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A. S. Bosh, C. B. Olkin (Lowell Obs.)
Saturn's F ring is inclined to the mean ring plane by 0.0067 +/- 0.0006 deg, as determined through analysis of stellar occultation data (Olkin et al., in prep). Because the F ring is only ~3500 km exterior to the outer edge of the A ring, during ring-plane crossings the F ring can be eclipsed or occulted by the main rings. Where these eclipses or occultations occur is determined by the location of the node, which regresses due to Saturn's non-spherical gravity field.
During the November 1995 solar crossing, the F ring was eclipsed by the main rings on the west ansa. This phenomenon was visible in HST WFPC2 images taken before and after the stellar occultation. During the May 1995 and February 1996 earth crossings, the line of nodes pointed almost directly at the Earth. Therefore, at crossing the portion of the F ring that was occulted was not visible as it was behind the planet; before crossing part of the F ring on the west ansa, and after crossing part on the east ansa were occulted. During the August 1995 earth crossing the line of nodes was angled away from Earth; at the time of crossing the F ring was occulted on the west ansa. Prior to crossing a section on the west ansa was occulted, and after crossing sections of both ansae were occulted. This changing appearance at each earth crossing may be related to the observed crossing time differences for the east/west ansae during the August event (Nicholson et al., 1996, Science 272, 509), while no such difference was observed in May (Bosh et al. 1997, Icarus 129, 555); photometric modeling of the crossings is necessary to determine if this is the dominant effect (Nicholson et al., 1999 DDA meeting). This result affects observations of possible F ring clumps; if these bodies are in the F ring, they too would be occulted. The predicted occulted regions correspond well with null detections of 1995 S5 by Roddier et al. (submitted).