Propagation of torsional Alfven waves in an inhomogeneous plasma

In ideal MHD, the behaviour of the torsional Alfven wave in a cylindrical, inhomogeneous plasma depends on the m spectrum of the wave, where m is the azimuthal mode number. It is well known that a pure m=0 mode is phase-mixed by a density gradient and that modes with integer m are coupled to the fast wave at an Alfven resonance layer. The author considers the propagation of a narrow, magnetically guided beam in an inhomogeneous, current-free plasma. Such a beam contains an infinite spectrum of m components. It is shown that a guided beam is decoupled from the fast wave, despite the fact that omega lies in the Alfven continuum, and it is not phase-mixed by the density gradient. The relation omega /k z = nu A (x,y) describing modes with zero or integer m is therefore not appropriate for a wave containing an infinite spectrum of m components.