The evolution of Saturn's radiation belts modulated by changes in radial diffusion

Globally magnetized planets, such as the Earth(1) and Saturn(2), are surrounded by radiation belts of protons and electrons with kinetic energies well into the million electronvolt range. The Earth's proton belt is supplied locally from galactic cosmic rays interacting with the atmosphere(3), as well as from slow inward radial transport(4). Its intensity shows a relationship with the solar cycle(4,5) and abrupt dropouts due to geomagnetic storms(6,7). Saturn's proton belts are simpler than the Earth's because cosmic rays are the principal source of energetic protons(8) with virtually no contribution from inward transport, and these belts can therefore act as a prototype to understand more complex radiation belts. However, the time dependence of Saturn's proton belts had not been observed over sufficiently long timescales to test the driving mechanisms unambiguously. Here we analyse the evolution of Saturn's proton belts over a solar cycle using in-situ measurements from the Cassini Saturn orbiter and a numerical model. We find that the intensity in Saturn's proton radiation belts usually rises over time, interrupted by periods that last over a year for which the intensity is gradually dropping. These observations are inconsistent with predictions based on a modulation in the cosmic-ray source, as could be expected(4,9) based on the evolution of the Earth's proton belts. We demonstrate that Saturn's intensity dropouts result instead from losses due to abrupt changes in magnetospheric radial diffusion.