Perturbations of a twisted solar coronal loop: The relation between surface waves and instabilities

Abstract

The spectrum of propagating waves and instabilities on a current-carrying, zero gas pressure, twisted magnetic flux loop is analysed for several models of the magnetic field structure. A surface wave mode of the fast Alfven wave is found to exist, with damping of the wave when Alfven resonance absorption occurs. If the loop is surrounded by a uniform, purely axial magnetic field, then the surface wave is always stable. If the loop is surrounded by a nonuniform field which is continuous with the loop's field, then the surface wave may connect to the unstable external kink mode.