Characteristics of Electron Holes Generated in the Separatrix Region during Anti-Parallel Magnetic Reconnection

Electron holes were found to be generated in the separatrix region during magnetic reconnection. In this paper, two-dimensional (2-D) particle-in-cell (PIC) simulations are performed to investigate the characteristics of these electron holes in anti-parallel magnetic reconnection. The electron holes with the bipolar structures of the parallel electric field are formed in the border between the electron inflow channel (where electrons move toward the X line) and outflow channel (where electrons flow away from the X line), where the electrons satisfy the bump-on-tail distribution. Quasi-monochromatic electrostatic waves are firstly excited by the electron bump-on-tail instability, which propagate with a speed near the bulk velocity of the fast electron beam. These waves then coalesce with each other, and at last electron holes (EHs) propagating away from the X line along the magnetic field lines are formed in the separatrix region.