The Origin and Propagation of Acoustic?Gravity Waves Ducted in the Thermosphere

Abstract

SummaryThe theory of acoustic-gravity waves in a thermoclinal atmosphere shows that damping decreases as the thermal gradient increases. If the thermal gradient is large enough it offsets the increase in amplitude caused by upward propagation into a region of decreasing ambient density and the motion is completely undamped, neglecting viscosity. Thus it appears that long distance propagation can occur in the principal duct at 150 km altitude, where the thermal gradient is almost large enough to eliminate nonviscous damping.It is shown that thermal perturbation of the principal duct by the long winter-solstice sunrise in the polar regions is a source of ducted gravity waves consistent with the seasonal variations in directions of travelling ionospheric disturbances (TID's) observed in Australia.The observed tilts of TID's are compared with theory and can be used to determine the period of a disturbance. The effect of ionospheric charge equilibration is discussed briefly, and a leaky· duct gravity wave model is proposed which reconciles the observations of tilted wave fronts and long distance propagation.

Airglow variations associated with nonideal ducting of gravity waves in the lower thermosphere region

“…wave may be internal in the region of local minimum of r B , and become evanescent at some distance either side of this where r• has increased [Hines, 1960;Wickersham, 1968;Hecht et al, 2001 ]. In this case, the wave would be thermally ducted.…”