Airglow variability in the context of the global mesospheric circulation

The oxygen airglow emission rate is modulated by gravity waves, by atmospheric tides, and by semi-annual, annual and longer-term variations including the solar cycle. This variability exhibits global patterns, including strong altitude dependence. In this investigation the previous relevant variability studies are reviewed, demonstrating the scope and nature of the airglow variations. Recent results are then presented, with a focus on the relationships between the different observables. The annual and semi-annual variations are found to be subject to tidal modulation. The annual variation is of particular interest as a signature of the large-scale circulation, leading to the familiar high latitude down-welling and consequent warm mesopause in winter with the opposite in summer. Atomic oxygen transported by the same circulation is therefore expected to result in airglow emission rates that are high in winter and low in summer, in parallel with the temperature variation. At high latitudes the airglow emission rate is indeed found to be highly correlated with temperature, as well as with the altitude of the emission peak, consistent with these up- and down-welling motions. These correlations, and the associated vertical airglow motions of about 6 km appear to underlie all of the scales of variability under investigation, from gravity waves to inter-annual variations. One exception to this correlation is noted in that a sharp “mid-summer peak” is superimposed on summer emission rate minimum, indicating an additional atomic oxygen mixing process near summer solstice.