Magnetic reconnection in collisionless plasmas: Prescribed fields

Abstract

The structure of the dissipation region during magnetic reconnection in collisionless plasma has been investigated. We consider a prescribed two‐dimensional magnetic x line configuration with an imposed reconnection electric field Ey. Particles are injected onto a computational grid, their orbits are integrated, and the moments of the distribution function are stored. The structure of the dissipation region depends on only two variables: a normalized reconnection electric field Ê and the opening angle θ of the separatrices of the magnetic field. An important conclusion of the work is that there is no linear relationship between the current sheet velocity νy and the electric field. For a small normalized electric field the maximum νy is localized away from the x line, is diamagnetic in origin (produced by local pressure gradients), and is independent of Ê as Ê → 0. For a large normalized electric field the effective life‐time of particles in the reconnection region scales as Ê−1/3 and νy, ∝ Ê2/3. In this limit, particles are ejected from the reconnection region as high‐velocity, gyrophase‐bunched beams. These beams produce an irregular filamentary current distribution in the outflow region. The beams which are ejected along the center of the outflow region are eventually trapped by the magnetic field while the beams ejected just downstream from the separatrix continue to move with high velocity out of the computational region. Analytic expressions for the current sheet velocity (νy), inflow and outflow velocities, and the scale size of the dissipation region are derived. Over the entire range of Ê, significant temperature anisotropies are produced, with T⊥ < T∥ in the inflow region and T⊥ ≫ T∥ in the outflow region. Implications of our results for understanding magnetic reconnection in magnetospheric plasma and the source of energetic particles measured in the plasma sheet boundary layer and in explaining recent observations of fine scale magnetic turbulence in the central plasma sheet are discussed.