ON THE PROPAGATION OF IDEAL, LINEAR ALFVEN WAVES IN RADIALLY STRATIFIED STELLAR ATMOSPHERES AND WINDS

The propagation of Alfven waves through isothermal, radially stratified, spherically symmetrical models of stellar atmospheres and winds is discussed. The transmission coefficient for the waves is calculated as a function of frequency, magnetic field base intensity, surface gravity and atmospheric temperature. When a wind is present, the wave energy flux is no longer conserved, but the conservation of the wave-action flux (Heinemann & Olbert 1980) allows the definition of an analogous transmission coefficient, giving the relative amount of waves reaching the super-Alfvenic regions of the wind. It is shown that for high-frequency waves the transmission coefficient for static and wind models is identical, while for low-frequency waves the presence of a wind enhances the transmission considerably. The latter are however totally reflected asymptotically, far from the stellar surface, a behaviour which is reminiscent of the observed evolution of the ''Alfvenicity'' of turbulence in the solar wind. Recent isotropic models for Alfvenic turbulence which display the same qualitative behaviour are compared to the low-frequency limit of the linear equations. Finally, it is argued that models for the mass loss of cool giants and super-giants which properly treat the reflection of Alfven waves might overcome the difficulties inherent in standard (WKB) wave-driven models.