Walker Fillius

Trapped radiation belts of Saturn: first look

Pioneer 11 has made the first exploration of the magnetosphere and trapped radiation belts of Saturn. Saturns magnetosphere is intermediate in size between Earths and Jupiters, with trapped particle intensities comparable to Earths. The outer region of Saturns magnetosphere contains lower energy radiation and is variable with time; the inner region contains higher energy particles. The pitch angle distributions show a remarkable variety of field-aligned and locally mirroring configurations. The moons and especially the rings of Saturn are effective absorbers of trapped particles; underneath the rings, the trapped radiation is completely absorbed. We confirm the discovery of a new ring, called the F ring, a new division, the Pioneer division, and a moon, called 1979 S 2. The latter has probably been seen from Earth. There may be evidence for more bodies like 1979 S 2, but at this stage the interpretation of the data is ambiguous. Using particle diffusion rates, we estimate that the cross-sectional area of the F ring is > 7 x 1013 square centimeters and that the opacity is > 10–5. Cosmic-ray albedo neutron decay should be looked into as a source of energetic particles in the inner magnetosphere of Saturn.

Cosmic ray gradients in the heliosphere

Abstract The spatial dependence of cosmic ray intensities in the heliosphere is slowly being revealed by intercomparisons among several spacecraft widely spread throughout interplanetary space. Up to the beginning of this year, important spacecraft included Helios 1 and 2 in to 0.3 AU, IMP 8 and ICE near 1 AU, Pioneer 11 out to 25 AU, Voyager 2 to 25 AU, Voyager 1 to 32 AU, and Pioneer 10 to 43 AU. The radial gradient has been studied most extensively, and recent determinations during the approach to solar minimum set its value from 1 to 3%/AU for integral gradients (above some threshold energy), with remarkable independence of the threshold. In the last couple of years the heliographic latitude spread of the spacecraft network has grown, with Voyager 1 climbing to 27°, Pioneer 11, 16°, Pioneer 10, 3° and Voyager 2, 0° degrees. In combination with a decline of the tilt angle of the heliospheric magnetic field, this has made it possible to obtain an unequivocal gradient with respect to heliographic latitude. When averaged over a solar rotation and expressed in %/AU, this value is near that of the radial gradient, but negative. No persistent longitudinal gradient has been identified. These spatial gradients can be variously extrapolated and interpreted to gain information regarding particle transport parameters, the predominant mechanism for modulation, and the scale size of the heliosphere.