Dones, L., Mitchell, C.J., Weiss, J.W., Porco, C.C. (2009). "Light Scattering Models of Spokes in Saturn's B Ring Observed by Cassini ISS " American Astronomical Society, DDA meeting #40, #24.01.


Abstract
Spokes - narrow, fast-evolving, nearly radial features in Saturn's B Ring - have been seen by Voyager, the Hubble Space Telescope, and Cassini, but most prominently when the solar elevation angle (B') above the rings has been small. From their observed photometric contrast, spoke particles are found to have a narrow size distribution, with radii ~ 0.6 microns (Doyle & Grun 1990; McGhee et al. 2004). These microscopic particles are believed to be levitated above the ring by electrostatic forces. Both the variations in spoke activity with B' and the size of the spoke particles can be explained by a model of particle charging in the plasma environment near the rings (Mitchell et al. 2006).

Cassini arrived at Saturn in 2004, near the time of maximum solar elevation angle. The extended "Cassini Equinox Mission" (CEM) began in July 2008. As we approach equinox in August 2009, spoke activity is increasing. Here we present data from five sets of spoke images taken during the CEM between August 21 and November 25, 2008, at |B'| = 4-5o and a range of spacecraft elevation angles. In order to determine how spoke activity - the area-integrated optical depth of the spokes (Porco and Danielson 1982) - varies with B' and ring longitude, we are performing light-scattering calculations for the geometries of the spoke images. Our models include both cases in which the thickness of the layer containing the spoke particles is much larger than, or is the same as, the layer containing the larger ring particles. We use both a classical "doubling" code and a ray-tracing code that can treat physically thin and clumpy rings (Porco et al. 2008). We will present our light-scattering results and the implications for models of spoke formation.