Abstract
Magnetic reconnection provides a physical mechanism for fast energy conversion from magnetic energy to plasma kinetic energy. It is closely associated with many explosive phenomena in space plasma, usually collisionless in character. For this reason, researchers have become more interested in collisionless magnetic reconnection. In this paper, the various roles of electron dynamics in collisionless magnetic reconnection are reviewed. First, at the ion inertial length scale, ions and electrons are decoupled. The resulting Hall effect determines the reconnection electric field. Moreover, electron motions determine the current system inside the reconnection plane and the electron density cavity along the separatrices. The current system in this plane produces an out-of-plane magnetic field. Second, at the electron inertial length scale, the anisotropy of electron pressure determines the magnitude of the reconnection electric field in this region. The production of energetic electrons, which is an important characteristic during magnetic reconnection, is accelerated by the reconnection electric field. In addition, the different topologies, temporal evolution and spatial distribution of the magnetic field affect the accelerating process of electrons and determine the final energy of the accelerated electrons. Third, we discuss results from simulations and spacecraft observations on the secondary magnetic islands produced due to secondary instabilities around the X point, and the associated energetic electrons. Furthermore, progress in laboratory plasma studies is also discussed in regard to electron dynamics during magnetic reconnection. Finally, some unresolved problems are presented.
Highlights
Magnetic reconnection provides a physical mechanism for fast energy conversion from magnetic energy to plasma kinetic energy
Magnetic reconnection provides a physical mechanism for fast energy conversion from magnetic energy to plasma kinetic energy, and is closely associated with many explosive phenomena in space plasma
Electron dynamics is very important for collisionless reconnection
Summary
Inside the ion inertial region of collisionless magnetic reconnection, while ions are decoupled from the magnetic field lines, electrons are still frozen-in along these lines. In anti-parallel magnetic reconnection, inside the electron inertial region, the convection and Hall terms in the generalized Ohm’s law are negligible due to the weak magnetic field. The reconnection electric field inside the electron diffusion region mainly comes from the off-diagonal term of the electron pressure. Even with the presence of a guide field, the magnetic field strength inside the reconnection plane is still very low so that the reconnection electric field within the electron diffusion region is mainly produced by the electron pressure gradient term. Outside the electron diffusion region, the reconnection electric field stems mainly from the convection and Hall terms, similar to anti-parallel reconnection. Electron and ion flow velocities inside the reconnection plane are smaller than those found in anti-parallel reconnection because of the guide field. These terms explain why the reconnection electric field diminishes as the guide field increases
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