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Cassini discovered Saturn's innermost radiation belt during the end of its mission. The belt is populated with relativistic protons, probably up to the trapping limit of approximate to 20 GeV. It extends from Saturn's dense atmosphere into and throughout the D-ring. The A-C rings separate this belt entirely from the previously known radiation belts, suggesting that the innermost radiation belt is populated entirely via cosmic ray albedo neutron decay. We find that the proton pitch angle distributions are consistent with being shaped by losses to the D-ring and the upper atmosphere rather than, for example, wave-particle interactions. This supports that the main loss process of this new radiation belt is energy loss in neutral material, different from Saturn's other radiation belts. This property constrains the overall scale height of Saturn's exosphere to <700 km and the average D-ring water molecule column density to being about 1 order of magnitude below the Enceladus gas torus.

Plain Language Summary A fundamental property that a planet with a magnetic field can have is if it is encompassed by radiation belts of energetic ions and electrons approaching light speed. It was the first discovery of the space age that this is the case for Earth. For Saturn, the Cassini satellite recently discovered an unknown radiation belt trapped between the planet and its rings. The physics of this radiation belt is as different to Saturn's previously known radiation belts, as Saturn's belts differ from Earth's. Here we seek the reason why the proton intensities in this new belt do not rise to extremely high values. We find that this is because the densities of Saturn's high atmosphere and inner rings are sufficiently high to deplete the protons as fast as they are produced.