Particle acceleration and fast magnetic reconnection

Abstract

Mathematics demonstrates that an ideally evolving magnetic field has an exponentially increasing sensitivity to non-ideal effects for all but truly exceptional evolutions. On a time scale that depends only logarithmically on the magnitude of non-ideality, an evolving magnetic field will generally reach a state of fast magnetic reconnection. The effects of fast magnetic reconnection proceed at a rate determined by Alfv\'enic, not resistive, physics. The best known of these effects are associated with the transfer of magnetic field energy to the plasma and the conservation of magnetic helicity, which limits the energy transfer. As will be shown, weak non-ideality implies helicity conservation in regions bounded by magnetic surfaces or rigid perfect conductors. But, weak non-ideality makes the rapid transfer of energy subtle. This transfer can be understood by the drive for Alfv\'en waves and two other effects, which are present even in an ideal evolution, an effective parallel electric field $\mathcal{E}_{||}$, which can accelerate particles despite the particle acceleration due to the true parallel electric field $E_{||}$ being negligible, and a coefficient $\nu_K$, which gives a rate for exponentiation of the kinetic energy of particle motion along the magnetic field. Classical theories of reconnection are two-dimensional and exclude the exponentially increasing sensitivity that is robustly present for magnetic fields in three-dimensional space.