Internal magnetic fields inferred from helioseismic data

Abstract

Measuring the internal solar magnetic fields and how they change over the course of a solar cycle is one of the key aims of helioseismology. We present the results of attempts to model the global mode splitting coefficients over solar cycle 23, assuming that the frequency splitting is only due to rotation and a large-scale magnetic field. The first results using only the a2 coefficients show that the data are best fit by a combination of a poloidal field and a double-peaked near-surface toroidal field. The toroidal fields are centered at r₀=0.999 R_⊙ and r₀=0.996 R_⊙ and are confined to the near-surface layers. The poloidal field is a dipole field. The peak strength of the poloidal field is 124±17 G. The toroidal field peaks at 380±30 G and 1.4±0.2 kG for the shallower and deeper fields, respectively. The field strengths are highly correlated with surface activity. We also examine the differences between the minima at the beginning and the end of solar cycle 23.