Electron Rolling-Pin Distributions Preceding Dipolarization Fronts

Electron rolling-pin distribution (RPDs), characterized by triple peaks at pitch angles 0°, 90°, and 180°, have recently been discovered in the terrestrial magnetosphere. Since the RPDs' formation is typically attributed to local betatron acceleration, RPDs have been believed to appear primarily inside strong magnetic regions, such as flux pileup regions (FPRs) behind dipolarization fronts (DFs). Different from such expectation, in this study we present unique observations of RPDs made by Cluster spacecraft in the terrestrial magnetotail, showing that RPDs are present inside weak magnetic field regions ahead of the DFs but absent in the strong magnetic field region behind them. The presence of RPD prior to the fronts may be atttributed to the combined effect of global betatron acceleration, global Fermi acceleration, and frontward transport driven by magnetic gradient drift. The atypical features of RPDs are important for fully understanding electron acceleration and transport in the magnetosphere.