Interstellar Scattering Disks. Dr. Gwinn will carry out a radio wavelength observational study of the shapes of scattering disks in the interstellar medium and of their temporal variations. The observations include Very Long Baseline Interferometry (VLBI) measurements of the sizes of the scattering disks of pulsars over periods of years; sensitive VLBI observations of the scattering disks of hydroxyl masers; and a search for speckles in the scattering disk of the Vela pulsar. Radio wavelength scattering in the interstellar plasma limits the resolution and positional accuracy of observations of many radio sources and will become more important as a limit as VLBI baselines are extended into space and the accuracy of astrometric VLBI approaches the microarcsecond level. Large scale plasma fluctuations, acting as lenses, can focus and defocus radiation from pulsars and quasars, producing the observed low frequency variability of these sources. Dr. Gwinn's research is directed toward improving our understanding of the properties of the scattering plasma.
