Tiscareno, M.S., Nicholson, P. D., Burns, J. A., Hedman, M. M., Porco, C. C. (2006). "Density Wave Metamorphosis." American Astronomical Society, DPS meeting #38, #38.07.

Spiral density waves raised in Saturn's rings by the co-orbital satellites, Janus and Epimetheus, are observed to have an unusual and complex structure, making their interpretation difficult. Furthermore, unlike most of the rings' spiral density waves, which rotate as a static pattern, the morphology of co-orbital waves changes on a timescale of months to years.
Janus and Epimetheus are in a mutual horseshoe libration, as they orbit Saturn, one satellite is slightly inward of the other, but they effectively switch places every 4 years. In most regions of the main rings, the rate at which waves carry information (the group velocity) is low enough that density waves propagate over a time that is comparable to the period of the satellites' libration.
We propose that the complex co-orbital density wave morphology can be explained by the superposition of multiple wave segments, starting and stopping at different locations as the perturbation from the satellites changes, and propagating outwards with group velocities as low as 6 km/yr. We will compare the results of our linear model against Cassini observations, and make predictions of future wave morphology.