Trapped and Leaking Energetic Particles in Injection Flux Tubes of Saturn's Magnetosphere

Abstract

In Saturn's magnetosphere, the radially-inward transport of magnetic fluxes is usually carried by localized flux tubes with sharply-enhanced equatorial magnetic fields. The flux tubes also bring energetic particles inward, which are expected to drift azimuthally and produce energy-dispersive signatures. Spacecraft observations, however, indicate the occurrence of energy-dispersionless signatures for perpendicular-moving particles. These unexpected features are attributed to the sharp magnetic gradient at the edge of the flux tubes, which significantly modifies the drift trajectories of perpendicular-moving particles to enable their trapping motion within the flux tubes. The bouncing particles are less affected by the gradient, and therefore, still display energy-dispersive signatures. It is the distinct particle behavior, together with different spacecraft traversal paths, that underlies the observational diversity. The results improve our understanding of particle dynamics in the magnetospheres of giant planets and indicate that pitch-angle information should be considered in the extraction of flux-tube properties from energetic particle observations.