Electron Acceleration In Guide Field Magnetic Reconnection

Magnetic reconnection is a basic and important process in space physics which relates to the rapid energy conversion.Magnetic energy stored in a current sheet is suddenly released,and then the plasma is accelerated and heated.In this dissertation,electrons acceleration is our main objective.By performing 2D Particle-in-cell simulations and 3D hybrid simulations,the results are shown below:1.The mechanisms of electron acceleration during multiple X line magnetic reconnection with a guide fieldThe interactions between magnetic islands are considered to play an important role in electron acceleration during magnetic reconnection.In this paper,two-dimensional particle-in-cell simulations are performed to study electron acceleration during multiple X line reconnection with a guide field.Because the electrons remain almost magnetized,we can analyze the contributions of the parallel electric field,Fermi,and betatron mechanisms to electron acceleration during the evolution of magnetic reconnection through comparison with a guide-center theory.The results show that with the magnetic reconnection proceeding,two magnetic islands are formed in the simulation domain.Next,the electrons are accelerated by both the parallel electric field in the vicinity of the X lines and the Fermi mechanism due to the contraction of the two magnetic islands.Then,the two magnetic islands begin to merge into one,and,in such a process,the electrons can be accelerated by both the parallel electric field and betatron mechanisms.During the betatron acceleration,the electrons are locally accelerated in the regions where the magnetic field is piled up by the high-speed flow from the X line.At last,when the coalescence of the two islands into one big island finishes,the electrons can be further accelerated by the Fermi mechanism because of the contraction of the big island.With the increase of the guide field,the contributions of the Fermi and betatron mechanisms to electron acceleration become less and less important.When the guide field is sufficiently large,the contributions of the Fermi and betatron mechanisms are almost negligible.2.On the gradient of the electron pressure in anti-parallel magnetic reconnectionWe first perform a two-dimensional particle-in-cell simulation of anti-parallel magnetic reconnection to verify that in the electron diffusion region the reconnection electric field is mainly balanced by the gradient of the electron pressure.Then,by following typical electron trajectories in the fixed electromagnetic field of antiparal1el reconnection,we calculate the gradient of the electron pressure.We find that the resulted gradient of the electron pressure is equal to the reconnection electric field.This indicates that in the electron diffusion region the reconnection electric field is balanced by the gradient of the electron pressure,which results from the electron nongyrotropic motions.Our result gives a microphysical explanation of the balance between the reconnection electric field and the gradient of the electron pressure.3.Electron acceleration in a secondary magnetic island formed during magnetic reconnection with a guide fieldSecondary magnetic islands may be generated in the vicinity of an X line during magnetic reconnection.In this paper,by performing two-dimensional(2-D)particle-in-cell simulations,we investigate the role of a secondary magnetic island in electron acceleration during magnetic reconnection with a guide field.The electron motions are found to be adiabatic,and we analyze the contributions of the parallel electric field and Fermi and betatron mechanisms to electron acceleration in the secondary island during the evolution of magnetic reconnection.When the secondary island is formed,electrons are accelerated by the parallel electric field due to the existence of the reconnection electric field in the electron current sheet.Electrons can be accelerated by both the parallel electric field and Fermi mechanism when the secondary island begins to merge with the primary magnetic island,which is formed simultaneously with the appearance of X lines.With the increase in the guide field,the contributions of the Fermi mechanism to electron acceleration become less and less important.When the guide field is sufficiently large,the contribution of the Fermi mechanism is almost negligible.4.Kinetic Alfven waves in the magnetopause reconnectionAlfven waves are believed to be fundamentally important in magnetic reconnection.The Kinetic Alfven Wave(KAW)takes the ion kinetic effect into consideration,the ion polarization drift caused by the time-varying electric field will lead to the charge separation in the scale of ion gyro-radius.By using 3D dayside hybrid simulation code,the electromagnetic perturbation in the reconnection region satisfield the Walen relation.The parallel electric field also can be found in the perturbed region.The parallel electric field and perpendicular electric field satisfied the KAW polarization relation,which means the perturbation in the dayside reconnection region can be identified as KAW.KAW carry the parallel Poyting flux along the ambient magnetic field,these energy will be pour into the magnetosphere at last.The process may relates to the particle heating in the magnetosphere and the generation of aurora.