Stability of negatively charged dust grains in Saturn's ring plane

Abstract

The boundary between the inner and outer parts of Saturn' s B ring is located at the theoretical limit of stability of dust grains with large negative charge to mass ratio. A grain inside of this stability limit will move along (pseudo) magnetic field lines and strike Saturn if given a slight velocity component normal to the ring plane. Outside of this marginal stability radius, a perturbed grain merely oscillates back and forth through the ring plane. The theoretical location of the marginal stability radius is at 1.625 Rs. Observations by Pioneer 11 and Voyager 2 in the infared see the boundary as a prominent change in ring brightness at this radius (within the spatial resolution). The occultation of &Scorpii by the rings in the ultraviolet seen by Voyager 2 shows about a factor of 2 change in optical depth beginning very close to this radius. This close agreement is conceivably a numerical coincidence. Whether this is the case, or whether the instability is actually the physical reason for the boundary's existence, requires further study of the implications.

Nonkeplerian dust dynamics at Saturn

“…This theory extended previous work [Northrop and Hill, 1982 Corresponding contour plots of a,(p,z) for dielectric grains based on eqs (9-11) and encompassing the region above and below the E-Ring are shown in Fig. 6 (a) for prograde and 6 (b) for retrograde orbits.…”

Photochemistry of Saturn's Atmosphere II. Effects of an Influx of External Oxygen

“…Enhanced erosion of the rings (and enhanced influx into the atmosphere) would be expected within these stability limits. Because Saturn's magnetic field is aligned with the planet's rotation axis, different latitude regions on Saturn can be mapped back to corresponding regions in the ring plane (e.g., Northrop and Hill 1982, 1983, Connerney 1986. As Connerney (1986) suggests, certain dark latitude bands on Saturn may correspond to regions of instability within the ring system; those planetary latitudes may be experiencing an enhanced water influx, helping to clear stratospheric aerosols from the atmosphere.…”

“…As we will discuss in Section 6 (below), an enhanced flux of oxygen-bearing ions or very tiny charged ice grains from Saturn's rings may be entering Saturn's atmosphere at latitudes where the magnetic field lines map back to certain instability regions within the ring plane (e.g., Connerney 1986, Northrop and Hill 1982, 1983. To simulate this effect, we now examine the consequences of an enhanced external water flux on the photochemistry of Saturn's stratosphere (the influx of all other oxygen compounds is kept the same magnitude as in our nominal model).…”

“…Theoretical studies of the stability of ions or very small charged grains in Saturn's ring plane (Ip 1983, Northrop and Hill 1982, 1983) predict that such particles located within a certain stability distance from Saturn can spiral along magnetic field lines to directly impact Saturn's atmosphere. Enhanced erosion of the rings (and enhanced influx into the atmosphere) would be expected within these stability limits.…”

Latitudinal variation of Saturn photochemistry deduced from spatially-resolved ultraviolet spectra

“…Hence, they interpreted the sharp difference in surface mass density between the C and B rings as indirect evidence for the occurrence of this electromagnetic erosion mechanism. Earlier work by Northrop and Hill (1982) and Ip (1983Ip ( , 1984 explored the stability of charged dust or ions in Saturn's equator plane for dipole field geometries. Then, Connerney (1986) identified dark bands in the atmosphere of the planet magnetically conjugated with the inner edge of the B ring and proposed they were the results of a locally enhanced source of water.…”