磁云传播的动力学演化过程研究进展

Advance in Dynamics of Magnetic Cloud Evolution

磁云因其独特的磁场结构经常是重大灾害性空间天气的驱动源.近来从磁云的边界层结构、环向通量、大尺度结构等方面关于磁云传播的动力学演化过程的研究取得了一些进展.在磁云边界存在一个由于磁场重联而形成的边界层结构.在磁云传播过程中,这种发生在边界处的磁场重联可能会把磁云的磁场剥蚀掉,进而引起其磁通量绳结构环向通量的减少以及不对称.在磁云内部,经常会观测到多个子通量绳结构.这些特性各异的子通量绳可以通过磁场重联而合并,进而引起磁云磁结构的改变.关于磁云大尺度磁场拓扑位形的演化机制,除了较早提出的交换重联外,目前的研究表明在行星际空间中,磁云边界处的重联过程也可以将磁云闭合或半开放的磁场线打开或断开.尽管在相关研究中已经取得了较大进展,但关于磁云传播的动力学演化过程还有许多问题尚不清楚.在行星际小尺度磁通量绳边界也发现了边界层结构,那么磁云是否会因剥蚀而成为小尺度通量绳?磁云内子通量绳结构在相互作用中会不会引起某些不稳定性而导致整个通量绳系统的崩溃?这些问题的解决还有待于进一步的理论、观测和数值模拟研究.

Owing to its special magnetic structures,magnetic cloud(MC)is the main driver of disaster space weather.This paper deals with the research of MC on three aspects including boundary layer,azimuthal flux and large-scale structure.The boundary layer exists in the boundary of MC due to magnetic reconnection taking place there.Such reconnection could peel off the magnetic field of MC as it propagates,causing the reduction and imbalance in the azimuthal flux of its magnetic flux rope structure.Multiple flux ropes are often detected within MC.The coalescence of these flux ropes could change the largescale structure of MC.In addition to exchange reconnection,magnetic reconnection taking place near the boundary of MC could also open the closed or half-closed magnetic field lines of MC.Although great progress has been made,many questions about the evolution of MC are still open.If MC could evolve into an interplanetary small magnetic flux rope due to the“peeling off”effect?Could the interaction of these sub-flux ropes within MC lead to collapse of the magnetic flux rope structure of MC?To answer these questions,further studies based on theory,observation and numerical simulation are needed.