Study On Magnetic Reconnection In A Force-free Current Sheet

In the space and laboratory plasma, magnetic reconnection plays a key role to explain plenty of explosive phenomena, such as solar flares, substorms in the Earth’s magnetosphere and fusion experiments in laboratory. Since Giovanelli R. G. firstly mentioned the definition of magnetic reconnection in the 1946, people have done numerous simulations, spacecraft observations and experiments about it. In this paper, using two dimensional (2D) particle-in-cell (PIC) simulations, we study the evolution of the ion diffusion region the and magnetic island during the collisionless magnetic reconnection in a force-free current sheet and compare the results with those of guide field reconnection and antiparallel magnetic reconnection in the Harris current sheet.1. The evolution of the ion diffusion region during magnetic reconnection in a force-free current sheetWith two dimensional PIC simulation, we find that the evolution of the ion diffusion region has two different phases. In the first phase, the electrons flow toward the X line along one pair of separatrices and away from the X line along the other pair of separatrices. Consequently, the out-of-plane magnetic field forms a distorted quadrupole structure as of the resulted Hall current, which is similar to that of a typical guide field reconnection in the Harris current sheet. In the second phase, the electrons move toward the X line along the separatrices, accelerated by the electric field near the X line and then flow away from the X line at the inner side of the separatrices, which leads to symmetric electron density depletion layers along the separatrices. In the ion diffusion region, the out-of-plane magnetic field exhibits a characteristic quadrupole pattern with a good symmetry, which is similar to that of antiparallel reconnection in the Harris current sheet.2. The structure of the magnetic island during magnetic reconnection in a force-free current sheetThe structure of the magnetic island, during magnetic reconnection in a force-free current sheet, is different from that of guide field reconnection in the Harris current sheet. In guide field reconnection, the out-of-plane magnetic field is enhanced in the magnetic island where there is a dip in the center, just like a crater; the electron density is enhanced in the center of the magnetic island. Whereas, during magnetic reconnection in a force-free current sheet, the out-of-plane magnetic field is enhanced in the center of the magnetic island and forms a quadrupole structure at the edge of magnetic island; the electron density is enhanced in the magnetic island where there is a dip in the center. After accelerated at the ends of magnetic island, both the electrons and ions move around the edge the magnetic island, with a ring formed. Besides, the electrons, nearby the ends of the of magnetic island, flow away from the X line at the inner side of the separatrices after they are accelerated in the vicinity of the X lines, moving around the outside of the magnetic island. Consequently, the electron density forms a ring aound the edge of the magnetic island. Besides, the system of electron flows and ion flows results in a quadrupole structure of the out-of-plane magnetic field at the edge of the magnetic island and enhanced configuration in the center.