Comment on “Gravitational Damping of Alfven Waves in Stellar Atmospheres and Winds”

Abstract

Khabibrakhmanov & Mullan considered Alfven wave propagation in a gravitational field. They pointed out that an ion can drift along the wave electric field, and gain energy if the wave propagates upward. They interpreted this result in terms of plasma heating and wave damping. We offer an alternative interpretation. We show that the Lorentz force associated with the drift in question is the force that guides the particle along the magnetic field lines. This force is analogous to the surface force exerted by an inclined plane on an object which is sliding on it. The electric field implies a moving magnetic field line, which is analogous to a moving inclined plane. The energy gained as a particle drifts along the electric field is shown to be analogous to the work done by the surface force which is exerted by a moving inclined plane. Thus the energy gain does not represent heating. It is simply the work done by the force which guides a particle along a moving magnetic field line.