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A Global Two-scale Helicity Proxy from pi-ambiguous Solar Magnetic Fields
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA; Carnegie Mellon University, USA.ORCID iD: 0000-0002-7304-021X
Number of Authors: 12019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 883, no 2, article id 119Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2019. Vol. 883, no 2, article id 119
Keywords [en]
dynamo, magnetic fields, polarization, techniques: polarimetric
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-175014DOI: 10.3847/1538-4357/ab3ec0ISI: 000488058300009OAI: oai:DiVA.org:su-175014DiVA, id: diva2:1367275
Available from: 2019-11-01 Created: 2019-11-01 Last updated: 2019-12-12Bibliographically approved

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