Lagrangian coherent structures in photospheric flows and their implications for coronal magnetic structure
Lagrangian coherent structures in photospheric flows and their implications for coronal magnetic structure
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DOI：
10.1051/0004-6361/201118278
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发表时间：
2011-10
影响因子：
6.5
通讯作者：
A. Yeates;G. Hornig;B. Welsch
中科院分区：
文献类型：
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作者：
A. Yeates;G. Hornig;B. Welsch
目标。我们展示了如何从光球速度数据直接推断太阳日冕中磁梯度的形成。这使得无需借助磁场外推即可计算诸如压扁因子之类的磁连通性度量。方法。假设日冕有一个理想的演化，并且初始磁场均匀，随后的场线映射是通过积分（随时间变化的）水平光球速度场的轨迹来计算的。该方法应用于由Hinode/SOT磁图获得的12 h高分辨率光球流序列。结果。我们发现在磁场中产生了一个准分离矩阵层网络，对应于光球速度下的拉格朗日相干结构。这些结构的视觉模式主要来自于光球流的发散部分，隐藏了旋转流成分的影响：这可以通过一个简单的光球对流分析模型来证明。我们将发散和旋转分量从观测流中分离出来，分别与纯发散和旋转模型在定性上一致。增加模型中的流动速度表明，我们的观测结果很可能给出由真实光球流动形成磁梯度的速率的下限。最后，我们用推断的拓扑构造了一个假设的磁场，可以用于未来重联和能量释放的研究。
Aims. We show how the build-up of magnetic gradients in the Sun’s corona may be inferred directly from photospheric velocity data. This enables computation of magnetic connectivity measures such as the squashing factor without recourse to magnetic field extrapolation. Methods. Assuming an ideal evolution in the corona, and an initially uniform magnetic field, the subsequent field line mapping is computed by integrating trajectories of the (time-dependent) horizontal photospheric velocity field. The method is applied to a 12 h high-resolution sequence of photospheric flows derived from Hinode/SOT magnetograms. Results. We find the generation of a network of quasi-separatrix layers in the magnetic field, which correspond to Lagrangian coherent structures in the photospheric velocity. The visual pattern of these structures arises primarily from the diverging part of the photospheric flow, hiding the effect of the rotational flow component: this is demonstrated by a simple analytical model of photospheric convection. We separate the diverging and rotational components from the observed flow and show qualitative agreement with purely diverging and rotational models respectively. Increasing the flow speeds in the model suggests that our observational results are likely to give a lower bound for the rate at which magnetic gradients are built up by real photospheric flows. Finally, we construct a hypothetical magnetic field with the inferred topology, that can be used for future investigations of reconnection and energy release.