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.