Backscatter of electromagnetic radiation from a turbulent plasma

Abstract

Any attempt at a quantitative description of the scattering of electromagnetic radiation from a turbulent plasma medium is bound to involve physical and mathematical approximations. The appropriateness of alternative approximation schemes is governed by a variety of parameters which characterize the radiation, the scattering geometry, and the structure of the plasma, e.g. the radar wavelength and angle of incidence, the mean electron density, the correlation function for electron density fluctuations, etc. A semiquantitative classification of the various regimes is presented here, and corresponding approximation schemes appropriate for each are described and assessed. For a sufficiently ‘underdense’ plasma a natural first estimate is represented by the Born approximation for scattering from turbulent eddies. In this approximation the scattering is a volume effect, independent of geometry. But the limits on the validity of the Born approximation, and the nature of the next-order corrections, are determined by geometry, multiple scattering, and other effects which are discussed at some length. At the opposite extreme, for a sufficiently ‘overdense’ plasma, the scattering is produced by surface fluctuations. This regime is discussed in some detail, and parameterization schemes are described. Throughout the entire discussion we concern ourselves only with the case of net radar scattering through very large angles, in fact, with the cross section for backscattering. Collision damping is ignored. For every regime particular attention is devoted to the dependence of the backscatter cross section on the angle of incidence of radar beam with respect to the surface of the plasma medium.