Latitudinal variation of helicity of photospheric magnetic fields
Latitudinal variation of helicity of photospheric magnetic fields
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DOI：
10.1086/187773
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发表时间：
1995-02
期刊：
影响因子：
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通讯作者：
A. Pevtsov;R. Canfield;T. Metcalf
中科院分区：
文献类型：
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作者：
A. Pevtsov;R. Canfield;T. Metcalf
利用1988-1994年的原始光球矢量磁图数据集以及已发表的数据，我们采用线性无力场参数α作为量度，研究了69个不同活动区的平均磁螺旋度。通过最小化计算的恒定α无力场和观察到的水平磁场之间的差异来确定该平均值。平均磁螺旋度在两半球的2 σ水平上有符号差异。在我们的数据集中，76%的活动区在北方半球有负螺旋度，69%的南半球，正。虽然数据显示从一个活动区到下一个活动区有相当大的变化，但数据集作为一个整体表明，平均螺旋度的大小随着太阳纬度的增加而增加，从赤道附近的零开始，在两个半球的15度-25度附近达到最大值，然后在35度-40度以上向较小的值回落。与已发表的模型的定性比较表明，这种纬向变化的平均磁螺旋度可能会导致湍流对流运动或差异旋转，虽然我们的旋转太阳黑子的研究导致我们倾向于前者。
Using a 1988-1994 data set of original photospheric vector magnetograms as well as published data, we have studied the average magnetic helicity of 69 diverse active regions, adopting the linear force-free field parameter alpha as a measure. This average value was determined by minimizing the differences between the computed constant-alpha force-free and observed horizontal magnetic fields. The average magnetic helicity shows a sign difference at the 2 sigma level in opposite hemispheres. In our data set, 76% of the active regions in the northern hemisphere have negative helicity, and 69% in the southern hemisphere, positive. Although the data show considerable variation from one active region to the next, the data set as a whole suggest that the magnitude of the average helicity increases with solar latitude, starting at zero near the equator, reaches a maximum near 15 deg - 25 deg in both hemispheres, and drops back toward smaller values avove 35 deg - 40 deg. Qualitative comparison with published models shows that such latitudinal variation of the average magnetic helicity may result from either turbulent convective motions or differential rotation, although our studies of rotating sunspots lead us to favor the former.