MHD Simulation Of The Structure Of Reconnection Layer In The Presence Of Shear Flow

Magnetic reconnection is believed to be a fundamental physical process for rapid energyrelease in space and laboratory fusion plasmas, and is of great interest in the research of otherareas such as topology, geometry and vector field dynamics. The theory of magneticreconnection was developed to explain various explosive processes observed in plasmas ofthe solar system and fast energy transmission in space and laboratory plasmas. Throughreconnection, the topology of magnetic field is changed, and high-speed plasma is ejected.Magnetohydrodynamic (MHD) shocks and discontinuities have been found in the outflowregion of reconnection. Not only these shocks or discontinuities can accelerate or heat theplasma, but their presence and structure have also been considered as important observationalevidence of reconnection. The physics of reconnection, from its triggering mechanism to itsresulting magnetic and plasma structures, however, is still far from well understood. Amongother unsolved problems is the effect of sheared plasma flows on reconnection.In this thesis, we systematically study structures of the outflow region of reconnection,i.e., the reconnection layer under a sheared plasma flow by using a resistive MHD model. Inparticular, we discuss the effects of shear flow tangential and perpendicular to the anti-parallelmagnetic field components. Cases with both "anti-parallel reconnection" and the so-called"component reconnection" are investigated. It is shown that the structure of MHDdiscontinuities and shocks, and thus the signature of reconnection, is significantly affected bythe shear flow.In ChapterⅠ, the background of our research is introduced, including the concept ofmagnetic reconnection, the reconnection models, and observations of reconnection. TheRiemann problem is also introduced as a powerful method of solving the problem fordisintegration of an initial current sheet associated with magnetic reconnection. Then, recentdevelopment of the reconnection in the presence of shear flow is outlined.In ChapterⅡ, by using a one-dimensional (1D) resistive MHD model, the Riemannproblem is solved numerically for the structure of reconnection layer under a sheared flowalong the anti-parallel magnetic field components B_z. The simulation is carried out for thegeneral cases with symmetric or asymmetric plasma densities and magnetic fields on the twosides of the initial current sheet, and cases with or without a guide magnetic field, as invarious space and fusion plasmas. The generation of MHD discontinuities in the reconnectionlayer is discussed, including time-dependent intermediate shocks, intermediate shocks, slow shocks, slow expansion waves, and the contact discontinuity. It is shown that the structure ofthe reconnection layer is significantly affected by the presence of the shear flow V_z0. For aninitial symmetric current sheet, the symmetry condition is altered due to the shear flow. Forcases with an asymmetric initial current sheet, as at the Earth's magnetopause, the strengths ofMHD discontinuities change significantly with the shear flow speed. Moreover, the generalresults for reconnection layers in the outflow regions on either sides of the X line arediscussed systematically for the first time.In the next chapter, the presence of a shear flow perpendicular to the anti-parallel fieldcomponents B_z is considered. The shear flow is in the y direction, or the guide magneticfield direction, with a total change of flow△V_y≠0 across the current sheet. Cases withsymmetric or asymmetric current sheet and various guide magnetic fields B_y are investigated.The simulation shows that in the reconnection layer, the structure of MHD discontinuitieschange significantly with the strength of the shear flow. The main findings are: (1) In the caseinitially with a zero guide field (B_y=0, for anti-parallel reconnection), the shear flow V_y0produces a finite B_y in the reconnection layer, and two time-dependent intermediate shocks(TDISs) with rotation angle of tangential magnetic field less than 180°. Besides, theasymmetry of the current sheet is still kept even in the presence of the shear flow V_y0. (2) Forinitial B_y≠0 (the "component reconnection") the shear flow V_y0 leads to very differentmagnetic field structures in the two outflow regions on the two sides of the X line. (3) In thecases with the initial B_y≠0, the existence of the sheared V_y can lead to the reversal of therotation sense of tangential magnetic field through the reconnection layer. The critical valueof△V_y for the occurrence of this field reversal is discussed. The general simulation resultscan be applied to space and laboratory plasmas.In the previous chapter, it is found that for cases with a finite guide field, theperpendicular shear flow V_y0 can lead to the reversal of the rotation sense of the tangentialmagnetic field in time-dependent intermediate shocks when shear flow exceeds the criticalvelocities. In ChapterⅣ, we discuss the detailed structure of reconnection layer in the generalcases where exist both a shear flow V_y0 and a shear flow V_z0. The critical shear flow speeds,V_c and V'_c, for the reversal of field rotations in TDIS and TDIS', respectively, are calculated.The structure at the critical shear flow velocities is also shown. For shear flow speed V_y0=V_c,the strong TDIS is replaced by a steady intermediate shock (IS), whereas at V_y0= V'_c anAlfvén wave pulse is present in the reconnection layer. The presence of tangential shear flow V_z0 alters not only the strength of TDIS and TDIS', but also the critical speeds V_c and V'_c.The critical shear flow speeds obtained from our simulation are found to agree very well withthose from the ideal MHD, in which the time-dependent intermediate shocks are replaced byrotational discontinuities.In ChapterⅤ, a three-dimensional (3D) MHD simulation is carried out for reconnectionwith a shear flow perpendicular to the anti-parallel magnetic field components. While varioustwo-dimensional MHD simulations of recormection have been conduced previously foreffects of shear flows along the anti-parallel magnetic field direction, the resulting magneticfield and plasma structure of reconnection in the presence of a shear flow in V_y shouldgenerally be of 3D nature. The purpose of this 3D simulation is to provide an initial trial forinvestigation of reconnection under an arbitrary shear flow, and to compare the results of 3Dreconnection to our 1D Riemann problem. The anti-parallel reconnection with no guide fieldis discussed. It is found that in the y=0 plane, the perpendicular shear flow does notdestroy the symmetry of the initial symmetric current sheet, which is in agreement with theresults of ChapterⅢbased on the 1D Riemann problem. The rotation sense of the tangentialmagnetic field is reversed due to the shear flow, consistent again with the 1D Riemannproblem. Nevertheless, it is also noted that asymmetry structures appear in the y≠0 planesin the 3D current sheet in the presence of the perpendicular shear flow. The occurrence ofKelvin-Helmholtz (KH) instability due to the shear flow is also discussed.Finally, a conclusion is given in ChapterⅥ.