Mitchell, C.J. and Porco, C.C. (2006). "New Spokes Observed in Saturn's B ring." Eos Trans. AGU 87(52), Fall Meet. Suppl., Abstract P23E-0107.


The spokes in Saturn's B ring, which have been absent since the Hubble Space Telescope last imaged them in 1998, were finally observed by the Cassini spacecraft in September 2005. Additional spokes were sighted immediately after Cassini's orbit left the ringplane in mid-2006. The current inventory of spoke observations by Cassini consists of 5 sets of images taken of both lit and unlit sides of the rings. The spokes have contrasts in the range from 0.02 to 0.2 and exhibit varying degrees of Kepler shear. The 2005 spoke sightings are the highest resolution observations we have obtained thus far. In that sequence, the spokes appeared in 3 consecutive Cassini ISS images spanning 27 minutes taken of the dark (north) side of the rings. They are approximately 6000 km in length, 1000 km wide and have contrast on the order of 20%. They appear nearly radial in the first image, indicating that they are likely quite young, and are located azimuthally within 30 degrees of the center of the magnetic field sector which gives rise to the peak Saturn Kilometric Radiation (SKR) as observed by Cassini (Kurth et al., Geophys. Res. Let., submitted, 2006), indicating a relationship to the magnetic field consistent with Voyager results (Porco and Danielson, Astron. J., 1982). Our kinematic analysis of the 2005 spokes shows that their motions deviate slightly from Keplerian. We show that this is consistent with the planet's electric and magnetic fields acting on the negatively charged dust particles which comprise the spokes. Some theories of spoke formation suggest that spokes will become visible with greater frequency during Cassini's present high orbital inclination sequence which began in late July 2006. We will present any additional observations of the spokes taken between the time of writing and the AGU meeting.