Fine light and dark radial markings discovered in Voyager images revolve around the planet along with the rings. Informally termed "spokes," these structures are seen as dark markings in reflected light, yet turn to bright streaks when lit from the rear. The spokes are probably formed by extremely small dust particles held in place by electrostatic forces.
Voyager 2 high resolution view of spokes in the B-Ring.
NASA Press Release #81-H-589
Bright spokes in the B ring as viewed by the Cassini orbiter.