Analysis and interpretation of airglow and radar observations of quasi-monochromatic gravity waves in the upper mesosphere and lower thermosphere over Adelaide, Australia (35 degrees S, 138 degrees E)

Observations of wave-driven fluctuations in emissions from the OH Meinel (O HM) and O-2 Atmospheric band were made with a narrow-band airglow imager lo cated at Adelaide, Australia (35S, 138E) during the period April 1995 to Ja nuary 1996. Simultaneous wind measurements in the 80-100 km region were mad e with a co-located MF radar. The directionality of quasi-monochromatic (QM ) waves in the mesopause region is found to be highly anisotropic, especial ly during the solstices. During the summer, small-scale QM waves in the air glow are predominately poleward propagating? while during winter they are p redominately equatorward. The directionality inferred from a Stokes analysi s applied to the radar data also indicates a strong N-S anisotropy in summe r and winter, but whether propagation is from the north or south cannot be determined from the analysis. The directionality of the total wave held (wh ich contains incoherent as well as coherent features) derived from a spectr al analysis of the images shows a strong E-W component, whereas, an E-W com ponent is essentially absent for QM waves. The prevalence of QM waves is al so strongly seasonally dependent. The prevalence is greatest in the summer and the least in winter and correlates with the height of the mesopause; wh ether it is above or below the airglow layers. The height of the mesopause is significant because for nominal thermal structures it is associated with a steep gradient in the Brunt-Vaisala frequency that causes the base of a lower thermospheric thermal duct to be located in the vicinity of the mesop ause. We interpret the QM waves as waves trapped in the lower thermosphere thermal duct or between the ground and the layer of evanescence above the d uct. Zonal winds can deplete the thermal duct by limiting access to the duc t or by negating the thermal trapping. Radar measurements of the prevailing zonal wind are consistent with depletion of zonally propagating waves. Dur ing winter, meridional winds in the upper mesophere and lower thermosphere are weak and have no significant effect on meridionally propagating waves. However, during summer the winds in the duct region can significantly enhan ce ducting of southward propagating waves. The observed directionality of t he waves can be explained in terms of the prevailing wind at mesopause alti tudes and the seasonal variation of distant sources. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.