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- Title
Energetic Radiation From Galactic Cosmic Ray Interactions With Saturn's Main Rings.
- Authors
Cooper, John F.; Sturner, Steven J.
- Abstract
Saturn's main rings have an energetic particle and gamma ray photon radiation environment produced by ring interactions of galactic cosmic ray (GCR) protons and heavier ions penetrating the planetary dipolar magnetic field. Accurate models of this radiation environment are important for interpretation of Pioneer 11 and Cassini in situ measurements near the rings and for constraints on radiolytic contributions to neutral gas production and ice chemistry. A GEANT (GEometry ANd Tracking) based simulation is used to model flux spectra of protons, electrons, positrons, charged pions, neutrons, and gamma ray photons emitted from GCR interactions with H2O ice spheres approximating the ring material. Dependent on location in the A to D rings within the planetary magnetic field of Saturn, only GCR protons above respective energies of 20 to 72 GeV can reach the rings without being deflected away by the magnetic field. Calculated differential and integral fluxes from our simulations have good agreement with in situ Pioneer‐11 measurements in selected energy channels. The charged particle and neutral radiation measurements are sensitive, respectively, to the sizes and areal mass densities of ring bodies. Computed gamma ray emission fluxes are 8% of our calculated limit for detection from the Earth by the Fermi Large Area Telescope. Addition of charged particle sensors and neutron‐photon imaging spectrometers to a future Saturn Ring Observer mission would provide valuable information on the ring mass structure. The present paper provides a foundation for modeling of Pioneer 11 and Cassini radiation measurements across the main rings and future measurements of radiation from the rings. Plain Language Summary: An invisible bath of GCRs continuously bombards Saturn's main rings and generates showers of secondary charged particles, neutrons, and gamma rays. The Pioneer 11 spacecraft first detected these emissions during traversal underneath the A‐B‐C rings during its 1979 flythrough of the Saturn system. The Cassini spacecraft later traversed the same region during its 2004 orbital insertion at Saturn and in 2017 also crossed the innermost ring, the D ring, during its final Grand Finale orbits prior to final atmospheric entry. Modeling of these energetic radiation emissions can provide lower limits on the total mass of the rings, on the production of neutral gases from chemical products of the ring interactions, and on the background radiation responses of the instruments to better define Pioneer 11 and Cassini measurements. The modeling confirms the earlier Pioneer 11 result that the A‐B rings are heavily populated by meter‐sized bodies of ice and that there should be a significant rate of production for neutral gases such as oxygen and carbon dioxide. Understanding of the Pioneer 11 and Cassini measurements of the radiation emissions from the main rings could be improved by usage of the computed radiation flux spectra for instrument response simulations. Key Points: The energetic radiation environment of Saturn's rings is dominated by products of galactic cosmic ray interactions with the ring materialFlux spectra of secondary emissions from various ring body sizes and positions have been modeled for radiation environment specificationWe confirm the determination that meter‐sized bodies dominate the total ring mass but cannot yet limit the total mass
- Subjects
GAMMA rays; PHOTON emission; GALACTIC cosmic rays; MAGNETIC fields; RINGS of Saturn
- Publication
Journal of Geophysical Research. Space Physics, 2018, Vol 123, Issue 9, p7473
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2018JA025583