Statistical characteristics of gravity waves observed by an all‐sky imager at Darwin, Australia

An all‐sky airglow imager with a cooled charge‐coupled device camera in place at Darwin (12.4°S, 131.0°E), Australia, since October 2001 has been used to obtain two‐dimensional gravity wave images in the mesopause region. Using airglow images of the OI (557.7 nm, emission altitude ∼96 km) and OH band (720–910 nm, ∼86 km) emissions obtained for October 2001 to August 2002, we investigated the wavelengths, phase velocities, and propagation directions of gravity waves. Wave occurrence in OH images (60–90%) is higher than in OI images (30–70%) for all seasons. The waves have wavelengths of less than 90 km (peak: 30–50 km) and phase velocities of less than 90 m/s (peak: 30–60 m/s). Most of the waves propagate in the meridional direction, and the directionality strongly depends on the season. In winter, waves propagate both poleward and equatorward, while in summer almost all waves propagate poleward. An examination of airglow‐imaging statistics at Adelaide (35°S, 138°E), Australia, obtained by Walterscheid et al. 1999 , leads us to conclude that this clear directionality is caused by the location of the wave sources and by the wave ducting processes; that is, poleward waves in summer come from an equatorial convective source through a thermal duct structure. The effect of wind filtering on the waves is also discussed for zonal wave propagation.