磁通量浮现驱动磁场重联的数值研究

应用二维时变可压缩磁流体动力学数值模拟，研究了双极－单极磁场中由于磁通量浮现驱动的磁场重联过程．结果表明，双极场与单极场间磁力线的重联形成上升的冷而密的等离子体团，磁场演变成鞭状结构．向上运动的等离子体团到达其最大高度后将回落和弥散.等离子体团最大上升速度达０．１４ＶＡ，等离子体最大上升速度达０．２７ＶＡ，ＶＡ为下边界处的Ａｌｆｖｅｎ速度．随着磁通量浮现幅度的增大，等离子体上升速度增加，重联过程发展得较快．背景等离子体β１值（β１为等离子体压力与磁压之比）越小，等离子体团中密度增量越大．磁Ｌｕｎｄｑｕｉｓｔ数Ｓ在１０３－１０６之间的改变对等离子体的速度和密度增量影响并不明显．与电阻撕裂模不稳定性引起的自发重联相比，磁通量浮现会更有效地驱动双极－单极场中的磁场重联过程，寻致日冕Ｈα冲浪和Ｘ射线喷流的形成．

A NUMERICAL STUDY OF MAGNETIC RECONNECTION DRIVEN BY AN EMERGING OF MAGNETIC FLUX

Using a two-dimensional, time-dependent, compressible magnetohydrodynamic simulation, magnetic reconnection driven by an emerging of magnetic flux in abipole-monopole magnetic field is studied numerically. The results show that anupward cold and dense plasmoid forms by reconnection of magnetic lines betweenbipole and monopole fields. The evolution of magnetic field leads to a whipconfiguration. The upward plasmoid falls and diffuses after reaching its maximumheight. The maximum of plasmoid velocity reaches 0.14VA and the maximum ofupward plasma velocity is about 0.27VA, where VA is the Alfven velocity at bottomboundary. The upward velocity of plasmoid increases and the process of magneticreconnection is faster when the amplitude of emerging magnetic flux increases. Thedensity enhancement in plasmoid is larger for smaller background plasma β1 （β1 isthe ratio between plasma and magnetic pressures）. The effects of magnetic Lundquistnumber S between 103-106 on plasma velocity and density enhancement are notobvious. Comparing with spontaneous reconnection caused by the resistive tearinginstability, the emerging of magnetic flux is more effective to drive magneticreconnection in bipole-monopole field and leads the formation of Hα　surges and X rayjets in the corona.