Frequency dependence of radio star scintillations

The amplitude scintillations of the radio flux from Cygnus and Cassiopeia have been studied at discrete frequencies in the VHF-UHF range (30, 63, 113, 228, and 400 Mhz) using a 150-foot and an 84-foot diameter steerable paraboloids, both independently and in an interferometer mode. Data at various combinations of these frequencies have been collected over a period of 3½ years and analyzed to establish the frequency dependence of ionospheric scintillations. For weak scintillations (the higher frequencies during periods of low magnetic activity) the scintillation index is found to be inversely proportional to (frequency)². For lower frequencies and high levels of ionospheric activity, the dependence approaches 1/ƒ. These results agree with predictions based upon a simplified theory of weak scattering in the medium and assuming various combinations of irregularity size and height of the disturbing layer. Measurements made with the two antennas in an interferometer configuration established that the diffraction field was truly random as assumed. Correlation coefficients were obtained for discrete sets of data and a distinction made between weak and strong scattering cases. Irregularity sizes of about 1–2 km were found to be consistent with the bulk of the observations, on the basis of weak scattering theory.