Dataset for "Helicity proxies from linear polarisation of solar active regions"

The \(\alpha\) effect is believed to play a key role in the generation of the solar magnetic field. A fundamental test for its significance in the solar dynamo is to look for magnetic helicity of opposite signs in the two hemispheres, and at small and large scales. However, measuring magnetic helicity is compromised by the inability to fully infer the magnetic field vector from observations of solar spectra, caused by what is known as the \(\pi\) ambiguity of spectropolarimetric observations. We decompose linear polarisation into parity-even and parity-odd E and B polarisations, which are not affected by the \(\pi \) ambiguity. Furthermore, we study whether the correlations of spatial Fourier spectra of B and parity-even quantities such as E or temperature T are a robust proxy for magnetic helicity of solar magnetic fields.  We analyse polarisation measurements of active regions observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics observatory. Theory predicts the magnetic helicity of active regions to have, statistically, opposite signs in the two hemispheres. We then compute the parity-odd EB and TB correlations, and test for systematic preference of their sign based on the hemisphere of the active regions. We find that: (i) EB and TB correlations are a reliable proxy for magnetic helicity, when computed from linear polarisation measurements away from spectral line cores, and (ii) E polarisation reverses its sign close to the line core. Our analysis reveals Faraday rotation to not have a significant influence on the computed parity-odd correlations. The EB decomposition of linear polarisation appears to be a good proxy for magnetic helicity independent of the \(\pi\) ambiguity. This allows us to routinely infer magnetic helicity directly from polarisation measurements.

The full article can be found at https://arxiv.org/abs/2001.10884

学界普遍认为，α效应（α effect）在太阳磁场的生成过程中发挥着核心作用。验证其在太阳发电机（solar dynamo）模型中的重要性的一项核心手段，便是在两个半球以及小、大尺度上寻找符号相反的磁螺旋度（magnetic helicity）。然而，磁螺旋度的测量工作面临瓶颈：偏振光谱观测（spectropolarimetric observations）中存在所谓的π歧义（π ambiguity），导致我们无法通过太阳光谱观测完全推知磁矢场（magnetic field vector），这直接制约了磁螺旋度的测量。

我们将线偏振（linear polarisation）分解为不受π歧义影响的偶宇称（parity-even）E偏振（E polarisation）与奇宇称（parity-odd）B偏振（B polarisation）。此外，我们探究了B与偶宇称量（如E偏振或温度T）的空间傅里叶谱（spatial Fourier spectra）之间的相关性，能否作为太阳磁场磁螺旋度的可靠替代指标。

我们分析了搭载于太阳动力学天文台（Solar Dynamics Observatory）的日震与磁像仪（Helioseismic and Magnetic Imager, HMI）所获取的活动区（active regions）偏振观测数据。理论预测，活动区的磁螺旋度在统计层面会于两个半球呈现相反的符号。随后我们计算了奇宇称的EB与TB相关性，并基于活动区所在的半球，检验其符号是否存在系统性偏好。

我们得到如下结论：（i）若从远离光谱线核（spectral line cores）的线偏振观测数据中计算，EB与TB相关性可作为磁螺旋度的可靠替代指标；（ii）E偏振在接近光谱线核时会发生符号反转。我们的分析显示，法拉第旋转（Faraday rotation）对所计算的奇宇称相关性无显著影响。线偏振的EB分解方法似乎可作为不受π歧义影响的磁螺旋度替代指标，这使得我们能够直接通过偏振观测常规推知磁螺旋度。

完整论文可于https://arxiv.org/abs/2001.10884 查阅。