Mitchell, C., Porco, C., Dones, L., Spitale, J. (2011). "The Behavior of Spokes in Saturn's B Ring" American Astronomical Society, DDA meeting #42 #8.07.


Abstract
We analyse the motion of spoke edges as observed in consecutive images taken by the Cassini spacecraft on timescales of tens of minutes, measuring the displacement of their edges and their shear rate, as well as their radial growth. We examine the edges of 28 spokes which either newly form on the morning portion of the rings, or which grow radially after leaving Saturn's shadow. Of these 28 spokes, 21 exhibit normal shearing behavior, with one edge undergoing Kepler shear while the other sheared more slowly. The other 7 exhibited more complex behavior, including 4 which sheared at different rates at different parts of the spoke and 2 which appeared to be sheared when they formed. We find that spoke radial growth at either terminus is typically less than 1 km/s. If one assumes the edge motion is due to the movement of individual particles, their charge to mass ratios would correspond to electrical potentials of up to approximately -1 kV for a one micron particle.

Additionally, we calculate as a function of time the spoke activity level, defined as the area-integrated optical depth of the spokes, and search for periodicities as were previously seen in Voyager data (Porco and Danielson, 1982) as well as correlations with magnetic longitude. We find that spoke activity on the northern side of the rings has a periodicity similar to the northern Saturn Kilometric Radiation (SKR)source (Kurth et al. 2008, JGR) and is confined in longitude. The southern-side spoke activity may vary with two periods: one equal to that of the southern SKR source and one equal to that of the northern SKR source.