At a horizontally homogeneous isothermal atmosphere approximation, we derive an ordinary six-order differential equation describing linear disturbances with consideration for heat conductivity and viscosity of medium. The wave problem may be solved analytically by representing the solution through generalized hypergeometric functions only at a nonviscous heat-conducting isothermal atmosphere approximation. The analytical solution may be used to qualitatively analyze propagation of acoustic and internal gravity waves (AGWs) in the real atmosphere: a) to classify waves of different frequencies and horizontal scales according to a degree of attenuation and thus according to their ability to appear in observations and in general dynamics of the upper atmosphere; b) to describe variations in amplitude and phase characteristics of disturbances propagating in a height region with dominant dissipation; c) to analyze applicability of quasi-classical wave description to a medium with exponentially growing dissipation. In this paper, we also present wave and quasi-classical methods for deriving waveguide solutions (dissipative ones corresponding to a range of internal gravity waves (IGWs)) with consideration of wave leakage into the upper atmosphere. We propose a qualitative scheme which formally connects the wave leakage solution to the wave solution in the upper dissipative atmosphere. Spatial and frequency characteristics of dissipative disturbances generated by a waveguide leakage effect in the upper atmosphere are demonstrated to agree well with observed characteristics of middle-scale traveling ionospheric disturbances (TIDs).
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