磁编织与平行电场
Magnetic Braiding and Parallel Electric Fields
摘要：
太阳日冕磁场通过光球运动编织--伴随着随后的弛豫和磁重联--是太阳物理学中最广泛争论的观点之一。根据三维磁重联理论的发展，我们对这一理论进行了重新讨论。已知沿场线的积分平行电场是决定重联率的关键量，而在二维情况下电场本身是重要量。我们证明，这种差异变得至关重要的足够复杂的磁场结构。数值方法被用来放松编织磁场的理想无力平衡，平衡被发现是光滑的，只有大规模的电流结构。然而，平衡被示出具有高度连续的集成的并联电流结构，具有极短的长度尺度。一个分析模型的开发表明，在日冕的情况下，与集成的平行电流结构的长度尺度将迅速减少的磁场的复杂性增加，或编织的程度。分析表明，对于任何有限的电阻率，这些长度尺度的减小最终将与无力场的稳定性不一致。因此，磁编织过程的必然结果是日冕场的平衡的损失，可能通过磁重联事件。
英文摘要：
The braiding of the solar coronal magnetic field via photospheric motions - with subsequent relaxation and magnetic reconnection -- is one of the most widely debated ideas of solar physics. We readdress the theory in the light of developments in three-dimensional magnetic reconnection theory. It is known that the integrated parallel electric field along field lines is the key quantity determining the rate of reconnection, in contrast with the two-dimensional case where the electric field itself is the important quantity. We demonstrate that this difference becomes crucial for sufficiently complex magnetic field structures. A numerical method is used to relax a braided magnetic field to an ideal force-free equilibrium; that equilibrium is found to be smooth, with only large- scale current structures. However, the equilibrium is shown to have a highly filamentary integrated parallel current structure with extremely short length- scales. An analytical model is developed to show that, in a coronal situation, the length scales associated with the integrated parallel current structures will rapidly decrease with increasing complexity, or degree of braiding, of the magnetic field. Analysis shows the decrease in these length scales will, for any finite resistivity, eventually become inconsistent with the stability of a force- free field. Thus the inevitable consequence of the magnetic braiding process is shown to be a loss of equilibrium of the coronal field, probably via magnetic reconnection events.
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