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Title:
Where did the Spokes go?
Authors:
Mitchell, C. J.; Horanyi, M.
Affiliation:
AA(Laboratory for Atmospheric and Space Physics, and Department of Physics, University of Colorado, Boulder, CO 80309-0392 United States ; ), AB(Laboratory for Atmospheric and Space Physics, and Department of Physics, University of Colorado, Boulder, CO 80309-0392 United States ; )
Publication:
American Geophysical Union, Spring Meeting 2005, abstract #P21C-07
Publication Date:
05/2005
Origin:
AGU
AGU Keywords:
6213 Dust, 6265 Planetary rings, 6275 Saturn
Bibliographic Code:
2005AGUSM.P21C..07M

Abstract

The intermittently appearing bright (in forward scattered light) or dark (in backscattered light) radial features above Saturn's B ring ("spokes") were discovered by the Voyager imaging experiment. These features represented a major challenge to our understanding of celestial mechanics in planetary rings. It was clear early on that, in addition to gravity, electromagnetic forces must play a role in the formation and evolution of spokes. Because of the competing theories about spoke formation, Cassini was scheduled to make a large number of dedicated observations. However, contrary to our high expectations no spokes to date have been observed by Cassini. Initially this was thought to be due to the geometry of the observations, however by now it is clear that the spokes are indeed absent in the current epoch. In this talk we argue that the absence of spokes is due to a seasonal modulation of the plasma environment in the rings. The photoelectron density above the rings is determined by solar irradiance, hence the elevation angle of the Sun. Following the argument of Nitter et al. (1998), we use simple physical model to follow the trajectory and the charge of small grains lofted from the rings into the plasma sheath. These calculations verify that the probability of a small grain rising above the photoelectron sheath is a function of the plasma density and, hence the solar elevation angle. Based on this seasonal variation of the spoke activity we expect that spokes will reappear towards the end of Cassini's main mission in 2008.
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