Mitchell, C., Porco, C.C., Dones, L. (2009). "The Behavior of Spokes in Saturn's B Ring" American Astronomical Society, DPS meeting #41, #25.09.


Abstract
We report on the analysis and modelling of the transient, quasi-radial features called spokes observed in Saturn's B ring by the Cassini ISS.

We analyze the kinematics and activity level, defined as the area integrated optical depth, of the spokes in images taken at regular intervals of the morning ansa of Saturn's B ring, where spokes were most frequently observed by Voyager.

For each image, we extract azimuthal brightness profiles through the spokes, each at a fixed radius, to which we simultaneously fit a linear combination of functions describing the local background brightness of the rings and the brightness profile of the spokes. We find that a gaussian function is sufficient for fitting most spokes. We calculate the activity level of each spoke by first converting I/F into optical depth using a doubling code to treat the spoke as an optically thin layer of dust above the ring plane. The fits are then area-integrated to produce the individual spoke's activity.

We find that most observed spokes are either increasing or decreasing in activity. Using the gaussian fits to determine the locations of the spokes' central peaks and edges in multiple time-lapse images, we find that those spokes with a decreasing activity move at the normal Keplerian rate for uncharged bodies, while spokes with increasing optical depth sometimes exhibit deviations toward corotation as well as radial spreading. We use these motions to calculate an average charge-to-mass ratio of the particles comprising the spokes. Finally, we check for correlations of activity with magnetic longitude as seen during the Voyager flybys, by using the latest magnetic coordinate system based on SKR emissions, SLSIII.