Nuclear cascades in Saturn's rings: Cosmic ray albedo neutron decay and origins of trapped protons in the inner magnetosphere

Abstract

The nearly equatorial trajectory of the Pioneer 11 spacecraft through Saturn's high energy proton radiation belts and under the main A‐B‐C rings provided a unique opportunity to study the radial dependence of the >30 MeV proton intensities in the belts in terms of models for secondary nucleon production by cosmic ray interactions in the rings, in situ proton injection in the radiation belts by neutron beta decay, magnetospheric diffusion, and absorption by planetary rings and satellites. Maximum trapped proton intensities measured by Pioneer 11 in the radiation belts are compared with calculated intensities and found consistent with trapping times ≈40 years and a radial diffusion coefficient DLL ≈ 10−15 L9 RS²/s. Differential energy spectra J(E) ∝ E−2 estimated from integral measurements of trapped protons with E > 10² MeV are consistent with the beta decay model, but an inferred turndown of the spectra toward lower energies and reported integral proton anisotropies of the form J(αeq) ∝ sin4–6 αeq both indicate the need for more realistic calculations of the neutron source from the rings and the radiation belt loss processes.

Mapping Magnetospheric Equatorial Regions at Saturn from Cassini Prime Mission Observations

“…Since ions and electrons are efficiently absorbed by ring particles (the term "ring particle" is used to refer to dust or larger particles comprising Saturn's rings), the energetic particle flux is very small over the main rings and is dominated by a very low flux of energetic ions produced by cosmic ray impacts (Cooper 1983). Therefore, the plasma-induced decomposition and sputtering rates are both very small.…”

Energetic Ion and Electron Irradiation of the Icy Galilean Satellites

“…11a for 100 MeV protons reaching centimeter depths. Energy deposition at meter depths would also arise from secondary neutrons produced by nuclear interactions of >100 MeV magnetospheric and cosmic ray protons in the water ice (Cooper 1983, Cooper et al 1985. The relatively low energy fluxes of deeply penetrating particles make these sources insignificant for the present work at optical depths but become important at regolith depths ≤10 1 m (Section 8.3).…”

Saturn Plasma Sources and Associated Transport Processes

“…The small fraction of those that will beta-decay within the strong dipole region, will populate the radiation belts with energetic electrons (mostly below 1 MeV) and protons. This mechanism was discussed by several authors (Cooper and Simpson, 1980;Fillius and McIlwain, 1980;Van Allen et al, 1980;Blake et al, 1983;Cooper, 1983;Cooper et al, 1985;Randall et al, 1994) and Figure 29 illustrates it. Blake et al (1983) proved theoretically that the high-energy component of the radiation belts originates from CRAND.…”