martian Upper Mesosphere/Lower Thermosphere From MAVEN/IUVS Stellar Occultations.

We report the first detailed study of the diurnal thermal structure of upper mesosphere/lower thermosphere (∼80 to 160 km) of Mars from stellar occultation observations by the Imaging Ultraviolet Spectrograph (IUVS) aboard the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Due to stray light contamination, analyses of this data set to date have been confined to the nighttime events. This study makes use of a revised algorithm for removal of stray light from occultation spectra to retrieve the dayside events as well. The dayside is observed to be warmer than the nightside, with the maximum day/night difference of ∼30 K in the lower thermosphere, ∼20 K around the mesopause, with little diurnal variations at lower altitudes. This is consistent with the radiative time constant which is of the order of 1 Mars day in the 3×10−4 $3\times {10}^{-4}$ to 3×10−3 $3\times {10}^{-3}$ Pa region. The data also shows that the regions at pressure less than 2×10−3 $2\times {10}^{-3}$ Pa are under strong solar control with no prominent migrating tidal signatures. In contrast, on Earth, the radiative time constant near the mesopause is ∼10 Earth days and the temperature variations due to tides are quite large. The Mars Climate Database shows a diurnal trend opposite to the data in the mesosphere, with the dayside mesopause predicted to be cooler than the nightside by ∼10 K along with signatures of a vertically propagating tide. The IUVS data set provides an unprecedented constraint on the structure of the Martian mesosphere. Plain Language Summary: The dayside Mars Atmosphere and Volatile EvolutioN/Imaging Ultraviolet Spectrograph stellar occultation observations are often contaminated by stray light and are not processed by the current data reduction pipeline. We have developed an improved algorithm to retrieve these dayside events as well, therefore expanding the usable data set. There have been very few observations of Mars in 80–160 km altitude range, a region that is influenced by the wave perturbations from lower atmosphere and solar forcing from above, with no data set providing a complete local time coverage to study the day/night differences. Therefore, we have enabled the first detailed study of the diurnal thermal structure of Mars in this region by salvaging the dayside events. We find the dayside atmosphere to be always warmer than the nightside at these altitudes. Though it is expected but has not been established for the Martian mesosphere. In contrast, the Mars Climate Database shows an opposite diurnal trend at the mesopause, along with inconsistent tidal signatures with the data. The mesopause is controlled by the energy balance between radiative processes. These data‐model inconsistencies thus imply that our current understanding of the dynamics and structure of the Martian mesosphere is inadequate and this data set can provide important constraints on the models. Key Points: First detailed study of the diurnal thermal structure of Mars for 10−5−10−2 ${10}^{-5}-{10}^{-2}$ Pa pressure levels from 3003 stellar occultationsDayside is observed to be warmer than the nightside at these pressure levels with no detectable migrating tidal signaturesGCM predictions are inconsistent with the data and show dayside mesopause cooler than the nightside, with a vertically propagating tide [ABSTRACT FROM AUTHOR]