GRAVITATIONAL DAMPING OF ALFVEN WAVES IN STELLAR ATMOSPHERES AND WINDS

We consider how gravity affects the propagation of Alfven waves in a s tellar atmosphere. We show that when the ion gyrofrequency exceeds the collision rate, the waves are absorbed at a rate proportional to the gravitational acceleration g. Estimates show that this mechanism can r eadily account for the observed energy losses in the solar chromospher e. The mechanism predicts that the pressure at the top of the chromosp here P(Tc) should scale with g as P(Tc) is-proportional-to g(delta), w here delta almost-equal-to 2/3; this is close to empirical results whi ch suggest delta almost-equal-to 0.6. Gravitational damping leads to d eposition of energy at a rate proportional to the mass of the particle s. Hence, heavier ion are heated more effectively than protons. This i s consistent with the observed proportionality between ion temperature and mass in the solar wind. Gravitational damping causes the local g to be effectively decreased by an amount proportional to the wave ener gy. This feature affects the acceleration of the solar wind. Gravitati onal damping may also lead to self-regulation of the damping of Alfven waves in stellar winds: this is relevant in the context of slow massi ve winds in cool giants.