Microarcsecond Radio Imaging using Earth‐Orbit Synthesis

The observed interstellar scintillation pattern of an intra-day variableradio source is influenced by its source structure. If the velocity of theinterstellar medium responsible for the scattering is comparable to theearth's, the vector sum of these allows an observer to probe the scintillationpattern of a source in two dimensions and, in turn, to probe two-dimensionalsource structure on scales comparable to the angular scale of the scintillationpattern, typically $\sim 10 \mu$as for weak scattering. We review the theory onthe extraction of an ``image'' from the scintillation properties of a source,and show how earth's orbital motion changes a source's observed scintillationproperties during the course of a year. The imaging process, which we callEarth Orbit Synthesis, requires measurements of the statistical properties ofthe scintillations at epochs spread throughout the course of a year.Comment: ApJ in press. 25 pages, 7 fig