Modulation of Electron Rolling-Pin Distribution Behind Anti-Dipolarization Front

The electron rolling pin distribution, showing electron pitch angles primarily at 0°, 90°, and 180°, has been recently observed behind dipolarization fronts (DFs) in the magnetosphere of Earth. However, the relation between such distribution and the leading edge of tailward magnetic reconnection jets, also known as anti-dipolarization fronts (ADFs), is still unclear. Here, by utilizing high-resolution data of the Magnetospheric Multiscale (MMS) mission, we provide the first observation of electron rolling pin distribution behind ADF. Such distribution of Maxwellian electrons appears in 1.3–5 keV and is modulated by firehose fluctuations: electron fluxes are high at wave troughs (|B|-minima) and are low at wave crests. The results of Liouville mapping and the electron loss cone angles are consistent with the spacecraft observations respectively, indicating such distribution is formed by the combination of global-scale Fermi acceleration and local-scale electron trapping. These findings highlight the importance of ADFs in magnetospheric convection.