overdense plasma heating. Underdense ionospheric heating corresponds to electromagnetic radiation with frequency greater than the ionosphere's maximum plasma frequency, so that the radio wave propagates through the ionosphere. The ionospheric critical frequency is typically 3 to 10 MHz; therefore, the ionosphere is underdense for the SPS microwave beam. Overdense heating occurs when the radiowave frequency is smaller than the ionosphere’s critical frequency, so that the wave resonates with the local plasma and in fact is reflected. Past HF experiments were almost entirely of this kind. Before further discussing thermal self-focusing, it is appropriate to mention some equivalent terminologies that can be found in the open literature. Interactions very similar to thermal self-focusing have also been referred to as thermal diffusive scattering, thermally-driven stimulated Brillouin scattering, or collisionally-coupled, purely-growing, parametric instability in ionospheric modification theories. In nonlinear optics, it is called stimulated Rayleigh scattering; as applied to laser technology, it is described as beam filamentation. The effect of the self-focusing instability, which we call ionospheric irregularities or plasma striations, is frequently referred to as “hot spots” in laser-plasma coupling, “solitons,” in astrophysical and controlled fusion research, and “cavitons” in some laboratory plasma studies. Nevertheless, despite subtle differences, the physics of self-focusing is essentially the same for each of these applications. Thermal self-focusing does not have an absolute threshold in the usual sense. The
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