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[33.04] HST Observations of Spokes in Saturn's B Ring

C. A. McGhee, R. G. French (Wellesley College)

Spokes, the transient streaks on Saturn's B ring, were revealed during the Voyager flybys. Their photometric properties have been interpreted in terms of small dust particles that become charged and interact with Saturn's magnetic field and plasma environment. From an examination of seven spokes in the Voyager images, Doyle and Grün (1990, Icarus 85, 168--190) concluded that the dust particles were approximately 0.6 \mum in radius with a fairly narrow size distribution. As part of a long--term study of Saturn's rings using WFPC2 as well as archival HST data, we have measured the photometric properties of spokes visible in a substantial number of images, particularly at low ring tilt angle. The high photometric accuracy of WFPC2 and extensive spectral coverage ranging from 255 to 973 nm have enabled us to extend Doyle and Grün's analysis over a much broader range of wavelengths than the blue end of the spectrum to which the Voyager cameras were sensitive. We searched for spokes in over 300 HST images from 1994 to 2002, and detected over 20 spokes in the full set of observations. They are visible on both east and west ansae, though they are generally more prominent on the east (or ``morning") ansa (cf. Porco and Danielson 1982, Astron. J. 87, 826--833). We follow the technique of Doyle and Grün (1990), where we model Mie particle extinction efficiency using a Hansen--Hovenier particle size distribution with an effective radius r_{\rm eff} and a size distribution variance b. To date, we have performed photometry on four spokes to determine the extinction efficiency Q_{\rm ext} as a function of size parameter 2\pi{r}/\lambda. >From our fits to the color dependence of the spoke contrast, which sample both the rising and falling slopes of the Mie scattering peak, we are able to determine tight constraints on both r_{\rm eff} and b. We obtain r_{\rm eff}~0.51-0.56\ {\mu}m and variance b~ 0.1-0.15. This strongly reinforces the conclusion that the particles comprising the spokes are very fine dust with a relatively narrow size range, an important observational constraint on any model of spoke formation. This work was supported in part by NASA PG&G grant NAG5--10197 and STSCI grant GO--08660.01A.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.