Relativistic electron acceleration at Saturn and Jupiter by global scale electric fields

Electrons in Saturn's radiation belts are distributed along discrete energy bands, a feature often attributed to the energisation of charged particles following their rapid injection towards a planet's inner magnetosphere. However, the mechanism that could deliver electrons deep into Saturn's radiation belts remains elusive, as for instance, the efficiency of magnetospheric interchange injections drops rapidly for electrons above 100 keV and at low L-shells. Using Cassini measurements and simulations we demonstrate that the banding derives from slow radial plasma flows associated to a persistent convection pattern in Saturn's magnetosphere (noon to midnight electric field), making the need for rapid injections obsolete. This transport mode impacts electron acceleration throughout most the planet's radiation belts and at quasi and fully relativistic energies, suggesting that this global scale electric field is ultimately responsible for the bulk of the highest energy electrons near the planet. We also present evidence from Galileo and Juno that the influence of Jupiter's inner magnetospheric convection pattern on its radiation belts is fundamentally similar to Saturn's but affects its higher energy ultra-relativistic electrons. The comparison of the two radiation belts indicates there is an energy range above which there is a transition from interchange to global scale electric field driven electron acceleration. This transiroty energy range can be scaled by the two planets' magnetic moment and strength of corotation, allowing us to study these two systems in complement.