Effective Resistivity In Collisionless Magnetic Reconnection

In space and laboratory environments,it is widely accepted that magnetic reconnection(MR)is one of the important mechanisms to transform magnetic energy into plasma kinetic and thermal energy in a short time scale.In most space and laboratory environment,plasmas are usually weak collisional or collisionless plasmas which means that the dissipation from the classical collisional resistivity is not enough for fast MR.In PIC simulations,it is indicated that the dissipation for fast MR is associated with the off diagonal terms of the electron pressure tensor.Since there is no simple physical model to describe the off diagonal terms of the electron pressure tensor,we are unable to implement the model of the electron pressure tensor into the magnetohydrodynamic(MHD)equations to study large scale magnetic reconnection.In the thesis,we propose a simple model of effective resistivity that has been verified by PIC simulation,and successfully implemented in MHD model,and used to explain some observational properties in different plasma environments.In the thesis,we first introduced our self-developed full Particle-in-Cell(PIC)code,including the frame of our code,field equations,particle pushing,and so on.The benchmark with other PIC code is also given.Then,we proposed the physical model of the effective resistivity(ER)associated with MR,and verified it with PIC simulation codes.The bent magnetic fields during MR process lead to scattering motion of charged particles inside diffusion region,which is similar to classical coulomb collision and provides a large plasma dissipation to result in fast MR in collisionless plasmas.Through investigation of the electron motion properties,we drived the analytical formulation of ER in collisionless plasma.The ER on X-line can be determined by a few of MR parameters.It is found that our model effective resistivity is in good agreement with the PIC simulations during the whole MR process.We further drove the analytical formulation of ER with a guiding field(GF),which is also cross-checked with the PIC simulations.Then,we implemented our theoretical model of ER into Hall MHD code.It is found that the results from Hall MHD simulations with ER have in qualitative agreement with the PIC ones.It should be pointed out that the electric field on X-line is self-consistently generated by ER in present Hall MHD simulations and is not from numerical diffusion as in previous Hall MHD simulations.The analytical model of ER is also successfully used to explain large dissipation in different collisionless plasma environments.