Milliarcsecond Structure of Microarcsecond Sources

We present the key findings of an ongoing program of Very Long Baseline Interferometry (VLBI) observations of scintillating and non-scintillating flat- spectrum extragalactic radio sources whose scintillation status was determined by the Micro-Arcsecond Scintillation-Induced Variability (MASIV) survey. Both single frequency (8.4 GHz) observations to compare morphologies, and multi-frequency observations (from 330 MHz through 22 GHz) to study scattering behaviour have been carried out. Though scintillation probes the stru cture of quasars on scales of tens of microarcseconds, information at the milliarcsecond scale probed by these VLBI observations is proving invaluable in furthering our understanding of th is phenomenon. Also, the common occurence of scintillation in compact radio sources might be the ultimate limitation for VLBI as- trometry (which among other things defines the Internationa l Celestial Reference Frame) in the sense that the final precision of reference frame source posi tions are likely to be influenced by the presence of scintillation.