Global oscillations in a magnetic solar model^*

II. Oblique propagation

¹ Solar System Physics Group, Institute of Mathematical and Physical Sciences, The University of Wales, Aberystwyth, Penglais Campus, Aberystwyth, Ceredigion, SY23 3BZ, Wales, UK e-mail: b.pinter@aber.ac.uk
² Solar Physics and Space Plasma Research Centre (SPRC), Department of Applied Mathematics, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7RH, England, UK e-mail: robertus@sheffield.ac.uk
³ Centre for Plasma-Astrophysics, Departement Wiskunde, Faculteit Wetenschappen, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium e-mail: Marcel.Goossens@wis.kuleuven.ac.be

Received: 9 July 2006
Accepted: 29 January 2007

Abstract

The coupling of solar global acoustic oscillations to a magnetised solar atmosphere is studied here. The solar interior – atmosphere interface is modelled by a non-magnetic polytrope interior overlayed by a planar atmosphere embedded in non-uniform horizontal atmospheric magnetic field. Pintér & Goossens (1999, A&A, 347, 321) showed that parallel propagating acoustic waves can couple resonantly to local magnetohydrodynamic (MHD) slow continuum modes only. In general, global acoustic modes can, however, propagate in arbitrary directions with respect to local atmospheric fields giving rise to an additional efficient coupling mechanism that has consequences on mode damping and atmospheric energetics. In this paper we study obliquely propagating global modes that can couple also to local MHD Alfvén continuum modes. The atmospheric magnetic effects on global mode frequencies are still much of a debate. In particular, the resulting frequency shifts and damping rates of global modes caused by the resonant interaction with both local Alfvén and slow waves are investigated. We found the coupling of global f and p modes and the Lamb mode, that penetrate into the magnetic solar atmosphere, will strongly depend on the direction of propagation with respect to the solar atmospheric magnetic field. These frequency shifts, as a function of the propagation direction, give us a further elegant tool and refinement method of local helioseismology techniques. Finally we briefly discuss the importance of studying obliquely propagating waves and discuss the results in the context of possible helioseismic observations.