If you've seen one magnetosphere, you haven't seen them all: Energetic particle observations in the Saturn magnetosphere

Abstract

The Goddard Space Flight Center/University of New Hampshire Pioneer 11 Cosmic ray experiment, with its ability to measure precisely proton and electron energy spectra and angular distribution and to identify heavier ions, was especially well suited for studying Saturn's magnetosphere. Pioneer 11 passed through the subsolar and dawn magnetospheres while a solar cosmic ray event was in progress and the solar wind pressure was enhanced. Energetic electrons (>0.16 MeV) and protons (E >0.2 MeV) were identified as major magnetospheric constituents, along with trace amounts of alpha particles (>0.65 MeV/nuc) and possible heavier nuclei. Three distinct regions were found inside the magnetopause: (1) an outer magnetosphere between 7.5 Saturn radii (Rs) and the magnetopause at 17.3 Rs inbound and 30 to 40 Rs outbound, (2) a slot region between 4 and 7.5 Rs which is dominated by the absorption due to three of Saturn's satellites, and (3) an inner region with an intense proton and high‐energy electron flux which stops abruptly at the outer edge of the A ring. The population in the outer magnetosphere consisted of solar cosmic rays (protons and alpha particles) and trapped protons and electrons. The access of solar cosmic rays to at least 9 Rs is interpreted in terms of an open magnetotail configuration. The phase space density shows that the trapped particles were diffusing inward, and the alpha‐to‐proton ratio at 0.65 MeV/nuc suggests that most of the protons must be of Saturn origin. The outer magnetosphere was found to be variable, presumably in response to changes in solar wind conditions. This variability was reflected not only in the particle fluxes, but more dramatically in changes of their pitch angle distributions. In the slot region the electron and proton fluxes are reduced by amounts that are strongly energy dependent. On the inbound pass the reduction factor for protons varies from 186±46 at 0.5 MeV to 39±5 at 2.1 MeV. Geometric absorption by the moons Dione, Tethys and Enceladus plays a major role, but additional processes may be necessary to explain the energy dependence. One possibility is energy losses in the material of the previously identified E ring. In the middle of the slot region (∼4.5 Rs), there commences a significant enhancement of 1 to 20 MeV protons which may be a precursor to the inner region. Electrons (0.75–1.6 MeV) whose gradient and curvature drift resonate with the satellite periods can diffuse past these moons and show no strong attenuation. In the inner magnetosphere, the total electron flux shows little change but becomes harder between 4 and 2.5 Rs. This is consistent with inward diffusion of the truncated electron population. The proton population shows deep absorption features at Mimas, 1979 S‐2 (Janus?), and the outer edge of the F ring. The phase space density of protons with constant first invariant demonstrates that there is a source of energetic protons in the inner region. Impulsive injection of trapped particles past the inner moons and cosmic ray albedo neutrons must both contribute to the inner region. From the present data it is difficult to determine the relative importance of the two processes.