Interplanetary Scintillations. I. Theory
The scintillations of small-diameter radio sources over time periods of the order of 1 sec are thought to be caused by electron-density fluctuations in the interplanetary medium, carried along by the solar wind. Some aspects of the theory had been presented by previous authors, all in the "thin phase screen" approximation, but not for all possible regimes. This theory is now extended to the regime where the rms phase fluctuations are large and distances to the screen are comparable with typical "focal lengths" Both the spatial autocorrelation function for intensity and also the distribution of intensities are discussed for all regimes. A simple formula is derived for the effect of a finite angular source size on the frequency spectrum of the scintillation autocorrelation function for any regime. Sufficient conditions for the validity of the "thin phase screen" approximation are derived The cross-correlation between scintillations at different radio frequencies is discussed. A lower limit is derived for the distance from the irregularities to Earth.