Detecting collisions and dust in Saturn's F ring from diffraction signatures in Cassini UVIS solar occultation data

We analyzed 11 solar occultations by Saturn's F ring observed by the Ultraviolet Imaging Spectrograph (UVIS) on Cassini. During four of these occultations we detected an unambiguous signal from diffracted sunlight. This signal appears as additional photon counts above the unocculted solar signal just before or after the occultation occurred. We compared the UVIS data with images of the F ring obtained by the Cassini Imaging Science Subsystem (ISS) and found that the ISS images showed collisional activity at each of the F ring longitudes where the diffraction signatures were detected. Furthermore, the ISS images revealed a quiescent region of the F ring at locations where diffraction signatures were conclusively not detected in the UVIS data. The strongest example of this correlation occurred in 2007, when UVIS serendipitously observed an occultation by a region of the F ring that had suffered a large collisional event several months prior (Murray, C.D. et al. [2008]. Nature 453, 739-744). The observation resulted in the detection of one of the most prominent diffraction signatures. We applied a forward model of the solar occultations to reproduce the observed UVIS light curves, varying the particle size distribution, optical depth, and radial width of the F ring. We found that the models that best reproduced the observations with a diffraction signature required a smaller effective particle size (aeff < 300 microns), while light curves without a diffraction signature were best reproduced using models with a larger effective particle size (aeff > 400 microns). We report that the diffraction signatures detected in the UVIS occultation data are indicative of recent collisional activity in the F ring, resulting in the release of a population of smaller (< 50 microns) ring particles.