If flux density variability is intrinsic to extragalactic sources, then the shortest time scale of the variability yields an extremely high angular resolution. There is, however, good evidence that some of the variability is due to refractive scintillation in our galaxy's interstellar plasma turbulence. The effect, initially discovered at low radio frequencies, is here shown to extend into the GHz band, which, up to now, has been assumed to display only the intrinsic variability. We conclude that further study in both the intermediate and high frequency bands needed to separate the intrinsic and extrinsic components of variability.

Low frequency variability of extragalactic radio sources is commonly interpreted in terms of refractive interstellar scintillation. In order to study the effect of the Earth orbital motion through the scintillation pattern we have analysed 408 MHz variability data on 43 sources in a search for statistically significant one year periodicity which could be attributed to such a motion. This analysis has been carried out by calculating an average flux density structure function for all sources, as well as for subgroups based on galactic latitude, galactic longitude, ecliptic latitude. In all classes we find a clear characteristic time scale of about 0.5 years corresponding to a one year modulation in the time series. The statistical significance of this result is confirmed by Monte Carlo simulations of variable sources and calibrators. This result is also consistent with numerical simulations of both thin and thick screen scattering models based on the refractive interstellar scintillation theory. A significant constraint on the propagation velocity of the turbulent irregularities has been obtained (V(irr) < 10 km s-1), suggesting that the motion between the Earth and the scattering pattern is, on average, mainly determined by the motion of the Earth-Sun system with respect to the Local Standard of Rest and the Earth orbital motion around the Sun.

Recent data on the variability of extragalactic sources at decimeter wavelengths are interpreted in terms of the author's earlier hypothesis that scintillations in their radio emission are produced by irregular interstellar structures in the Galaxy: loops, spurs, ridges. The behavior of the scintillation index over a wide frequency range (0.325--2.7 GHz) can be explained by the theory of scintillation at large-scale irregularities taking place in the weak-focusing regime at the boundary of the strong-focus zone. Corrections are necessary for the finite angular size of the source (caused by the interstellar scattering) or for self-absorption of the scintillating component at the low-frequency limit of the band observed.