In late September 2004, Cassini made a movie in search of ring spokes, but found none. Spokes comprise dust particles less than about 1 micron in size that collect electrostatic charges in the plasma environment of the rings, and become subject to electric and magnetic forces. In certain conditions, the particles become negatively charged and are levitated en masse 100 kilometres away from the surface of the rings for brief periods, forming dark features against the illuminated side of the rings and bright features against the shadowed side. However, the light-scattering geometry changes as a result of Saturn's axial tilt and orbital motion, with the formation of spokes being more likely when the Sun is near the ring plane. The spokes had last been seen on 18 October 1998 by the Hubble Space Telescope, when the angle of illumination was 15 degrees.175 But at the time of Cassini's arrival the rings were more 'open' to the Sun and conditions were less favourable.176
On 6-7 October, with Cassini inbound and 6 million kilometres from Saturn, the highspeed photometer element of the Ultraviolet Imaging Spectrograph sampled at 1,000 counts per second to observe an occultation of the 4th magnitude star Xi Ceti by the ring
system. Occultation measurements are particularly well-suited to the low to intermediate optical depth regions of the 'C' and 'F' rings and gaps such as the Cassini Division, but less so to the regions of higher optical depth. There were great variations in the distribution of the material. The particles in individual ringlets were 'bunched', with the distance between the presence or absence of material at the edge of some ringlets - especially in the 'C' ring - being as little as 50 metres. The sharp edges dramatically illustrated the dynamics that constrain the ring processes against the tendency of material to spread out to occupy nearby empty space.177 This was the first of 80 such observations to be made during the 4-year primary mission.
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