Electron Heating and Acceleration in 2D Collisionless Reconnection

Electron heating in collisionless magnetic reconnection is explored in particle-in-cell (PIC) simulations with non-zero guide fields so that electrons remain magnetized. We address two important sources of particle energization: electric fields parallel to the magnetic field, and electric fields parallel to the curvature-drift (the latter associated with acceleration by Fermi reflection). In the case of a small guide field (20% of the magnitude of the reconnecting field) the curvature drift is the dominant source of electron heating, while for a larger guide field (equal to the magnitude of the reconnecting component) the electron acceleration by the curvature drift is comparable to that of the parallel electric field. We find that in both simulations, however, acceleration at high electron energies is primarily due to the curvature-drift, which suggests that this mechanism may be more important for the generation of energetic (nonthermal) electrons. Additionally, heating by the curvature-drift is distributed over a large area, while the parallel electric field is localized near X-lines. This in turn suggests that acceleration by parallel electric fields may play a smaller role in large systems where the X-line occupies a vanishing fraction of the system. Finally, both simulations exhibit a significant electron temperature anisotropy due to the parallel heating produced by both curvature-drift acceleration and parallel electric fields. This anisotropy is more pronounced in the simulation with the larger guide field where the electrons are more strongly magnetized.