The resonant absorption of p-modes by sunspots with twisted magnetic fields

Abstract

The observed absorption of solar p-mode flux by sunspots could provide a valuable diagnostic tool for probing the subsurface structure and evolution of the magnetic field of solar active regions once the physics of the process is understood. We consider a simplified inhomogeneous sunspot model with an axial current (twisted magnetic field). The absorption of incoming acoustic modes in a narrow resonance layer inside the sunspot flux tube is investigated, and the energy loss is estimated. For nonaxisymmetric modes the results are consistent with previous calculations. However, contrary to previous work, it is demonstrated that the existence of an azimuthal component of the magnetic field can lead to significant absorption of even the axisymmetric modes. If the absorption rate calculated in this paper is used in conjuction with the observed wavelength dependence of the absorption coefficient, it is found that the sunspot flux tube must have significant twist in the subsurface layers (B_0θ/B_0Z~ 0.56). Furthermore, the presence of twist in the magnetic field leads to natural explanation for the observed dependence on m, the azimuthal wave mode number, and the magnitude of the absorption coefficient can be accounted for in a self-consistent way.