The observable effects of gravity waves on airglow emissions

The interaction of gravity waves and airglow emissions is explored with a view to elucidating the observable effects of such waves. The results obtained are applicable to ground-based photometric observations, where the total line-of-sight emission, at an arbitrary viewing angle, is measured. Explicit relations are developed for the dependence of the magnitude and phase of the observable parameter {dollar}eta{dollar}, the ratio of brightness fluctuations to temperature fluctuations. Both magnitude and phase depend on dynamical parameters (wavelength and period), the viewing angle at which observations are made, and chemical parameters related to the temperature dependence of the reaction chemistry, and the number of bodies involved in the net reaction. Methods of (i) distinguishing between evanescent and internal gravity wave modes, and (ii) determining the vertical wavelength and direction of propagation for internal modes, are proposed. The former may be of some importance observationally, since the effects of evanescent and internal modes on airglow are quite different. The latter is a measurement which is typically difficult to make directly.; The specific cases of O{dollar}sb2(bsp1Sigma{dollar}) and OH airglow are examined in detail, for different proposed chemical production mechanisms. Attention is given to nonequilibrium chemistry in the latter case, since the chemical time constants of some implicated species are comparable to gravity wave periods. Relations for the dependence of {dollar}eta{dollar} are presented in a generalized form which is readily applicable to other emissions. It is shown that gravity wave observations may place new observational constraints on possible mechanisms for airglow production, constraints which are not obtainable by other means.