Cross-sectional area and intensity variations of sausage modes^⋆

Centre for mathematical Plasma Astrophysics, Mathematics Department, KU Leuven, Celestijnenlaan 200B bus 2400, 3001 Leuven, Belgium
e-mail: michael.moreels@wis.kuleuven.be

Received: 22 March 2013
Accepted: 2 May 2013

Abstract

Context. The observations obtained using the Rapid Oscillations in the Solar Atmosphere instrument (ROSA) show variations in both cross-sectional area and intensity for magnetic pores in the photosphere.

Aims. We study the phase behaviour between cross-sectional area and intensity variations for sausage modes in a photospheric context. We aim to determine the wave mode by looking at the phase difference between the cross-sectional area and intensity variations.

Methods. We used a straight cylinder as a model for the flux tube. The plasma is uniform both inside and outside the flux tube with a possible jump in the equilibrium values at the boundary, the magnetic field is directed along the flux tube. We derived analytic expressions for the cross-sectional area variation and the total intensity variation. Using these analytic expressions, we calculated the phase differences between the cross-sectional area and the intensity variations. These phase differences were then used to identify the wave mode.

Results. We found that for slow sausage modes the cross-sectional area and intensity variations are always in phase, while for fast sausage modes the variations are in antiphase.