Particle Acceleration by Magnetic Reconnection

Abstract

This is a review of theoretical models for particle acceleration by DC electric fields in reconnecting current sheets during solar flares. Particular emphasis is placed on models for collisionless acceleration in a large-scale reconnecting current sheet with a nonzero magnetic field and a highly super-Dreicer electric field of order a few hundred V m^—1. Theoretical arguments and observational evidence for such electric fields are also discussed. An approximate analytical approach is employed to identify the effects of the electric and magnetic fields on particle orbits. The magnetic field structure in the sheet is shown to determine both the electron to proton ratio for the accelerated particles and their typical energies and spectra. Formulae for the particle energy gains and acceleration times are presented. Recent numerical calculations of particle orbits are described, stressing the use of exact MHD solutions for the magnetic fields and plasma flows in the sheet. The analytical and numerical results form the basis for electric field acceleration models in solar flares. In particular, physical conditions can be identified that lead to either electron acceleration to gamma-ray energies of a few tens of MeV in electron-rich flares or the generation of protons with energies up to several GeV in large gradual events.