Electron Dynamics In Collisionless Magnetic Reconnection

Magnetic reconnection is a fundamental physical process in plasma. It isconsidered to be associated with many bursts in the magnetosphere, theinterplanetary space, the solar atmosphere, and the laboratory plasma. Magneticreconnection provides a physical mechanism for fast conversion from magneticenergy to plasma kinetic and thermal energy. The concept of reconnection was firstsuggested by Giovanelli (1946) as a mechanism for particle acceleration in solarflares. In the past half century, the concept of reconnection has developed throughadvances in numerical simulations and spacecraft observations. However, there arestill something unclear. With two-dimensional (2D) particle-in-cell (PIC) simulationsand spacecraft observations, we study the structure of diffusion region, the evolutionof electron current sheet (ECS), secondary islands, electron acceleration and thestructure of island. The results are shown as follows.1. Structure of diffusion region in collisionless magnetic reconnectionIn collisionless reconnection, the magnetic field near the separatrix is strongerthan that around the X-line, so an electron-beam can be formed and flows toward theX-line, which leads to a decrease of the electron density near the separatrix. Havingbeen accelerated around the X-line, the electrons flow out along the magnetic fieldlines in the inner side of the separatrix. A quadruple structure of the Hall magneticfield is formed by such a current system. Furthermore, the position of the peak of theHall magnetic field is found to be between the separatrix and the center of thecurrent sheet, which is verified by Cluster observations.2. Electron diffusion region and secondary islands in collisionless magneticreconnectionThe ECS is formed by electrons accelerated by the inductive electric feld in thevicinity of the X line, which is then extended along the x direction due to theimbalance between the electric feld force and Ampere force. The tearinginstability is unstable when the ECS becomes suffciently long and thin, and severalseed islands are formed in the ECS. These tiny islands may coalesce and form alarger secondary island in the center of the diffusion region. The length to heightratio of the secondary island is 2:1, and the out-of-plane magnetic field is enhanced,which is consistent with the Cluster observation. 3. Electron acceleration in collisionless magnetic reconnectionElectrons can be accelerated both in the vicinity of the X line and the pileupregion by the reconnection electric feld. Most of the electrons accelerated in thevicinity of the X line come from the region just outside of the separatrices, whilemost of the electrons accelerated in the pileup region come from the region inside ofthe separatrices. In guide feld reconnection, electrons can be accelerated by theparallel electric feld in the vicinity of the X line, as well as when they move towardthe X line. Most of these energetic electrons come from the region outside of thenegative separatrices. The effciency of such an acceleration mechanism is obviouslyhigher than that in the antiparallel reconnection, and in both cases the mechanisms ofelectron acceleration favor the electrons with higher initial energy. The process of anaccelerated electron has three stages: fowing into the vicinity of the X line,acceleration in the vicinity of the X line, and fowing out of the vicinity of the X line.4. Structure of island in guide field magnetic reconnectionThe out-of-plane magnetic field has a dip in the center of magnetic islandformed during multiple X line guide field reconnection. Such structures areconsidered to be produced by the current system in the magnetic island. At the edgeof the magnetic island, there exists a current anti-parallel to the in-plane magneticfield, while the current is parallel to the in-plane magnetic field inside the magneticisland. Such a dual-ring current system, which is attributed to the electron dynamicsin the magnetic island, leads to the dip of the out-of-plane magnetic field in thecenter of the island. The above demonstration is verified by the ion and electronpitch-angle distributions in a typical crater flux transfer event (C-FTE) observed bythe THEMIS mission.