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

A. A. Vidotto, L. T. Lehmann, M. Jardine, A. A. Pevtsov

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Abstract

In this 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 January to 2018 April. 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  per cent 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 (⟨B2tor⟩) and poloidal (⟨B2pol⟩) energies are related as ⟨B2tor⟩  α ⟨B2pol1.38±0.04. Similar to the stellar sample, the large-scale field of the Sun shows a lack of toroidal non-axisymmetric field.
Original languageEnglish
Pages (from-to)477-487
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Volume480
Issue number1
Early online date19 Jul 2018
DOIs
Publication statusPublished - 11 Oct 2018

Keywords

  • Methods: analytical
  • Magnetic topology
  • Surface magnetism
  • Magnetic fields

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