ELECTRON ACCELERATION IN MAGNETIC RECONNECTION

Abstract

Magnetic reconnection is a ubiquitous plasma physics process responsible for

the explosive release of magnetic energy. It is thought to play a fundamental role

in the production of non-thermal particles in many astrophysical systems. Though

MHD models have had some success in modeling particle acceleration through the

test particle approach, they do not capture the vital feedback from the energetic

particles on the reconnection process. We use two and three-dimensional kinetic

particle-in-cell (PIC) simulations to self-consistently model the physics of electron

acceleration in magnetic reconnection. Using a simple guiding-center approxima-

tion, we examine the roles of three fundamental electron acceleration mechanisms:

parallel electric fields, betatron acceleration, and Fermi reflection due to the re-

laxation of curved field lines. In the systems explored, betatron acceleration is an

energy sink since reconnection reduces the strength of the magnetic field and hence

the perpendicular energy through the conservation of the magnetic moment. The

2D simulations show that acceleration by parallel electric fields occurs near the mag-

netic X-line and the separatrices while the acceleration due to Fermi reflection fills

the reconnection exhaust. While both are important, especially for the case of a

strong guide field, Fermi reflection is the dominant accelerator of the most energetic

electrons. In a 3D systems the energetic component of the electron spectra shows

a dramatic enhancement when compared to a 2D system. Whereas the magnetic

topology in the 2D simulations is characterized by closed flux surfaces which trap

electrons, the turbulent magnetic field in 3D becomes stochastic, so that electrons

wander over a large region by following field lines. This enables the most energetic

particles to quickly access large numbers of sites where magnetic energy is being

released.