Particle acceleration at sites of magnetic reconnection

Abstract

Electric fields induced by the changing magnetic field at sites of magnetic reconnection can efficiently accelerate charged particles in the solar corona. This review begins with estimates for the electric field magnitude in flare models and presents some of the theoretical results for the electron and proton acceleration in reconnecting current sheets in 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 V cm−1. Particle orbits in model current sheets are discussed using an approximate analytical approach that allows one to identify the effects of both the electric and magnetic field components on the particle motion. Formulas for the particle energy gains and acceleration times are presented. Given a super-Dreicer electric field in the sheet, it is the magnetic field structure in the sheet that determines both the electron to proton ratio for the accelerated particles and their typical energies and spectra. The analytical results form the basis for the electric field acceleration models in solar flares. In particular, physical conditions can be identified that lead to either flares in which electrons primarily generate hard X-rays in the energy range of tens of keV or flares with unusually large electron fluxes at gamma-ray energies extending up to a few tens of MeV.