February 15: Joel Dahlin (NASA Goddard)

Particle Acceleration Mechanisms during Magnetic Reconnection

Magnetic reconnection is a fundamental plasma process that drives explosive conversion of magnetic energy into bulk flows, heat, and nonthermal particles. Reconnection is a promising mechanism for particle acceleration in a wide range of astrophysical phenomena including solar and stellar flares, pulsar wind nebulae, and AGN jets. We present results from three-dimensional full-particle kinetic simulations that demonstrate two primary particle energization processes operating during reconnection: (1) a Fermi-type mechanism associated with contracting magnetic ‘islands’, and (2) electric fields parallel to the local magnetic field. The Fermi mechanism drives volume-filling energization that scales strongly with particle energy, whereas parallel electric fields are localized to micro-scale dissipation regions and scale weakly with energy, instead generating bulk heating.

We also show that a background axial or ‘guide’ field plays a vital important role in controlling the acceleration efficiency. In the absence of a guide field, reconnection dynamics are effectively two-dimensional, and particles become trapped in stagnant island cores where acceleration ceases. In a system where the guide field is much stronger than the reconnecting field, magnetic field contraction is inhibited and the Fermi mechanism is suppressed. The most efficient acceleration occurs with a moderate guide field (comparable to the reconnecting field) that drives turbulent transport, enabling particles to escape island cores and re-accelerate while not substantially suppressing the Fermi mechanism. We present a model for the macro-scale evolution of the guide field during a solar/stellar flare and demonstrate consistency with solar hard X-ray observations.