Mars airglow observations from the Trace Gas Orbiter and MAVEN

The UVIS (UV and Visible Spectrometer) channel of the NOMAD (Nadir and Occultation for MArs Discovery) spectrometer on board the ExoMars Trace Gas Orbiter covers the spectral range 200-650 nm at a spectral resolution between 1.2 and 1.6 nm. It has made limb observations of the Martian dayglow since April 2019. Two pointing modes have been applied: (1) in the inertial mode, the spectrometer scans the atmosphere twice down to near the surface and provides altitude profiles of the dayglow; (2) in the tracking mode, the atmosphere is scanned at varying latitudes at a nearly constant altitude. Feature that have been identified include the OI 557.7 nm and 297.2 nm lines, the OI 630-636.4 nm red doublet, the CO Cameron bands, and the CO _{2} <SUP>+</SUP> B→X UV doublet and the A→X FDB bands. A statistical study of the vertical and seasonal distribution of the oxygen green and red lines at 557.7 nm and 630 nm indicates that both the brightness and altitude of the green line emission respond to seasons. The peak altitude of the green line emission increases seasonally when the Sun-Mars distance decreases. The lower peak of the green line statistically drops by 15-20 km between perihelion and aphelion at mid- to high altitude. It is significantly brighter and more pronounced in the southern hemisphere than in the north in response to the stronger Lyman-alpha solar flux near perihelion. These seasonal effects will be compared with the 297.2 nm characteristics observed the IUVS instrument on board the MAVEN orbiter. Results from a photochemical model simulating the variations of the oxygen dayglow observed will be compared with observations. Model fits to the OI 557.7 nm limb profiles are also a tool to determine the CO_2 density and its seasonal variations. MUV UVIS spectra will be compared with Mariner and MAVEN/IUVS limb observation and with the visible component of the UVIS spectra.