Evolution of the frequency spectrum of an extraordinary wave near the upper hybrid resonance in a turbulent plasma

A study is made of the formation of the frequency spectrum of an extraordinary wave during its multiple small-angle scatterings along the path to the upper hybrid resonance, in the upper hybrid resonance region, and behind the conversion point. The formation of the spectrum is investigated both approximately (by the eikonal method) and exactly (in the limit of large-scale plasma density fluctuations responsible for small-angle scattering). It is demonstrated that these two approaches yield the same results in the common range of their applicability. It is shown that, in the vicinity of the upper hybrid resonance, the broadening of the frequency spectrum of a probing wave is proportional to its wavenumber. This circumstance and the predicted amount by which the spectrum broadens make it possible to consider small-angle scattering as one of the main effects responsible for a very large spectrum broadening observed in experiments.