The observable effects of gravity waves on airglow emissions

Abstract

The influence of gravity waves on 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 zenith angle, is measured. Explicit relations are developed for the dependence of the magnitude and phase of the observable parameter η, the ratio of brightness fluctuations to temperature fluctuations. Both magnitude and phase depend on dynamical parameters (wavelength and period), the zenith angle at which observations are made, and a chemical parameter which is specific to the reaction chemistry. Methods of (i) distinguishing between evanescent and internal gravity wave modes, and (ii) determining the vertical wavelength and sense of vertical propagation for internal modes, are proposed. The former may be of considerable importance observationally, since the effects of evanescent and internal modes on airglow are quite different. The latter is a measurement that is typically difficult to make directly. It is shown that, for an isothermal atmosphere, the dependence of η on the characteristics of the perturbing wave is partially separable from the dependence on the emission chemistry, making it possible to draw conclusions about the effects of gravity waves on airglow brightness and temperature that are independent of any specific emission mechanism, as well as conclusions that are independent of specific wave characteristics.