No. 21: May-Jun 1982
The strange dark radial spokes seen in Voyager's photos of Saturn's rings wax and wane with a period of about 621 minutes. This is very close to the rotation period of Saturn's magnetic field. Somehow, the rotating magnetic field interacts with the particles making up the rings, forcing density or reflectivity changes that we see as transitory spokes. It is not yet clear how this magnetic tune is played.
(Proco, C.C., and Danielson, G.E.; "The Periodic Variation of Spokes in Saturn's Rings," Eos, 63:156, 1982.)
Comment. Brightness changes have also been noted in the zodiacal light, which is light reflected from particles surrounding the sun. After reading about Saturn's variable spokes, one wonders whether solar-system magnetic fields might be the cause of these mysterious variations in the zodiacal light.