Particle Acceleration during Magnetic Reconnection in Highly Magnetized Plasmas

We present two- and three-dimensional fully kinetic simulations of fast magnetic reconnection in the regime with low plasma beta. The magnetic energy efficiently converts into kinetic energy of nonthermal relativistic particles in a power-law spectrum. For large closed systems, the power law slope approaches "-1". The dominant acceleration mechanism is a first-order Fermi process accomplished through the curvature drift motion in magnetic flux tubes along the electric field induced by reconnection outflows. We have developed an analytical theory to describe the power-law spectrum. We demonstrate that both continuous inflow and Fermi-type acceleration lead to the power-law distributions. A general condition for the formation of power-law distributions in magnetic reconnection is derived. The role of anisotropic pitch-angle distribution is evaluated. For a system that allows particle escape from the acceleration region, the spectra get softer. The work shows that power-law distributions are a common feature in magnetic reconnection region with low plasma beta, which may be important to explain fast and efficient nonthermal particle acceleration in solar flares, Earth's magnetotail, and other astrophysical reconnection sites. Reference: Guo et al. arXiv:1405.4040, PRL in review.