Mitchell, C., Porco, C., Dones, L., Spitale, J. (2010). "The Temporal Variability of Spokes in Saturn's B Ring" American Astronomical Society, DPS meeting #42, 19.05.


Abstract
We report on the variability of the spoke activity in Saturn's B ring as observed in Cassini ISS images. Two sets of observations are analyzed. The first consists of 6 movies of the southern side of the rings prior to Saturn's northern vernal equinox on August 11, 2009 while the second consists of 13 movies of the northern side taken after equinox. Both are of the illuminated side of the rings. First, we find photometric evidence that the particles comprising the spokes are irregular in shape. Next, power spectral analysis of the northern-side spoke observations produces a distinct peak at a period of 637 +/-9 minutes, very similar to the Cassini-era period for the Saturn Kilometric Radiation (SKR) arising from the planet's northern auroral zone (Gurnett et al. 2009, GRL). Furthermore, histograms of northern-side spoke activity versus longitude in the coordinate system defined by the Cassini SKR indicates that the longitudinal sector in which spoke activity reaches a maximum coincides with the northern SKR-active sector at the time of our observations, as was found during the Voyager era (Porco and Danielson 1982; 1984).

Though southern-side spoke observations were too sparse to yield a period using spectral analysis, histograms of southern spoke activity, created using an array of periods between 620 minutes and 660 minutes, indicate that southern-side spoke activity may vary with two periods: one equal to that of the southern SKR source (Kurth et al. 2006, JGR) and one equal to that of the northern SKR source. Interestingly, the northern SKR source was most active during the time of our southern-side spoke observations, perhaps explaining the presence of both periods in the variability of spokes on the southern side of the rings.