Two‐component proton spectra in the inner Saturnian magnetosphere

The Low Energy Charged Particle (LECP) experiment onboard the Voyager‐2 spacecraft performed extensive measurements of energetic (∼28 keV ≲ E p ≲ 150 MeV; ∼ 22 keV ≲ E e ≲ 20 MeV) protons and electrons during spacecraft passage through the inner magnetosphere of Saturn in August 1981. Analysis of the measurements inside the orbits of Enceladus and Mimas reveals the presence of a two‐component proton spectrum consisting of a low‐energy population at ≲ 500 keV identified for the first time and described by dj/dE = KE −2.5 , and a high‐energy part at E ≳ 16 MeV whose spectral form is similar to that expected from cosmic ray neutron albedo decay (CRAND), as previously inferred by the Pioneer‐11 investigators from integral measurements. In addition, there exist large fluxes of high (≳ 1.5 MeV) and low (22 to 35 keV) energy electrons (∼3.7 × 10 5 and ∼1.1 × 10 5 cm −2 sec −1 sr −1 , respectively) throughout this region. The pitch angle distribution of low energy protons changes from a simple sin 2/3 α dependence to one where intensities near 90° become depleted as the spacecraft approached the planet’s ring plane at ∼ 2.88 R S . It is likely that the pitch angle depletion is due to proton interactions with the tenuous G‐ring. These observations also resolve a number of uncertainties regarding the presence of energetic protons and electrons inside the orbits of Enceladus and Mimas, arising from the analysis of Pioneer‐11 measurements.