Radio scintillations at large elongation angles: Contributions of waves versus discontinuities

Formulas for the phase fluctuation of an electromagnetic wave moving though a random time dependent medium, which generalize existing theory by including retardation effects due to the finite time of propagation of radio signals are presented. Formulas to compute the phase correlation in a complete round trip Doppler link between Earth and a spacecraft are given. Formulas for statistics of phase fluctuations are separately applied to the case of discontinuities and two MHD waves, with reasonable assumption on their wavelength spectra. Numerical results are shown, and the role of waves versus discontinuities in the correlation spectrum of phase fluctuation is discussed as a function of elongation angle and of frequency.