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Abstract

The L shell distributions of MeV electrons in Saturn's radiation belt are investigated orbit by orbit for the 13‐year exploration of Cassini. It is found that in addition to the monotonic decrease profiles, there are orbits showing superimposed transient extensions. The extensions are found to stand out above the background population during 50% of belt crossings. We estimate that there is high probability (>72%) that transient extensions contribute more than 10% electron content in the radiation belt. The high occurrence frequency of one extension every 2–3 weeks, together with the relative content, demonstrate that the extensions constitute a regular and fundamental process populating and sustaining the electron belts of Saturn. The transients regularity excludes interplanetary coronal mass ejections as dominant trigger and implies corotation interaction regions and/or internal processes as candidates. Statistical results suggest that the communication of electrons between the middle magnetosphere and the radiation belts is largely through convective radial transport, which produces transient radiation belt extensions.