Absorption of energetic protons by Saturn's ring G

Abstract

A restudy has been made of Pioneer 11 data on the distribution of energetic protons Ep > 80 MeV in Saturn's inner magnetosphere. An improved value of the ratio of the cosmic ray albedo neutron decay source strength $ to the radial diffusion coefficient D is 6.9 × 10−24 cm−5 at r ∼ 2.67 Rs (1 Rs = Saturn's equatorial radius = 60,000 km). Using the recently calculated lower limit on $ by Blake et al., one finds an upper limit on the mean residence time T against diffusion of 4.6 × 108 s (15 years) in the major peak of the distribution, whereas using our earlier estimate of $, one finds T ∼ 2.2 × 108 s (7.0 years). The two corresponding determinations of D are >1.3 × 10−11 and ∼ 2.8 × 10−11 Rs² s−1, respectively. A generous upper limit on D is 2 × 10−10 Rs² s−1 as found from study of the slot region associated with the co‐orbiting satellites 1979 S2 (1980 S3) and 1980 S1. The mean lifetime τ of energetic protons against absorption by particulate matter in Ring G is < 1.1 × 108 s (3.5 years) and ∼ 5.1 × 107 s (1.6 years) corresponding to the two choices of $. Then, using the Voyager data on normal optical opacity η ∼ 3 × 10−5 and on radial width Δr ∼ 500 km, one finds that the particulates in Ring G have an effective radius R ≳ 0.035 cm, an areal mass density σ > 1.4 × 10−6 g cm−2, and an areal number density n < 8 × 10−3 cm−2. It is unlikely that R exceeds 0.1 cm. The foregoing values of R (for assumed spherical particulates of water ice, all of the same size) are in effect the ratio 〈R³〉/〈R²〉 for a distribution of sizes, where the angle brackets denote the mean value over the distribution. It is reasonably certain that there are no satellites having radii of the order of a kilometer or larger associated with Ring G and that objects having radii of ≳10 cm contribute less than 10−2 of its opacity. The mean radius of Ring G is 170,200±900 km.