ICARUS 60, 409-415 (1984) Evidence for Material between Saturn's A and F Rings from the Voyager 2 Photopolarimeter Experiment A. L. GRAPS, A. L. LANE, AND L. J. HORN Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 AND K. E. SIMMONS Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80309 Received January 6, 1984; revised July 6, 1984 The region in the Saturn system between the F ring and the outer edge of the A ring is an area that appears, in images from the imaging experiment, to be virtually devoid of material except for three small satellites. Near the orbit of 1980S28, Atlas--the innermost satellite--the Voyager Photopo- larimeter Stellar Occultation data show a discontinuity in count rate which marks a boundary between the tenuous material near the outer edge of the A ring and the orbit of Atlas. The data pertaining to this region have been examined with the aid of statistics and models generated from other similar ring structures. It is concluded that the discontinuity is real, implying the existence of tenuous material of normal optical depth of 0.01 to 0.006 in this region. © 1984Academic Press.Inc. INTRODUCTION The Voyager 1 and 2 Saturn encounters revealed the Saturn ring system as a dy- namic and complex structure. Many theo- ries have successfully explained the more obvious features in the rings, such as the resonant forcing of ring particles by Sat- urn's moons to generate gaps and excite spiral density waves (Goldreich and Tre- maine, 1978; Cuzzi et al., 1981). The less obvious features are still being found in the immense data sets resulting from the Saturn encounters. The Voyager 2 Photopolarimeter Stellar Occultation Experiment provided a single continuous cut through the entire ring sys- tem with an extremely fine radial resolu- tion. It is from this data set that we report our findings on the region between the A and the F rings. DESCRIPTION OF STELLAR OCCULTATION EXPERIMENT The Voyager 2 Stellar Occultation Ex- periment took place on August 25-26, 1981 409 UT, during Voyager 2's closest approach to Saturn. Observations of the BOV-type star, 6-Scorpii, were made by the Photopolarim- eter Subsystem (PPS) as the star was oc- culted by Saturn's rings. The PPS moni- tored the starlight as it emerged from behind Saturn's darkened disk and traveled through the shadowed portion of the D, C, B, A, and F rings. Data were acquired usinog the PPS 1 ° circular aperture, the 2640-A filter (300 A full width at half-maximum), and the 45° linear polarizer (Lillie et al., 1977). The instrument recorded the star brightness and thus the ring opacity at 10- msec intervals, with successive data points separated by a radial distance of approxi- mately I00 m (Lane et al., 1982). The PPS instrument has a very fast f/l optical system but suffers from poor rejec- tion of light emanating from off the viewing axis (light from Saturn itself and nonsha- dowed rings, in this case) (Esposito et al., 1983). Since this off-axis light comes from a large region, and thus changes very slowly, we could make a reasonable estimate of this source and remove its contribution. (This 0019-1035/84 $3.00 Copyright© 1984 by Academic Press, Inc. All rightsof reproduction in any formreserved.