Particle-in-cell simulations of collisionless magnetic reconnection with a non-uniform guide field

F Wilson,M G Harrison,M Hesse,T Neukirch,C R Stark

doi:10.1063/1.4942939

Abstract

Results are presented of a first study of collisionless magnetic reconnection starting from a recently found exact nonlinear force-free Vlasov–Maxwell equilibrium. The initial state has a Harris sheet magnetic field profile in one direction and a non-uniform guide field in a second direction, resulting in a spatially constant magnetic field strength as well as a constant initial plasma density and plasma pressure. It is found that the reconnection process initially resembles guide field reconnection, but that a gradual transition to anti-parallel reconnection happens as the system evolves. The time evolution of a number of plasma parameters is investigated, and the results are compared with simulations starting from a Harris sheet equilibrium and a Harris sheet plus constant guide field equilibrium.

Highlights

Magnetic reconnection is one of the most fundamental plasma processes and plays an important role in the magnetic activity of many astrophysical and laboratory plasmas.1,2 It allows the conversion of stored magnetic energy into bulk flow, thermal, and non-thermal energy, through changes in magnetic connectivity
It is found that the reconnection process initially resembles guide field reconnection, but that a gradual transition to anti-parallel reconnection happens as the system evolves
At t 1⁄4 40 and at subsequent times, a modified quadrupolar structure of By can be seen around the X-point. This structure is qualitatively similar to that seen in Harris plus constant guide field simulations,8,10 and so we do not see a transition to the quadrupolar structure seen in Harris sheet simulations

Summary

INTRODUCTION

Magnetic reconnection is one of the most fundamental plasma processes and plays an important role in the magnetic activity of many astrophysical and laboratory plasmas. It allows the conversion of stored magnetic energy into bulk flow, thermal, and non-thermal energy, through changes in magnetic connectivity. 8, 10–17,19, and 21) is the influence of a guide field on the reconnection process Most of these studies have used a Harris sheet with a constant guide field as an initial current sheet configuration. We present results of a 2.5D particle-incell (PIC) simulation, in which we use an exact self-consistent equilibrium for the force-free Harris sheet as an initial condition.. The initial particle distribution functions were taken to be drifting Maxwellian distributions, which do not represent an exact initial equilibrium for this configuration This leads us to discuss another motivation for our work— we are not aware of any previous study of collisionless reconnection for which exactly force-free initial conditions have been used for a nonlinear force-free field. To analyse the expected transition from guide field to anti-parallel reconnection in the force-free Harris sheet case, we will present results from two other simulation runs: one which starts from a Harris sheet, and the other from a Harris sheet plus uniform guide field of By 1⁄4 B0

Normalisation and parameters

Evolution of magnetic field and current density

Electron Larmor radius and bounce width

The reconnection electric field

Pressure tensor components

SUMMARY AND CONCLUSIONS