Instability layers and airglow imaging

[1] For over 30 years it has been recognized that atmospheric gravity waves ( AGWs) in the 80 - 110 km region significantly perturb the basic atmospheric state, perhaps causing instability regions and subsequent turbulence. It has also been recognized for nearly as long that AGWs cause strong fluctuations in the airglow emissions that originate in this altitude region. Airglow images have been obtained since 1973, and they have shown structures that have mainly been attributed to the passage of AGWs through this region as predicted theoretically. The AGWs have been assumed to originate largely in the troposphere because of either convective activity or the flow of air over large mountain ranges. However, intensive analysis of the properties of a class of small-scale features currently known as ripples [Taylor et al., 1997; Nakamura et al., 1999] suggests that these features are not AGWs but are rather instability features generated in situ. The basis for this hypothesis is examined in this review, and it is concluded that while there is support for the instability hypothesis as the origin of ripple features, at present the exact nature of the instabilities causing these features is not known.