Geometrical control of the ratio of intensity and phase scintillation indices

Abstract

Early observations of complex-signal scintillation revealed a sizeable difference in the ratio of the intensity scintillation index, S 4, to the phase scintillation index, σ φ , when measured at mid-latitude, auroral-zone, and equatorial stations. The differences observed between auroral and equatorial stations receiving a beacon signal from a polar-orbiting satellite have now been found to persist, for non-saturated values of S 4. The phase-screen theory for production of scintillation is employed in this paper to identify four factors that control the S 4 σ φ ratio, and the behavior of two of them is explored for models describing both sheetlike and rodlike plasma-density irregularities. The analysis permits rejection of the effects of static diffraction by field-aligned irregularities (whether rodlike or sheetlike) as the factor controlling the different behaviours observed. It is shown, on the other hand, that geometrical control of the effective outer scale imposed by detrend filtering in the presence of highly anisotropic irregularities can readily explain the observed behavior. Such effects arise in virtually all experiments and radio systems operating in the presence of a power-law spectrum with a very large outer scale, and they are important in controlling the relationship between phase and intensity scintillation.