Five stellar occultations by Neptune: Further observations of ring arcs

Stellar occultations on 18 April 1984 and 20 August 1985 have yielded additional observations of incomplete rings, or "arcs," around Neptune. The latter represents only the second unambiguous observation of a Neptunian arc to be confirmed by observations at more than one telescope, and thus strongly corroborates the discovery of the first of this class of features by Hubbard et al. (1986, Nature 319, 636). Diffraction model fits to the observed light curves indicate radial widths widths for the arcs of 8-26 km, and normal optical depths of 0.07-0.14, on the assumption that the arcs lie in Neptune's equatorial plane. The 20 August 1985 arc shows a lower optical depth "shoulder" at its inner edge, reminiscent of the Uranian δ ring. The observation of 18 April 1984 shows three distinct features, within a radial range of 180 km. Using the Neptune pole direction determined from Voyager navigation analyses, we confirm that the arc observed on 20 August 1985, together with two previously observed arcs (Hubbard et al., 1986, Nature 319, 636; Covault et al. 1986, Icarus 67, 126), appear to match in radius the outermost ring (1989N1R) observed in Voyager images (Smith et al. 1989, Science 246, 1422). The 18 April 1984 observation does not match any narrow Voyager feature, but does fall within the 6000-km-wide zone denoted 1989N4R by Smith et al. (1989). A combination of the stellar occultation data and data from Voyager imaging observations of the 1989N1R arcs leads to a refined value for the arc mean motion of 820.1185 deg/day, corresponding to a semi-major axis of 62932.3 km.