Acoustic-gravity modes and large-scale traveling ionospheric disturbances of a realistic, dissipative atmosphere

Abstract

The guided acoustic-gravity modes that can propagate in a realistic dissipative atmosphere are computed for periods between 30 sec and 2 hours. The analysis differs from previous treatments by including dissipation (viscosity and thermal conductivity) and by using a realistic sound speed profile throughout the thermosphere. The presence of realistic dissipation allows a unique determination of the acoustic-gravity mode spectrum free from ambiguities related to the choice of the upper boundary condition. Since the atmospheric model below the thermosphere is similar to those used in previous studies, the characteristics of the lower atmospheric modes (e.g., the Lamb mode) are essentially unchanged from previous analyses of fully ducted modes in dissipationless atmospheres. The attenuation distances of these lower atmospheric modes, defined as the distances required for the modes to be attenuated by a factor of 1/e, are of the order of the earth's circumference or greater. The upper atmospheric modes, on the other hand, exhibit several characteristics that disagree with the results of previous analyses. The calculations reported here reveal the existence of several long-period modes with average speeds between 300 and 700 m/sec, all composed at F region heights of internal gravity waves with nearly horizontal phase fronts. Their attenuation distances are of the order of 1/8 of the way around the earth or less. All the properties of these upper atmospheric modes agree qualitatively with the observed characteristics of large-scale F region traveling disturbances (TID's). Ion drag is not found to be particularly important, but changes of thermospheric temperature due to diurnal and solar cycle variations are found to have quite appreciable effects on the upper atmospheric modes.