Magnetic reconnection in large-scale astrophysical systems

Magnetic reconnection mediates the conversion of magnetic field energy into plasma kinetic energy through the production of bulk flows, heating, and the acceleration of energetic particles. In high energy astrophysical systems such as pulsars, gamma-ray bursts, and active galactic nuclei jets, reconnection is expected to occur in the relativistic regime where the energy density of the magnetic field exceeds that of the plasma rest mass. Relativistic reconnection thus plays a critical role in the global evolution of astrophysical systems and the production of nonthermal particle distributions that are observed through their radiation. Bridging the gap between the microscopic diffusion regions where magnetic field topology changes occur and the global scales of realistc systems is a major challenge models of reconnection in astrophysical contexts. Understanding the sacrifices in fidelity as the physical model is reduced from a kinetic Vlasov-Maxwell system to a simplified magnetohydrodynamic fluid is critical for the accurate modelling of large-scale realistic systems. Using kinetic particle-in-cell simulations with a fully relativistic Coulomb collision operator, we study the transition from collisionless to collisional regimes of relativistic reconnection for the first time. At sufficiently high collisionalities, we reproduce the relativistic generalization of the Sweet-Parker scaling of reconnection rate with Lundquist number. In intermediate regimes, the collisionality significantly impacts plasmoid formation and particle acceleration. Finally, we discuss the use of two-dimensional kinetic equilibria for modelling compact object magnetospheres.