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

<p>The <span class="math-tex">\(\alpha\)</span>&nbsp;effect is believed to play a key role in the generation&nbsp;of the solar magnetic field. A fundamental test for its significance in&nbsp;the solar dynamo is to look for magnetic helicity of opposite signs&nbsp;in the two hemispheres, and at small and large scales. However, measuring magnetic helicity is compromised by the inability to fully infer the&nbsp;magnetic field vector from observations of solar spectra,&nbsp;caused by what is known as the <span class="math-tex">\(\pi\)</span>&nbsp;ambiguity of&nbsp;spectropolarimetric observations.&nbsp;We decompose linear polarisation into parity-even and parity-odd <em>E</em> and <em>B</em> polarisations,&nbsp;which are not affected by the <span class="math-tex">\(\pi \)</span> ambiguity.&nbsp;Furthermore, we study whether the correlations of spatial Fourier&nbsp;spectra of <em>B</em>&nbsp;and parity-even quantities such as <em>E&nbsp;</em>or&nbsp;temperature <em>T</em> are a robust proxy for magnetic helicity of solar magnetic fields.&nbsp; We analyse polarisation measurements of active regions observed by the&nbsp;Helioseismic and Magnetic Imager on board the Solar Dynamics observatory. Theory predicts&nbsp;the magnetic helicity of active regions to have, statistically, opposite signs in the two hemispheres.&nbsp;We then compute the parity-odd <em>EB</em> and <em>TB</em> correlations, and test for systematic preference of&nbsp;their sign based on the hemisphere of the active regions.&nbsp;We find that: (i) <em>EB</em> and <em>TB</em> correlations are a reliable proxy for magnetic helicity, when&nbsp;computed from linear polarisation measurements away from spectral line cores, and (ii)&nbsp;<em>E</em>&nbsp;polarisation reverses its sign close to the line core. Our analysis reveals Faraday&nbsp;rotation to not have a significant influence on the computed parity-odd correlations.&nbsp;The <em>EB</em>&nbsp;decomposition of linear polarisation appears to be a good proxy for magnetic helicity&nbsp;independent of the <span class="math-tex">\(\pi\)</span>&nbsp;ambiguity. This allows us to routinely infer magnetic helicity&nbsp;directly from polarisation measurements.</p> <p>The full article can be found at&nbsp;https://arxiv.org/abs/2001.10884</p>