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P. D. Nicholson, D. B. Campbell (Cornell), R. G. French (Wellesley College), J-L. Margot (Caltech), G. J. Black (NRAO), M. Nolan (NAIC)
The first radar echoes from Saturn's rings were obtained at a wavelength of 12.6~cm by Goldstein and Morris (1973). In October 1999 we used a frequency-stepped technique similar to that used in the mid-70s by Ostro etal. (1982) to make the first true radar images of the rings. In November 2000 and again in December 2001 we repeated this experiment, using the Arecibo S-band radar. With a pulse length of 70~msec, the range resolution of these data is 10,000~km; the Doppler spectra were processed to a frequency resolution of 2~kHz, corresponding to a radial resolution at the ring ansae of 2000~km. To date we have obtained images at ring opening angles B of -19.9, -23.6 and -25.9~deg. Images from all three years show a pronounced azimuthal asymmetry in the ring reflectivity, which is seen in both circular polarizations. The analogous phenomenon at visual wavelengths is ascribed to gravitational `wakes' generated by individual large ring particles, which are distorted by keplerian shear into elongated structures trailing at angles of 70~deg from the radial direction (Franklin and Colombo 1978). Such wakes are diagnostic of the rings' gravitational stability parameter, Q, and are expected to have characteristic length scales of 30-100~m in the A ring. To the radar, the rings appear brighter when the wakes are seen sideways, and fainter when they are viewed end-on. Fits of a numerical model by Salo and Karjalainen (1999) to our data show that the asymmetry is concentrated in the A ring, where its amplitude is 25% of the average reflectivity. This is twice the model prediction --- which is based on a dynamical simulation employing a realistic ring particle size distribution used as input to a Monte Carlo light scattering code --- and about three times the amplitude measured in HST images obtained at a wavelength of 439~nm and the same opening angle. The large amplitude of the radar asymmetry is difficult to reproduce with current models, although the phase of the asymmetry is within 7~deg of that predicted by the models. This work is supported by NASA's PG&G program.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.