Flux-freezing breakdown in high-conductivity magnetohydrodynamic turbulence
Flux-freezing breakdown in high-conductivity magnetohydrodynamic turbulence
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DOI：
10.1038/nature12128
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发表时间：
2013-05-23
期刊：
影响因子：
64.8
通讯作者：
Szalay, Alexander
中科院分区：
文献类型：
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作者：
Eyink, Gregory;Vishniac, Ethan;Szalay, Alexander
理想等离子体的“冻结”磁力线的想法（1）有助于解释各种天体物理现象（2），例如通过冻结到星际介质中的磁力线的扭曲，原恒星的多余角动量脱落。然而，冻结的场线排除了在高电导率下观察到的磁拓扑结构的快速变化，如太阳耀斑（2，3）。微物理等离子体过程是对观测到的高速率的一种解释（4-6），但这是一个悬而未决的问题，这种过程是否可以迅速重新连接天体物理通量结构的程度远远大于几千离子回旋半径。另一种解释（7，8）是湍流理查森平流（9）使场线从相距很远的地方聚在一起，达到回转半径的分离。在这里，我们报告了一个模拟的磁流体动力学湍流在高电导率，表现出理查森色散的分析。在空间中不可微的粗糙速度场中的平流效应导致了完全不确定或“自发随机”的线运动，正如分析研究所预测的那样（10-13）。在大于离子回旋半径的尺度上，标准通量冻结的湍流破裂可以解释观测到的（太阳耀斑和日冕物质抛射）和预测到的（内日鞘、吸积盘、伽马射线爆发等）非常大尺度通量结构的快速重联。对于层流等离子体流与光滑的速度场或低湍流强度，随机通量冻结减少到通常的冻结条件。[图形]。
The idea of 'frozen-in' magnetic field lines for ideal plasmas(1) is useful to explain diverse astrophysical phenomena(2), for example the shedding of excess angular momentum from protostars by twisting of field lines frozen into the interstellar medium. Frozen-in field lines, however, preclude the rapid changes in magnetic topology observed at high conductivities, as in solar flares(2,3). Microphysical plasma processes are a proposed explanation of the observed high rates(4-6), but it is an open question whether such processes can rapidly reconnect astrophysical flux structures much greater in extent than several thousand ion gyroradii. An alternative explanation(7,8) is that turbulent Richardson advection(9) brings field lines implosively together from distances far apart to separations of the order of gyroradii. Here we report an analysis of a simulation of magnetohydrodynamic turbulence at high conductivity that exhibits Richardson dispersion. This effect of advection in rough velocity fields, which appear non-differentiable in space, leads to line motions that are completely indeterministic or 'spontaneously stochastic', as predicted in analytical studies(10-13). The turbulent breakdown of standard flux freezing at scales greater than the ion gyroradius can explain fast reconnection of very large-scale flux structures, both observed (solar flares and coronal mass ejections) and predicted (the inner heliosheath, accretion disks, gamma-ray bursts and so on). For laminar plasma flows with smooth velocity fields or for low turbulence intensity, stochastic flux freezing reduces to the usual frozen-in condition.[GRAPHICS].