diffracting screen for this reason is referred to as the phase changing screen. The amplitude fluctuations will develop as the wave propagates away from the screen. The diffraction method employs Rayleigh-Sommerfeld diffraction theory to calculate the behavior of the amplitude and phase of the wave as a function of distance from the phase screen. Analytic solutions have been obtained to the diffraction problem usually under the assumption that the rms phase deviation imposed on the incident wavefront by the screen does not exceed one radian. This is the shallow-screen (weak-scatter) approximation. Under this approximation, the amplitude and phase fluctuations correspond respectively to the in-phase and quadrature-phase fluctuating components (relative to the mean undeviated component) of the wave field. The two components become approximately equal when the observations are made in the far-field of the diffracting screen. In the scattering approach, the wave at the observing point is considered to be the sum of the unscattered wave and the waves scattered by the irregularities in the medium. This method can also be successfully applied in treating a case of weak scattering where the effects of multiple scattering may be neglected. It was shown that both the diffraction and the scattering methods are equally valid and provide identical results. The model computations for the statistical description of the wave field presented in Section 3 are based upon the diffraction method. In the following, the discussion will therefore be limited to this method. A. 2 MODEL FOR ELECTRON DENSITY IRREGULARITIES It is necessary to specify a model for the electron density irregularities in the ionosphere in order that the fluctuations in the radio wave field can be derived using the diffraction theory. A simple model which is often used considers the irregularities to be field aligned, axially symmetric and randomly distributed. They are described in terms of a Gaussian spatial correlation function defined such that
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