Department of Applied Mathematics & Statistics, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
Nature. 2013 May 23;497(7450):466-9. doi: 10.1038/nature12128.
The idea of 'frozen-in' magnetic field lines for ideal plasmas is useful to explain diverse astrophysical phenomena, 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. Microphysical plasma processes are a proposed explanation of the observed high rates, 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 is that turbulent Richardson advection 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. 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, γ-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.
理想等离子体的“冻结”磁场线的概念对于解释各种天体物理现象很有用，例如，通过将冻结在星际介质中的磁场线扭曲，从原恒星中排出多余的角动量。然而，冻结的磁场线排除了在高电导率下观察到的磁场拓扑的快速变化，例如在太阳耀斑中。微观等离子体过程是对观察到的高速度的一种解释，但仍有一个悬而未决的问题，即这种过程是否可以快速重新连接比几千个离子回旋半径大得多的天体物理通量结构。另一种解释是，湍动理查森平流将磁场线从很远的距离压缩在一起，达到回旋半径量级的距离。在这里，我们报告了对高电导率下磁流体力学湍流的模拟分析，该模拟显示了理查森弥散。这种在空间上不可微的粗糙速度场中的平流效应导致了线运动，这些线运动完全是不确定的或“自发随机的”，正如分析研究所预测的那样。标准通量冻结在大于离子回旋半径的尺度上的湍动破坏可以解释非常大尺度通量结构的快速重联，包括观察到的（太阳耀斑和日冕物质抛射）和预测的（内日鞘、吸积盘、γ射线爆发等）。对于具有平滑速度场的层流等离子体流动或低湍流动强度，随机通量冻结简化为通常的冻结条件。
