The behavior of spokes in Saturn’s B ring

We present the first results from Cassini ISS observations aimed at determining both the short-term and long-term behavior of the spokes in Saturn’s B ring. We have observed multiple spokes which appear between images where there was not a spoke before. The radial termini of these new spokes expand radially at ∼0.5 km/s in both the inward and outward directions and both towards and away from corotation. Defining a spoke’s activity as the area-integrated optical depth over the region of the ring it covers, we find that the majority of spokes which are found in multiple images are either increasing or decreasing in activity. In addition, in analyzing the shapes and motions of spokes, we find the azimuthal profiles to be well-fitted by Gaussians for the majority of spokes. We have found that several of the imaged spokes were undergoing an “active” phase during which the spoke’s optical depth increases and the spoke grows both azimuthally and radially. We interpret these motions to be either due to the Lorentz force acting on dust grains charged by a very high temperature plasma or the group velocity of an advancing spoke-forming front. The spokes’ light scattering behavior suggests that the particles comprising them are not spherical but instead are irregularly shaped. rch for periodicities in the longer-term temporal variability of spoke activity, and for correlations between spoke activity and other processes ongoing in Saturn’s magnetosphere and atmosphere. Using the latest results on the periods and locations of the sources of Saturn’s Kilometric Radiation (SKR) obtained by the Cassini Radio and Plasma Wave experiment (RPWS) (Gurnett, D.A., Groene, J.B., Averkamp, J.B., Kurth, W.S., Ye, S.-Y., Fischer, G. [2011]. Planet. Solar Heliosph. Radio Emis. PRE VII, 51–64), we find that spoke activity observed on both sides of Saturn’s rings occurs with a period equal to, within all uncertainties, the period of the SKR emissions arising from the northern SKR source, though a period equal to that of the southern SKR source seems also to be present. The period for of the Saturn Electrostatic Discharges (SEDs) falls within the uncertainty of the spoke activity on the northern side of the rings. However, spokes have been observed during SED-quiescent periods, which strongly suggests that there is no connection between the two phenomena (Fischer, G., Gurnett, D.A., Kurth, W.S., Zarka, P., Moore, L., Dyudina, U.A., 2011. J. Geophys. Res.). y, we use the latest SLS4 coordinate systems and locate the peak spoke activity on both sides of the rings at ≈200° in both the northern and southern SLS4 coordinate systems on both sides of the rings. This peak is ≈100° away from the locations expected from Voyager observations in both coordinate systems.