Formation of the energetic electrons at the dipolarization front and the trailing flux pileup region during magnetic reconnection

The dipolarization front (DF) and the flux pileup region (FPR) are crucial downstream structures in magnetic reconnection, where significant energetic electrons are frequently observed.  Using a two-dimensional particle-in-cell simulation model, we investigate the formation of energetic electrons in both the DF and the trailing FPR. Our results demonstrate that the energetic electrons at pitch angles near 90° at both regions undergo a two-stage acceleration process: an initial non-adiabatic acceleration by the reconnection electric field at the reconnection site followed by downstream adiabatic acceleration. We find that the 90° pitch-angle energetic electrons in the FPR reach substantially higher energies than those at the DF, as they encounter a stronger reconnection electric field at the reconnection site in the first stage. Furthermore, two populations of energetic electrons with distinct energy ranges at pitch angles near 0° and 180° are identified at the DF. The lower-energy population exhibits energies close to the magnitude of the parallel potential at the DF, which dominates the formation of this population byaccelerating the electrons towards the DF and providing the trapping mechanism. The higher-energy population is energized via Fermi mechanism through multiple reflections within the contracting magnetic island downstream. These findings provide new insights into the generation of energetic electrons during magnetic reconnection.