Characteristics of electron holes generated in the separatrix region during antiparallel magnetic reconnection

In this paper, two-dimensional particle-in-cell simulations are performed to investigate the characteristics of these electron holes generated in the separatrix region of antiparallel magnetic reconnection. The electron holes with 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, which propagate with a speed near the bulk velocity of the fast electron beam, are first excited by the electron bump-on-tail instability. These waves then coalesce with each other, and at last electron holes are formed in the separatrix region which then propagate away from the X line along the magnetic field lines.