Airglow intensity variations induced by gravity waves. Part 2: comparisons with observations

Abstract

The generalized gravity wave–airglow interaction theory presented in the first paper of this two part series by Wang et al. (2001) is further developed for comparisons with observations. As a counterpart of Krassovsky's airglow intensity-to-temperature ratio η, we propose the ratio of the line-of-sight (LOS) perturbation wind to the intensity, η w. The behaviors of these ratios are determined by the intrinsic wave frequency, rather than the observed frequency. The Doppler-shifting effects are investigated and found to be quite important. For a given observed frequency, an increasing (decreasing) magnitude and phase angle of η is generally expected, in response to the downward (upward) Doppler-shifting of the intrinsic frequencies for the waves propagating along (against) a mean flow. In a wind-temperature-stratified atmosphere, the magnitude of η may vary by a factor of ∼2 or more, with larger values corresponding to small-scale waves, and the phase angles may change by ∼40–90°, with larger values occurring for either large-scale or very small-scale wave modes. For η w, which is proportional to 1/ η, its Doppler response generally exhibits a tendency opposite to that of η. These ratios are also dependent on zenith angle θ, at which observations are made. In particular, θ-dependence of η w is primarily controlled by different contributions of horizontal and vertical motions to the LOS wind. For limb scans, small-scale gravity waves with relatively large vertical perturbation velocities are shown to play an important role in producing the observed large airglow intensity variations. In order to explore the importance, we simulate the green line airglow images observed by WINDII, the WIND Imaging Interferometer on the Upper Atmosphere Research Satellite (UARS), by numerically integrating the Michelson equation of the instrument. The model-calculated ratios and simulated images are compared with realistic observations from the ground and space in good agreements.