Mode classification and wave propagation in a magnetically structured medium

Abstract

The authors examine the structure of motions that can occur in a vertical magnetic flux tube with a rectangular cross-section. A polytropic stratification is assumed in the vertical direction. The authors use a gauged version of Helmholtz's theorem, the decompose the perturbations into an irrotational component and a solenoidal component, which they further split into the sum of poloidal and toroidal components. These components are identified with p, g and toroidal modes of a fluid. The normal modes of the tube are determined using a Rayleigh-Ritz variational technique. The authors' technique efficiently isolates all the modes to high orders. They first consider some special cases, in order to highlight some interesting properties of the modes. Then, they choose a parameter range to study the properties of oscillations in intense flux tubes on the Sun.