Helicity and winding fluxes as indicators of twisted flux emergence
Helicity and winding fluxes as indicators of twisted flux emergence
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DOI：
10.1080/03091929.2020.1740925
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发表时间：
2020-03-20
影响因子：
1.3
通讯作者：
Prior, C.
中科院分区：
文献类型：
--
作者：
MacTaggart, D.;Prior, C.
到目前为止，太阳大气中出现扭曲通量管的证据来自间接特征。在这项工作中，我们通过研究螺旋度和缠绕磁通直接研究扭曲磁通管出现的拓扑输入。在磁流体动力学模拟中，域从对流区顶部跨越到较低的电晕，我们模拟了具有一系列不同初始场强的扭曲通量管的出现。这项工作的一个重要特征是在光球层下方包含了一个对流不稳定层。我们发现在考虑的不同场强的螺旋度输入中近似自相似的行为。当管上升并到达光球层时，由于我们考虑左手扭曲管，因此存在强烈的负螺旋性输入。此阶段之后是负输入的减少，并且对于低初始场强，净正螺旋度输入。这一阶段对应于对流对场的日益增长的影响以及出现期间蛇形场结构的发展。缠绕通量可用于检测管的扭曲芯何时到达光球层，从而提供有关拓扑复杂磁场输入太阳大气的清晰信息。简而言之，螺旋度和缠绕通量可以提供有关磁场如何出现的大量信息，而这些信息无法从其他来源（例如磁图）直接获得。在评估这些模拟的螺旋度内容时，我们测试了多种创建合成磁图的方法，包括考虑不断演变的几何形状和光球层有限范围的方法。虽然每种情况下的一般定性行为是相同的，但不同形式的平均确实会定量地影响螺旋度和缠绕输入。
Evidence for the emergence of twisted flux tubes into the solar atmosphere has, so far, come from indirect signatures. In this work, we investigate the topological input of twisted flux tube emergence directly by studying helicity and winding fluxes. In magnetohydrodynamic simulations with domains spanning from the top of the convection zone to the lower corona, we simulate the emergence of twisted flux tubes with a range of different initial field strengths. One important feature of this work is the inclusion of a convectively unstable layer beneath the photosphere. We find approximately self-similar behaviour in the helicity input for the different field strengths considered. As the tubes rise and reach the photosphere, there is a strong input of negative helicity since we consider left-handed twisted tubes. This phase is then followed by a reduction of the negative input and, for low initial field strengths, a net positive helicity input. This phase corresponds to the growing influence of convection on the field and the development of serpentine field structures during emergence. The winding flux can be used to detect when the twisted cores of the tubes reach the photosphere, giving clear information about the input of topologically complex magnetic field into the solar atmosphere. In short, the helicity and winding fluxes can provide much information about how a magnetic field emerges that is not directly available from other sources, such as magnetograms. In evaluating the helicity content of these simulations, we test numerous means for creating synthetic magnetograms, including methods which account for both the evolving geometry and the finite extent of the photosphere. Whilst the general qualitative behaviours are same in each case, the different forms of averaging do affect the helicity and winding inputs quantitatively.