Spoke formation under moving plasma clouds

Goertz and Morfill [Goertz, C.K., Morfill, G., 1988. Icarus 53, 219–229] propose that spokes on Saturn's rings form under radially moving plasma clouds produced by meteoroid impacts. We demonstrate that the speed at which a plasma cloud can move relative to the ring material is bounded from above by the difference between the Keplerian and corotation velocities. The radial orientation of new spokes requires radial speeds that are at least an order of magnitude larger than this upper limit, thus the model advanced by Goertz and Morfill fails to make radial spokes.