Saturn's Spokes: Lost and Found
C. J. Mitchell,1
M. Horányi,1*
O. Havnes,2
C. C. Porco3
The spokes are intermittently appearing radial markings in Saturn's
B ring that are believed to form when micrometer-sized dust
particles are levitated above the ring by electrostatic forces.
First observed by the Voyagers, the spokes disappeared from
October 1998 until September 2005, when the Cassini spacecraft
saw them reappear. The trajectories of the charged dust particles
comprising the spokes depend critically on the background plasma
density above the rings, which is a function of the solar elevation
angle. Because the rings are more open to the Sun now than when
Voyager flew by, the charging environment above the rings has
prevented the formation of spokes until very recently. We show
that this notable effect is capable of stopping spoke formation
entirely and restricting the size of the particles in the spokes.
2 Department of Physics, The Auroral Observatory, University of Tromsø, 9000 Tromsø, Norway.
3 Cassini Imaging Central Laboratory for Operations, Space Science Institute, Boulder, CO 80301, USA.
* To whom correspondence should be addressed. E-mail: horanyi{at}colorado.edu
