A radiation belt of energetic protons located between Saturn and its rings

INTRODUCTION Most magnetized planets are known to possess radiation belts, where high-energy charged particles are trapped in large numbers. The possibility that a radiation belt could exist also in the confined region between Saturn and its main rings has been proposed on the basis of remote sensing observations and simulations. It was not until the final 5 months of the Cassini mission that in situ measurements were obtained from this region with the Magnetosphere Imaging Instrument (MIMI). This paper provides an overview of these measurements and their interpretation. RATIONALE Saturn’s main rings prevent the inward transport of trapped charged particles in the magnetosphere. Material from the outer radiation belts cannot directly access the low-altitude region within the rings. The isolation of this region allows the study of energetic particle source and loss processes because it is only indirectly coupled to the dynamics of the rest of the magnetosphere. Potential sources include cosmic ray albedo neutron decay (CRAND) and multiple-charge exchange, whereas losses are likely dominated by energy deposition and scattering of trapped particles by dust and atmospheric neutrals. All of these mechanisms involve charged particle interactions with materials in space, meaning that MIMI measurements can provide information to probe the material itself—particularly the tenuous D-ring, the innermost component of Saturn’s main rings, which is difficult to constrain by remote sensing observations. RESULTS We observed an inner radiation belt extending between 1.03 and 1.22 Saturn radii (1 R S = 60,268 km) at the equatorial plane, dominated by protons with energies from 25 MeV up to the giga–electron volt range. This belt is limited by the atmosphere at its inner edge and by the D73 ringlet (at 1.22 R S ), a component of the D-ring, at its outer boundary. Another ringlet (D68 at 1.12 R S ) splits the trapped particle population in two. The outer sector overlaps with the extended D-ring, and its intensity is reduced compared with that of the inner sector, owing to proton losses on ring dust. The proton angular distributions are highly anisotropic with fluxes that are orders of magnitude higher near the magnetic equator compared with fluxes of particles that can reach high latitudes. No time variability could be discerned in the >25-MeV proton population over the 5-month period of the observations. Trapping of lower-energy (tens of kilo–electron volt) protons was clearly observed in at least one case by imaging the emission of energetic neutral atoms (ENAs) coming from below ~1.06 R S (altitude CONCLUSION The radial profile, the stability of the >25-MeV proton fluxes, and the lack of heavy ions are features consistent with a radiation belt originating from CRAND. The strong anisotropy of the proton distributions is primarily the result of proton losses in collisions with atmospheric neutrals, though an anisotropy in the production of CRAND protons from Saturn’s rings may also contribute. The low-altitude, kilo–electron volt proton population is transient and derives from charge stripping of planetward ENAs, which are generated at the variable magnetospheric ring current.