First Observation of Electron Rolling‐Pin Distribution in Jupiter's Magnetosphere.

The electron rolling‐pin distribution, showing electron pitch angles primarily at 0°, 90°, and 180°, has been widely studied in the Earth's magnetosphere, but has never been reported in other planetary environments. Here, by utilizing the Jupiter Near‐polar Orbiter (Juno) measurements, we report for the first time the electron rolling‐pin distribution in Jupiter's magnetosphere. We reveal the energy range of such distribution and find it appears only above 19.5 keV, falling well into the suprathermal energy range. Moreover, we quantitively reproduce the formation processes of such distribution by using an analytical model. Gratifyingly, the distribution derived from the analytical model agrees well with the Juno observations, indicating such distribution is formed by the combination of global‐scale Fermi acceleration and local‐scale betatron acceleration. These results, demonstrating that the electron rolling‐pin distribution exists beyond the Earth, can improve our knowledge of electron dynamics in planetary magnetosphere. Plain Language Summary: Examining the electron pitch angle distribution can help us better understand the electron dynamics in planetary magnetosphere. Up to now, it still remains unknown whether the rolling‐pin distribution, which shows pitch angles primarily at 0°, 90°, and 180°, exists beyond the Earth. Here using the Juno measurements, for the first time, we report such distribution in the Jupiter's magnetosphere. We find the rolling‐pin distribution appears only in the suprathermal electrons, and further explain its formation processes by using an analytical model. Our study is useful to understand electron dynamics in planetary magnetosphere. Key Points: We for the first time provide the evidence of electron rolling‐pin distribution in Jupiter's magnetosphereThe rolling‐pin distribution appears only above 19.5 keV, falling well into the suprathermal energy rangeThe rolling‐pin distribution is formed by the combination of global‐scale Fermi acceleration and local‐scale betatron acceleration [ABSTRACT FROM AUTHOR]