# A global two-scale helicity proxy from $\pi$-ambiguous solar magnetic fields

## TL;DR

This paper introduces a novel global two-scale helicity proxy using spin-2 spherical harmonics to analyze solar magnetic fields, overcoming pi ambiguity issues and revealing large-scale magnetic helicity signs.

## Contribution

The authors develop a new method employing spin-2 spherical harmonics to measure global magnetic helicity spectra from polarized solar magnetic field data.

## Key findings

- Detected negative EB helicity proxy at degree ~6, indicating positive large-scale magnetic helicity.
- Method successfully distinguishes large-scale helicity signs despite pi ambiguity.
- Potential applicability to stellar and Galactic polarization observations.

## Abstract

If the alpha effect plays a role in the generation of the Sun's magnetic field, the field should show evidence of magnetic helicity of opposite signs at large and small length scales. Measuring this faces two challenges: (i) in weak-field regions, horizontal field measurements are unreliable because of the pi ambiguity, and (ii) one needs a truly global approach to computing helicity spectra in the case where one expects a sign reversal across the equator at all wavenumbers. Here we develop such a method using spin-2 spherical harmonics to decompose the linear polarization in terms of the parity-even and parity-odd E and B polarizations, respectively. Using simple one- and two-dimensional models, we show that the product of the spectral decompositions of E and B, taken at spherical harmonic degrees that are shifted by one, can act as a proxy of the global magnetic helicity with a sign that represents that in the northern hemisphere. We then apply this method to the analysis of solar synoptic vector magnetograms, from which we extract a pseudo-polarization corresponding to a "pi-ambiguated" magnetic field, i.e., a magnetic field vector that has no arrow. We find a negative sign of the global EB helicity proxy at spherical harmonic degrees of around 6. This could indicate a positive magnetic helicity at large length scales, but the spectrum fails to capture clear evidence of the well-known negative magnetic helicity at smaller scales. This method might also be applicable to stellar and Galactic polarization data.