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Description
We report a detailed statistical study of the intensity variations in 412 extragalactic radio sources randomly selected from the ~2000 sources which have been observed with the Culgoora circular array at 80 and 160 MHz during the interval 1970-84. Our results are: (i) About 47 per cent of the sample displays significant variability on time scales of months or years. (ii) A similar degree of variability is shown by QSOs, radio galaxies and sources in blank optical fields. (iii) Sources with flatter radio spectra are more variable. (iv) There is a weak increase in variability for the lower-galactic-latitude sources. (v) There is a correlation between month-to-month variability and year-to year variability in the same sources. (vi) The degree of variability at 80MHz is related to that at 160MHz. (vii) Most of the sources in both the variable and non-variable categories have a component with angular diameter <1arcsec. Our results are partly consistent with an origin in refractive scintillation by large-scale electron density turbulence in the intervening medium. There are, however, serious difficulties in explaining, from the existing theory of slow galactic scintillation, the short time-scale of the scintillation. Our results are probably more in accord with the focusing and occulting effects of compact interstellar ionized structures recently identified at microwave frequencies.
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