Effects of Downwelling in the Martian Atmosphere as Observed by MAVEN/IUVS Stellar Occultations

The ultraviolet nitric oxide nightglow emissions in 50-100 km altitude range have been detected in nadir images and limb scans by the Imaging Ultraviolet Spectrograph (IUVS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, as well as by the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) experiment on board Mars Express (MEx). These emissions are produced when N and O atoms, created on the dayside, are transported to the nightside and down to the mesosphere where they radiatively recombine, serving as a tracer of Hadley circulation and downward transport. However, the models do not match the observations in critical aspects such as the altitude and local time of peak emission, implying that the downwelling and its effects are not well-understood. Previous studies have also reported the presence of recurring nightside mesospheric warm layer in the 70-90 km altitude range, that is related to the signature of thermal tidal propagation. However, it is not established if the warming could also be due to air subsidence of dayside-to-nightside circulation.