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C. Ferrari, S. Charnoz, A. Brahic, P. Thebault (CEA Saclay)
Located at the Roche limit of Saturn, the F ring is very intriguing : its apparent complex radial and azimuthal structure (multiple strands, clumps, braids) proves a complex dynamical evolution. We have studied the evolution of the azimuthal distribution of structures with Voyager 1 and 2 images and with 1995 Saturn RPCs images made with CFHT and UHAO adaptive optics system. Voyager 2 images in 1981 show that the brightest azimuthal structures are located on slightly different orbits (Da~90±15 km). Their azimuthal distribution is then expected to significantly evolve over few months. This was not detected in Voyager 1 images nine months earlier. The lifetime of these structures is at least 15 days. A new mean motion of the ring is estimated to be 140212 km ± 15 km, as an average of mean motions measured at Voyager 1 and Voyager 2 epochs. This value is consistent with former estimate by Synnott et al. (1983) of 140175 km ± 32 km with fewer Voyager 2 images. We have reported the discovery of several new objects close to the F ring during August 1995 Earth RPC (Roddier et al. 1996, Ferrari et al. 1997). We present here the detection of four new elongated structures three months later, on November 20th 1995 during the Sun RPC. They have been seen by HST the day after. Combining both data sets provides an estimated orbit for these structures of 140050 km ± 60 km. They are certainly F ring azimuthal structures. The mean orbit of the F ring might have moved then by 150 km towards Saturn compared to Voyager epoch. The azimuthal distribution has probably evolved over the three months. Only Cassini spacecraft observations will definitely constrain lifetime and dynamical evolution of these structures on shorter timescales.
The author(s) of this abstract have provided an email address for comments about the abstract: ferrari@discovery.saclay.cea.fr