35th Meeting of the AAS Division on Dynamical Astronomy, April 2004
Session 9 Satellites \& Rings
Oral, Friday, April 23, 2004, 2:20-5:35pm,

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[9.06] Radar Observations of Wakes in Saturn's Rings.

P.D. Nicholson, D.B. Campbell (Cornell Univ.), R.G. French (Wellesley College), H.J. Salo (Univ. of Oulu, Finland)

Starting in 1999, annual delay-Doppler images of Saturn's rings have been made using the Arecibo S-band (12.6 cm wavelength) planetary radar facility. With a frequency resolution corresponding to a radial resolution at the ring ansae of 2000~km, we easily resolve the classical A, B and C rings. To date we have measured the radar cross-section and depolarization ratio of the A and B rings at ring opening angles |B| = 20.1, 23.5, 25.8 and 26.7~deg. No echoes have been detected from the C ring or the Cassini Division. Images from all four years in both circular polarizations show a pronounced m=2 azimuthal asymmetry in the reflectivity of the A ring. The analogous phenomenon at visual wavelengths is ascribed to gravitational `wakes' generated by individual large ring particles or arising from internal instabilities, 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 expected to have characteristic wavelengths of 30 to 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. When compared with a numerical model fitted successfully to Voyager and HST data (Salo et al 2004; French et al 2004) the phase of the radar asymmetry is found to match that of the model to within a few degrees, whereas the amplitude is found to be larger by almost a factor of two. The model is based on a local dynamical simulation employing a realistic ring particle size and elasticity used as input to a Monte Carlo light scattering code (Salo et al. 1995; 2003). Further experiments indicate that the unexpectedly large radar asymmetry amplitude may be due to the strongly forward-scattering phase function of meter-size ice particles at radio wavelengths, which enhances the sensitivity to optical depth variations. This work was supported by NASA's Planetary Geology and Geophysics program.

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Bulletin of the American Astronomical Society, 36 #2
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