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abstract
Extreme Scattering Events (ESEs) are a recently discovered form of extreme variation in the gigahertz light curves of a number of extragalactic sources. These variations cannot be explained by current models of intrinsic variability or normal refractive interstellar scintillation. ESEs may be characterized as a sequence of smooth flux density variations consisting of a minimum surrounded by symmetric maxima. These variations have no resemblance to flux density variations normally observed in a given source. Fiedler et al. (1994, ApJ, 430, 581) present a simple model for ESEs based on diffraction or refraction through a Galactic region of high electron density turbulence which has an angular extent comparable to that of the background source. We present multiple epoch, multiple frequency VLBI observations of nine extragalactic sources whose light curves have exhibited ESEs. A comparison between the milliarcsecond scale intrinsic structure of these sources and their ESE radio light curves is made in the context of the Fiedler et al. model.
