Threshold of Non-potential Magnetic Helicity Ratios at the Onset of Solar Eruptions
Threshold of Non-potential Magnetic Helicity Ratios at the Onset of Solar Eruptions
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DOI：
10.3847/1538-4357/aacdfc
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发表时间：
2018-07
期刊：
影响因子：
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通讯作者：
F. Zuccarello;E. Pariat;G. Valori;L. Linan
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文献类型：
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作者：
F. Zuccarello;E. Pariat;G. Valori;L. Linan
相对磁螺旋度是一个经常用来描述非孤立磁场（如太阳活动区的磁场）纠缠程度的量。本文的目的是探讨不同种类的光球边界流积累相对磁螺旋度的日冕，以及如何以及磁螺旋度相关的数量确定爆发的开始。我们使用一系列的三维，参数磁流体动力学模拟的磁通绳的形成和喷发。所有的模拟都是在相同的网格上进行的，使用相同的参数，但是它们的特征在于不同的驱动光球流，即，剪切、会聚、拉伸以及外围和中心扩散流。对于每一个模拟，爆发的开始时刻是仔细确定使用一系列的松弛运行。我们发现，当在边界处施加剪切流时，磁能和总相对螺旋度主要被注入，而与日冕电流相关的磁能和螺旋度增加，而不管光球流的种类如何。我们还发现，在爆发的开始，非潜在的磁螺旋度和总的相对磁螺旋度之间的比率具有相同的值为所有的模拟，这表明存在一个阈值在这个数量。对于其他量，例如与磁能相关的量，没有观察到这样的阈值。
The relative magnetic helicity is a quantity that is often used to describe the level of entanglement of non-isolated magnetic fields, such as the magnetic field of solar active regions. The aim of this paper is to investigate how different kinds of photospheric boundary flows accumulate relative magnetic helicity in the corona and if and how well magnetic-helicity-related quantities identify the onset of an eruption. We use a series of three-dimensional, parametric magnetohydrodynamic simulations of the formation and eruption of magnetic flux ropes. All the simulations are performed on the same grid, using the same parameters, but they are characterized by different driving photospheric flows, i.e., shearing, convergence, stretching, and peripheral- and central- dispersion flows. For each of the simulations, the instant of the onset of the eruption is carefully identified by using a series of relaxation runs. We find that magnetic energy and total relative helicity are mostly injected when shearing flows are applied at the boundary, while the magnetic energy and helicity associated with the coronal electric currents increase regardless of the kind of photospheric flows. We also find that, at the onset of the eruptions, the ratio between the non-potential magnetic helicity and the total relative magnetic helicity has the same value for all the simulations, suggesting the existence of a threshold in this quantity. Such a threshold is not observed for other quantities as, for example, those related to the magnetic energy.