Nitric oxide nightglow and Martian mesospheric circulation from MAVEN/IUVS observations and LMD-MGCM predictions

Abstract : We report results from a study of nitric oxide nightglow over the northern hemisphere of Mars during winter, the southern hemisphere during fall equinox and equatorial latitudes during summer in the northern hemisphere based on observations of the δ and γ bands between 190 and 270 nm by the Imaging UltraViolet Spectrograph (IUVS) on the MAVEN spacecraft. The emission reveals recombination of N and O atoms dissociated on the dayside of Mars and transported to the nightside. We characterize the brightness (from 0.2 to 30 kR) and altitude (from 40 to 115 km) of the NO nightglow layer, as well as its topside scale height (mean of 11 km). We show the possible impact of atmospheric waves forcing longitudinal variability, associated with an increased brightness by a factor 3 in the 140 - 200∘ longitude region in the northern hemisphere winter and in the -102∘ to -48∘ longitude region at summer. Such impact to the NO nightglow at Mars was not seen before. Quantitative comparison with calculations of the LMD-MGCM (Laboratoire de Météorologie Dynamique - Global Circulation Model) suggests that the model globally reproduces the trends of the NO nightglow emission and its seasonal variation, but also indicates large discrepancies (up to a factor 50 fainter in the model) in northern winter at low to mid-latitudes. This suggests that the predicted transport is too efficient towards the night winter pole in the thermosphere by ∼20∘ latitude north.