LAUSR

The rolling-pin distribution of suprathermal electrons (40-200 keV), showing electron pitch angles primarily at 0°, 90°, and 180°, has recently been reported behind dipolarization fronts (DFs) both in observations and simulations. The formation of such type of distribution, however, has been unclear so far. In this study, we present an observation of such type of distribution by Cluster in the magnetotail behind a DF. We interpret the formation of such distribution using the global-scale Fermi acceleration together with local-scale betatron acceleration. We quantitatively reproduce these two processes and therefore the rolling-pin distribution of suprathermal electrons using an analytical model. We further reveal that only at energies higher than 26 keV can such distribution be formed. This study, quantitatively explaining the formation of rolling-pin distribution, can improve the understanding of electron dynamics behind DFs.