A global twoscale helicity proxy from piambiguous solar magnetic fields 

Axel Brandenburg Submitted: 20191015 22:42
If the α 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 weakfield 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 spin2 spherical harmonics to decompose the linear polarization in terms of the parityeven and parityodd E and B polarizations, respectively. Using simple one and twodimensional 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 pseudopolarization corresponding to a "piambiguated" 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 wellknown negative magnetic helicity at smaller scales. This method might also be applicable to stellar and Galactic polarization data.
Authors: Axel Brandenburg
Projects: CORONASF/RESIK

Publication Status: Astrophys. J. 883, 119 (2019)
Last Modified: 20191016 12:42


