Electron Rolling-Pin Distributions Preceding Dipolarization Fronts

Electron rolling-pin distribution (RPDs), characterized by triple peaks at pitch angles 0 & DEG;, 90 & DEG;, and 180 & DEG;, 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. Electrons usually experience efficient acceleration and transport in space, and account for many explosive phenomena, such as magnetospheric substorms. The accelerated electrons typically exhibit certain velocity distributions, such as rolling-pin distributions (RPDs), which are key to understanding the underlying acceleration mechanisms. Electron RPD has been traditionally believed to develop inside strong magnetic field regions, such as inside FPRs behind dipolarization fronts (DFs) in the terrestrial magnetotail. Different from the conventional knowledge, we present in this study the first observation of electron RPD inside weak magnetic field regions preceding DFs inside plasma jets, by using data collected by ESA' Cluster spacecraft. Taking advantage of multipoint measurements provided by Cluster, we have investigated in detail the underlying acceleration and transport processes responsible for such unique feature of RPDs. We report unique observations showing that rolling-pin distribution (RPDs) is present before dipolarization fronts (DFs) but absent behind themPresence of RPD before the fronts may be attributed to combined effect of global betatron and Fermi accelerationAbsence of RPD behind the fronts may be explained by electron loss due to three-dimensional gradient drift