Formation of Rolling-Pin Distribution of Suprathermal Electrons Behind Dipolarization Fronts

The rolling-pin distribution of suprathermal electrons, exhibiting electron pitch angles primarily at 0°, 90°, and 180°, receives attention in both observations and simulations recently. However, its formation mechanism has rarely been studied. In this work, we report an observation of such distribution with two substructures behind the dipolarization front (DF) and reveal that only Fermi mechanism is contributory to suprathermal electron acceleration. We present a new explanation for its formation. The Fermi mechanism accelerates parallel and antiparallel suprathermal electrons, the spatially varying magnetic bottles control the emergence of perpendicular electrons, and electrons whose pitch angle around 45° and 135° drift away in the middle magnetotail. Under the combination of these mechanisms, the rolling-pin distribution with two substructures is formed behind DFs. This formation mechanism is useful to understand the electron dynamics behind DFs.