One of the curious discoveries made by the Voyager spacecraft was the presence of dark radial features that move in curious patterns on the B ring. These features have been named spokes due to their likeness of spokes on a wheel.
This picture shows a 35 minute sequence (top to bottom) of the change in spoke appearence. It also shows the swift formation of a new, radially aligned spoke (see arrow in the bottom image).
Spokes are thought to be microscopic dust particles that have levitated away from the ring plane. They appear on both sides, near the densest part of the B ring. They rotate at the same rate as Saturn's magnetic field indicating they are affected by electromagnetic forces. They form about 104,000 kilometers from Saturn's center and extend out to the Cassini division. They are shapped somewhat like an hour glass with the narrow centers located near the synchronous orbit radius. Perhaps they are formed by meteorites that punch through the rings, by tiny moonlets as they plunge through the rings, or dust particles that have taken on a charge and levitated off the larger ring bodies