Noncircular features in Saturn\u27s rings I: The edge of the B ring

A comprehensive investigation of all available radio and stellar occultation data for the outer edge of Saturn\u27s B ring, spanning the period 1980-2010, confirms that the m = 2 distortion due to the strong Mimas 2:1 inner Lindblad resonance circulates slowly relative to Mimas in a prograde direction, with a frequency Omega(L) = 0.1819 degrees d(-1). Our best-fitting model implies that the radial amplitude of this distortion ranges from a minimum of 3 km to a maximum of 71 km, with short-lived minima recurring every 5.42 yrs. In addition to the dominant m = 2 pattern, the edge of the B ring also exhibits at least four other perturbations. An m = 1 component with a radial amplitude of similar to 20 km rotates at a rate very close to the expected local apsidal precession rate of (pi) over dot(B) similar to 5.059 degrees d(-1), while smaller perturbations are seen with in = 3 (amplitude 12.5 km), in = 4 (5.9 km), and m = 5 (5.6 km), each of which has a pattern speed consistent with that expected for a spontaneously-generated normal mode (French, R.G. et al. [1988]. Icarus 73, 349-378). Our results for m = 1, in = 2 and m = 3 are compatible with those obtained by Spitale and Porco (Spitale, J.N., Porco, C.C. [2010]. Astron. J. 140, 1747-1757), which were based on Cassini imaging data. The pattern speed of each normal mode slightly exceeds that expected at the mean edge radius, supporting their conclusion that they may represent a series of free modes, each of which is trapped in a narrow region between the mode\u27s resonant radius and the ring\u27s edge. However, both our model and that of Spitale and Porco fail to provide complete descriptions of this surprisingly complex feature, with post-fit root-mean-square residuals of 8 km considerably exceeding typical measurement errors of 1 km or less. (C) 2013 Elsevier Inc. All rights reserved