The magnetic field vector of the Sun-as-a-star. II. Evolution of the large-scale vector field through activity cycle 24

TL;DR

This study examines the evolution of the Sun's large-scale magnetic field during cycle 24, revealing polarity reversals, energy distribution, and correlations with sunspot activity, and compares these findings with stellar magnetic trends.

Contribution

It introduces a spherical harmonic decomposition method to analyze the Sun's large-scale magnetic field and compares solar magnetic properties with stellar magnetic observations.

Findings

Polarity reversal observed in radial and meridional components.

Large-scale field remains mainly poloidal (>70%).

Correlation between toroidal energy and sunspot number.

Abstract

In the present work, we investigate how the large-scale magnetic field of the Sun, in its three vector components, has evolved during most of cycle 24, from 2010 Jan to 2018 Apr. To filter out the small-scale field of the Sun, present in high-resolution synoptic maps, we use a spherical harmonic decomposition method, which decomposes the solar field in multipoles with different l degrees. By summing together the low-l multipoles, we reconstruct the large-scale field at a resolution similar to observed stellar magnetic fields, which allows the direct comparison between solar and stellar magnetic maps. During cycle 24, the `Sun-as-a-star' magnetic field shows a polarity reversal in the radial and meridional components, but not in the azimuthal component. The large-scale solar field remains mainly poloidal with > 70% of its energy contained in the poloidal component. During its evolution, the large-scale field is more axisymmetric and more poloidal when near minima in sunspot numbers, and with a larger intensity near maximum. There is a correlation between toroidal energy and sunspot number, which indicates that spot fields are major contributors to the toroidal large-scale energy of the Sun. The solar large-scale magnetic properties fit smoothly with observational trends of stellar magnetism reported in See et al. The toroidal (Etor) and poloidal (Epol) energies are related as Etor ~Epol^{1.38 \pm 0.04}. Similar to the stellar sample, the large-scale field of the Sun shows a lack of toroidal non-axisymmetric field.