Exceptional bright OH airglow night at Cerro Paranal, Chile, with high wave activity and sudden brightness depletion

The dynamics in the atmosphere, especially the upper mesosphere and mesopause are significantly driven by atmospheric gravity waves. OH airglow offers an unique possibility to observe atmospheric dynamics in this altitude region with a high spatio-temporal resolution simultaneously using imager and spectrometer systems. Especially, characteristics of gravity waves as well as features like wave breaking and wave-wave interaction can be observed. Spectroscopic observations allow observing rotational temperature changes. Thus, both instrument types complement each other very well.

Since November 2022 two airglow imagers (FAIM) and one airglow spectrometer (GRIPS) with high temporal resolution (1 image every 2 seconds, 1 spectrum every 15 seconds) started routine observations during each night in cooperation with and at ESO’s Very Large Telescope (VLT) in the Atacama Desert at Cerro Paranal, Chile (24.6°S, 70.4°W).

During the night from 31^st July to 1^st August 2023 we observed an exceptional bright night that is much brighter than any other we observed so far: a single wave front propagates from West to East with an observed phase speed of about 60m/s. After the passing of the wave front the OH intensity decreases by around 50% within only one hour. Pronounced wave activity of small-scale waves is observed especially before the passing of the event. Similar events in literature are often stated as “wall events”, but seem to occur very rarely in the extent observed.

We present and interpret the wall event and discuss the observed phenomenon and its causes using data from multiple instruments and data sources.