The application of the double weighted Fourier transform (DWFT) method is suggested for extracting the linear integral of the electron density in the inhomogeneous plasma from microwave or IR sounding data. The virtue of DWFT is its ability to localize the measured linear integral in the area, narrow as compared with the radius of the Fresnel zone, that is to realize super-Fresnel resolution. The advantages of the DWFT method are illustrated, firstly, by numerical simulation of a wave propagation through the Gaussian inhomogeneity in conditions of weak scattering, when a small angle Born approximation is applicable, and secondly, by theoretical estimates of resolution in conditions of strong scattering. The method under discussion promises to be helpful for studying the fine structure of turbulent plasma.
Abstract. New integral representation of a wave field ,in a continuously inhomogeneous random medium is suggested in the form of double-weighted Fourier transformation, performed simultaneously with respect to coordinates of the source and the observer. The integral representation under consideration takes into account both the diffraction effects and the multiray effects. It incorporates many results of known techniques of wave propagation description in continuously inhomogeneous media: the methods of geometrical optics, smooth perturbations, phase screen, and two-scale expansions. The method delivers new opportunities to retrieve small-scale inhomogeneous structure of ionosphere plasma from radio-sounding data and can serve as the basis for diffraction tomography of the ionosphere.
In considering HF propagation in a random inhomogeneous ionosphere it is necessary to take into account regular and random caustics. Regular caustics connected with regular refraction of radio waves in the ionosphere form a skip zone and determine the maximum usable frequency (MUF) and the maximum of the oblique incidence backscatter sounding (OBS) signal. Random fluctuations of ionospheric radio rays “wash out” field enhancement in the vicinity of MUF and the maximum OBS signal. The presence of random caustics results in strong intensity fluctuations of ionospheric radio waves. The above mentioned problems are considered by the interference integral method. A scintillation index formula for strong fluctuations of oblique ionospheric radio waves is obtained. Average intensity and average pulse signal which form in the vicinity of MUF are investigated. The peculiarities of oblique multihop radio wave propagation, taking into account random ionospheric inhomogeneities, terrestrial surface roughness, and caustics focusing in skip distance are discussed.
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