Particle acceleration by magnetic reconnection from fluid to kinetic regimes

Magnetic reconnection is widely believed to play a critical role in the production of nonthermal energetic particles in a variety of systems in astrophysics and space physics. Several acceleration mechanisms including direct acceleration by the reconnection electric field at X-points and Fermi acceleration associated with contracting and merging plasmoids are known to contribute to energization. These mechanisms differ greatly in the complexity of the physical model required to model them, and their relative importance remains disputed. Due to the large scales of realistic systems and the computational expense associated with fully kinetic simulations, it is important to determine the simplest physical model that can accurately model the particle acceleration in a given system. Using kinetic particle-in-cell simulations with a relativistically correct Coulomb collision operator, we study the changes in particle energization that occur in the transition from a collisionless plasma to the collisional fluid regime. A detailed analysis of the generalized Ohm's law and self-consistent particle trajectories reveal the roles of ideal and non-ideal electric fields in particle energization. We discuss the implications of these results for modelling large-scale astrophysical systems.