Relativistic electrons in Saturn's inner magnetosphere and an estimate of their synchrotron emission

Abstract

Energetic electron data obtained by the University of Iowa instrument during Pioneer 11's two‐way traversal of Saturn's inner magnetosphere in September 1979 are reviewed. There were substantial differences between inbound and outbound observations. It is argued that the inbound data are more likely to represent the time‐stationary state. Adopting these inbound data, we develop a quantitative model for the spatial and spectral distributions of relativistic electrons. The principal features of this model are as follows: (1) The radial dependence of omnidirectional intensity J in the equatorial plane is given by J = k′ exp(−1.05 x)[1 − exp(−6.5 x)] where k′ = 1.28 × 107 (cm² s)−1 and x = (r−2.30) with r the radial distance in units of the planet's radius 60,000 km. This distribution is applicable to the range 2.30 ≤ r ≤ 3.60. (2) The latitudinal dependence of J is derived from observed pitch‐angle distributions. These distributions are reasonably well represented by j(α) = j(90°)sin1.5 α (where j is the unidirectional intensity and α is the pitch angle), if one ignores the relatively small depletion of intensity near α = 90°, such depletion being significant in other contexts. (3) As shown previously, the energy spectrum is a relatively narrow one with characteristic energies E in MeV at various r as follows: 0.69 at 5.0; 1.10 at 4.0; 1.89 at 3.0; 2.62 at 2.5; and 3.05 at 2.30. The synchrotron emission of the entire population of relativistic electrons in Saturn's inner magnetosphere is estimated to be about 1 kW with a spectral maximum at 720 kHz. It is shown that this radiation will be very difficult, if not impossible, to observe, even in the near vicinity of the planet. The paper includes a crude but instructive explication of the truly enormous differences between the synchrotron emissions of the inner magnetospheres of Jupiter and Saturn. Indeed, this comparison was an underlying motive for undertaking the present study.