The Sun's Mean Lineofsight Field
Abstract
In this paper, I regard the Sunasastar magnetic field (i.e., the mean field) as a filter for the spherical harmonic components of the photospheric field, and then calculate the transmission coefficients of this filter. The coefficients for each harmonic, Y _{ l } ^{ m }, are listed in three tables according to their dependence on B _{0}, the observer's latitude in the star's polar coordinate system. These coefficients are used to interpret the 46 yr sequence of daily meanfield measurements at the Wilcox Solar Observatory. I find that the nonaxisymmetric part of the field originates in the ${Y}_{1}^{1}$ , ${Y}_{2}^{2}$ , and a combination of the ${Y}_{3}^{3}$ and ${Y}_{3}^{1}$ harmonic components. The axisymmetric part of the field originates in ${Y}_{2}^{0}$ plus a B _{0}dependent combination of the ${Y}_{1}^{0}$ and ${Y}_{3}^{0}$ components. The power spectrum of the field has peaks at frequencies corresponding to the ~27 day synodic equatorial rotation period and its second and third harmonics. Each of these peaks has fine structure on its lowfrequency side, indicating magnetic patterns that rotate slowly under the influence of differential rotation and meridional flow. The sidebands of the fundamental mode resolve into peaks corresponding to periods of ~28.5 and ~30 days, which tend to occur at the start of sunspot maximum, whereas the ~27 day period tends to occur toward the end of sunspot maximum. We might expect similar rotational sidebands to occur in magnetic observations of other Sunlike stars and to be a useful complement to asteroseismology studies of convection and magnetic fields in those stars.
 Publication:

The Astrophysical Journal
 Pub Date:
 October 2022
 DOI:
 10.3847/15384357/ac86d6
 arXiv:
 arXiv:2208.03216
 Bibcode:
 2022ApJ...937...87S
 Keywords:

 Solar magnetic fields;
 Solar rotation;
 Solar cycle;
 Stellar magnetic fields;
 1503;
 1524;
 1487;
 1610;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 27 pages, 16 figures