  The co-orbital satellites of Saturn, Janus and Epimetheus, swap radial
positions every 4.0 years. Since \textit{Cassini} has been in orbit about
Saturn, this has occurred on 21 January in 2006, 2010, and 2014. We describe
the effects of this radial migration in the Lindblad resonance locations of
Janus within the rings. When the swap occurs such that Janus moves towards
Saturn and Epimetheus away, nonlinear interference between now-relocated
density waves launches a solitary wave that travels through the rings with a
velocity approximately twice that of the local spiral density wave group
velocity in the A ring and commensurate with the spiral density wave group
velocity in the B ring.
