Structure of the electron diffusion region in magnetic reconnection with small guide fields

Abstract

Observations in the Earth's magnetotail and kinetic simulations of magnetic reconnection have shown high electron pressure anisotropy in the inflow of electron diffusion regions. This anisotropy has been accurately accounted for in a new fluid closure for collisionless reconnection. By tracing electron orbits in the fields taken from particle-in-cell simulations, the electron distribution function in the diffusion region is reconstructed at enhanced resolutions. For antiparallel reconnection, this reveals its highly structured nature, with striations corresponding to the number of times an electron has been reflected within the region, and exposes the origin of gradients in the electron pressure tensor important for momentum balance. The addition of a guide field changes the nature of the electron distributions, and the differences are accounted for by studying the motion of single particles in the field geometry. Finally, the geometry of small guide field reconnection is shown to be highly sensitive to the mass ratio.