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The thermal self-focusing of a wave beam in an underdense plasma. Part 1. The wave spectrum

The present series of papers is concerned with the problem of the thermal selffocusing of an electromagnetic wave in an underdense plasma which is examined within the context of ionospheric modification by high-power radio waves. We formulate this problem as a coherent four-wave interaction between a pump wave, Stokes side-band waves and a thermally driven density perturbation. Our treatment assumes that the heating of the electrons in the interference pattern of the pump and side-band waves is in equilibrium with longitudinal heat conduction and energy transfer to the ions and, in addition, that the density perturbation is subject to decay on account of longitudinal ambipolar diffusion, ambient plasma drift motion, recombination and the stabilizing influence of perturbations to the electrical conductivity.