Abstract
Context. The α 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 both between the two hemispheres as well as between 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 π ambiguity of spectropolarimetric observations. Aims. We decompose linear polarisation into parity-even and parity-odd E and B polarisations, which are not affected by the π 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. Methods. We analysed 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 computed the parity-odd EB and TB correlations and tested for a systematic preference of their sign based on the hemisphere of the active regions. Results. 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 that Faraday rotation does not have a significant influence on the computed parity-odd correlations. Conclusions. The EB decomposition of linear polarisation appears to be a good proxy for magnetic helicity independent of the π ambiguity. This allows us to routinely infer magnetic helicity directly from polarisation measurements.
Highlights
Astrophysical bodies such as stars, galaxies, and planets are known to posses magnetic fields, typically on scales as large as those systems themselves
The study is motivated by an earlier work by Brandenburg et al (2019), who demonstrated that the EB decomposition of linear polarisation can, under inhomogeneous conditions, be a proxy for magnetic helicity
We looked at individual active regions (ARs) from both hemispheres, observed with Solar Dynamics Observatory (SDO)/HMI, and recovered significant EB correlations, and a systematic dependence of its sign on hemisphere
Summary
Astrophysical bodies such as stars, galaxies, and planets are known to posses magnetic fields, typically on scales as large as those systems themselves. A more global approach, taking into account the change in sign of helicity across the equator was developed by Brandenburg et al (2017), called the two-scale approach after Roberts & Soward (1975) This was followed by a systematic study employing this two-scale approach over a large sample of Carrington rotations from solar cycle 24 by Singh et al (2018). Brandenburg et al (2019) used this EB decomposition and tested it with full disk polarisation data from the Vector SpectroMagnetograph (VSM) instrument of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project They did not find significant nonzero parity-odd correlations from their analysis.
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