The Role of the Guide Field in Electron Acceleration during Magnetic Reconnection

Magnetic reconnection is thought to be an important driver of energetic particles in a variety of astrophysical phenomena including solar flares and magnetospheric storms. Kinetic particle-in-cell (PIC) simulations of collisionless reconnection reveal that the efficiency of electron acceleration is highly sensitive to magnitude of the guide field (the magnetic field component perpendicular to the reconnection plane). In reconnection where the guide field is smaller than the reconnecting component, the dominant electron accelerator is a Fermi-type mechanism that preferentially energizes the most energetic particles. In strong guide field reconnection, the field-line contraction that drives the Fermi mechanism becomes weak. Instead, parallel electric fields are primarily responsible for driving electron heating but are ineffective in driving the energetic component of the spectrum. Three-dimensional simulations reveal that the stochastic magnetic field that develops during 3D guide field reconnection plays a vital role in particle acceleration and transport. In 2D systems electrons are trapped within stagnant magnetic island cores so that acceleration is suppressed, whereas in 3D the stochastic magnetic field enables energetic electrons to freely sample regions where energy release is taking place. In 3D systems with a weak guide field, however, transport is diminished and electron acceleration is suppressed as in the 2D case. These results suggest that the most efficient electron acceleration occur in reconnection with a moderate guide field (comparable to the reconnecting component) so that both (a) the Fermi mechanism is an efficient driver and (b) energetic electrons may freely access acceleration sites. This has important implications for understanding electron acceleration in solar flares and reconnection-driven dissipation in astrophysical turbulence.