Electromagnetic angular momentum transport in Saturn's rings

Abstract

The observed ‘spokes’ in Saturn's rings have been interpreted as consisting of elevated, sub-micrometre sized dust particles¹. Arguments in favour of this interpretation are, for example, the photometric properties (spokes are dark in backscattered, bright in forward scattered light), the dynamics (approximate keplerian rotation) and lifetime (less than half an orbital period). We show here that submicrometre dust particles sporadically elevated above the ring are subject to electromagnetic forces which will reduce their angular momentum inside synchronous orbit and increase it outside. When the dust is reabsorbed by the ring the angular momentum of the ring is decreased (increased) inside (outside) of synchronous orbit. For the case of the spokes in Saturn's B-ring we estimate that the timescale for transporting ring material due to this angular momentum coupling effect is comparable to the viscous transport time or even smaller. We suggest that the minimum in the optical depth of the B-ring at synchronous orbit is due to this effect