General characteristics of hot plasma and energetic particles in the Saturnian magnetosphere: Results from the Voyager spacecraft

Abstract

The low energy charged particle (LECP) experiment on the Voyager 1 and 2 spacecraft made measurements of the intensity, energy spectra, and spatial distributions of ions (30 keV ≲ E ≲ 150 MeV) and electrons (22 keV ≲ E ≲ 20 MeV) during encounters with the Saturnian magnetosphere in November 1980 and August 1981, respectively. Detailed analysis of the data has revealed the following: (1) Energetic ions are present in the interplanetary medium both upstream (to ∼200 Rs) and off the dawn bow shock (to ∼400 Rs) of the magnetosphere, with maximum energies ∼100 keV. (2) Low‐energy (≳22 keV) electrons are generally depleted inward of L ∼ 10 Rs, while low‐energy (≳30 keV) ions are greatly enhanced in the same region. (3) The composition of low‐energy ions is most likely dominated by protons in the outer magnetosphere but is consistent with oxygen in the inner (L ≲ 9) magnetosphere. (4) The ion spectrum is described well by the κ distribution with characteristic temperatures kTH ranging from ∼15 to ∼55 keV; the hot plasma region is generally confined between the L shells of Tethys and Rhea but exhibits substantial variability. (5) The electron energy spectrum at L ≲ 10 develops a secondary peak at E ≳ 200 keV that shifts to higher (∼1 MeV) energies inside the orbits of Enceladus and Mimas, indicative of electron resonance interactions with the planetary satellites. (6) There is a noon‐dawn asymmetry in ion and electron intensities with peak fluxes near the Rhea‐Dione L shells at local morning; this is the region in local time where Saturn kilometric radiation is modulated by the presence of Dione. (7) The ion energy density (≳30 keV) represents a significant fraction of the field energy density in the outer magnetosphere of the planet (L ≳ 13 Rs), with values of β ranging from 0.1 up to ∼4, when projected to the equator. (8) Comparison of electron and ion intensities measured by Voyagers 1 and 2 in the inner (L ≲ 6) magnetosphere at common points in B, L space shows that the radiation belts are substantially stable over periods of ∼9 months; both ion and electron intensities compared well with Pioneer 11 observations in 1979. It is evident from the results that the inner satellites of Saturn play a dominant role in the determination of intensity and spectral features of energetic particles at L ≲ 10. These aspects of the data are discussed in the context of proposed physical mechanisms expected to be operating within the magnetosphere of Saturn.