High‐energy trapped radiation penetrating the rings of Saturn

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Abstract
Electrons and protons in the energy ranges 2–25 MeV and >67 MeV, respectively, have been discovered throughout the entire equatorial region inward from the outer edge of the A ring at L=2.3 to the periapsis of the Pioneer trajectory at L ∼1.3. The trapped radiation which populates Saturn's magnetosphere beyond L=2.3 is totally absent in this region. The electron measurements include (1) a differential energy spectrum ∝ E−0.6, (2) an L dependence consistent with L2.8; and (3) an intensity of ∼0.05 el/cm² s sr near L=2 for the energy range 7–17 MeV (a factor of 5 times the interplanetary quiettime flux in this energy range). The proton measurements display an L dependence of L² with a flux level of ∼6 × 10−2 protons/cm² s sr above 67 MeV, just inside the edge of the A ring. The pitch angle distributions of both the electrons and protons are consistent with isotropy in the dipole magnetic field. It is argued from these results that the electrons and protons are trapped and thus penetrate the A‐B‐C rings. However, from the above experimental evidence it is concluded that this trapped radiation is not remnant radiation from the trapped radiation region beyond L=2.3. We find that these measurements are consistent with a model for splash albedo production of electrons and protons resulting from the bombardment of the atmosphere and/or rings of Saturn by cosmic ray protons with energies above the Störmer cut‐off at the magnetic latitudes of production. These secondary particles are then observed as trapped radiation propagating along the appropriate field lines crossing the ring plane. We also show that electron production may occur through π± → µ± → e± decay chain which yields an L dependence of L2.8 for pions. Our measurements and calculations are in reasonable agreement with extrapolations of electron albedo measurements from earth's upper atmosphere to the physical conditions at Saturn. We conclude that the absorption of the magnetospheric trapped radiation by Saturn's A, B, and C rings provides a set of physical conditions not present in other planetary magnetospheres for investigating the splash and reentrant albedo problems which can then be applied in the near‐earth space.