The Process Of The Coalescence Of Magnetic Islands

Magnetic reconnection is a basic process in space plasmas.It provides an effective mechanism for converting magnetic field energy into plasma kinetic and thermal energy.Therefore,many explosions in space plasmas can be explained by magnetic reconnection,such as substorms in the Earth’s magnetosphere and solar flares.The generation of high-energy electrons is an important feature of collisionless magnetic reconnection.The excitation of high energy electrons occurs not only in the reconnection diffusion region,but also in the acceleration of the contracted magnetic island and the Dipolarization front(DF)region.In recent years,Magnetic Island merging is also closely related to electron acceleration.Observations show that there are some obvious electron acceleration processes in the process of magnetic island merging.It has been found that a large number of high-energy electrons are produced when several magnetic islands are finally merged into a large magnetic island.Therefore,it is very important to study the physical process of Magnetic Island merger and the mechanism of electronic acceleration.In this paper,two-dimensional(2D)particle-in-cell(PIC)simulations are performed to investigate the coalescence process of magnetic islands in collisionless plasma with the magnetic island chain in the elongated current sheet as the initial state.We find that the merging of magnetic islands have two stages:In the first stage,the two islands approach each other slowly due to the attractive force between the homodromous currents.The electrons around the merging line are accelerated by the out-of-plane electric field,and then a thin current sheet is formed,where the magnetic field is also piled up.In the second stage,the fast magnetic reconnection occurs in the thin current sheet around the merging line,and the resulted electromagnetic structures around the merging line are similar to those in reconnection diffusion region formed in a Harris current sheet,where the most characteristic is the quadrupole structure of the out-of-plane magnetic field.