Damping of helioseismic modes in steady state

¹ School of Science and Mathematics, Hallam University, Howard Street, Sheffield S1 1WB, UK e-mail: r.new@shu.ac.uk
² Space & Atmosphere Research Center, Dept. of Applied Mathematics, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK e-mail: Robertus@sheffield.ac.uk

Corresponding author: B. Pintér, b.pinter@shu.ac.uk

Received: 26 March 2001
Accepted: 12 April 2001

Abstract

The effects of an equilibrium flow in the internal regions of the Sun are studied on the damping of helioseismic f- and p-modes. The Sun is modeled as a multi-layered plasma, where the upper parts, representing the chromosphere and corona, are embedded in a horizontally unidirectional though vertically inhomogeneous magnetic field, while the lower part, representing the sub-photospheric polytropic region, is in a steady equilibrium state. The steady state sub-surface region can be considered as a first approximation of dynamic motions (e.g., differential rotation, sub-surface flows, meridional flows, convective motion, etc.). The frequencies and the line-widths of eigenmodes are affected by sub-surface flow and atmospheric magnetic fields. A key contribution to the effects comes from the universal mechanism of resonant absorption. When both atmospheric magnetic field and sub-surface flows are present, a complex picture of competition between these two effects is found. The theoretically predicted frequency shifts in a steady state show promise of explaining the observed effects. Changes in damping of f- and p-modes caused by changes (e.g. cyclic, if any) of steady state flows are predicted.