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G. Black (NRAO), D. Campbell, P. Nicholson (Cornell), R. Sault (ATNF)
Three point-like extragalactic radio sources have been observed passing behind the rings of Saturn, and the varying attenuation of the background sources' intensities result in radial profiles of ring opacity. These events took place in September 1998, February 1999, and October 1999, with the opening angle of the rings varying from -16\circ to -20\circ. The first and last events were observed with the Very Large Array and the middle one with the Australian Telescope Compact Array. Collectively the occultation geometries probe all ring radii. All three events were observed at 20~cm wavelength with one event also observed at 90~cm wavelength, and hence they probe the distribution of ring particles with sizes larger than ~10~cm. The source fluxes at \lambda20~cm are 6~mJy, 30~mJy, and 25~mJy respectively, compared to the (extended) Saturn thermal emission of order 100~mJy. The relative weakness of the first source was compensated for by the slower apparent motion of Saturn at opposition which occurred near that event. Thermal emission from the rings was not detectable due to their extremely low emissivity at these wavelengths. The events were predicted by a search of point sources in the NRAO VLA Sky Survey catalog [Condon et al.\ (1998), AJ 115, 1693], and subsequent high resolution mapping confirmed their positions and compactness. For an unresolved source, our radial resolution in the rings is ultimately limited by the Fresnel zone diameter of \sqrt{2\lambda D} = 720\sqrt{\lambda/21\,{\rm cm}}~km. This work extends the radio occultation experiment carried out by the Voyager I spacecraft [Marouf et al.\ (1983), Icarus 54, 189], which determined opacity profiles of the rings at \lambda4~cm and \lambda13~cm. Observations of occultations of natural radio sources by Saturn at longer wavelengths provide a similar direct probe of the ring particle sizes in the dynamically-important 30~cm--10~m size range.
The author(s) of this abstract have provided an email address for comments about the abstract: gblack@nrao.edu