On over-reflection and generation of Gravito-Alfven waves in solar-type stars

Abstract

The dynamics of linear perturbations is studied in magnetized plasma shear flows with a constant shearing rate and with gravity-induced stratification. The general set of linearized equations is derived and the two-dimensional case is considered in detail. The Boussinesq approximation is used in order to examine relatively small-scale perturbations of low-frequency modes: Gravito-Alfven waves (GAW) and Entropy Mode (EM) perturbations. It is shown that for flows with arbitrary shearing rate there exists a finite time interval of non-adiabatic evolution of the perturbations. The non-adiabatic behavior manifests itself in a twofold way, viz. by the over-reflection of the GAWs and by the generation of GAWs from EM perturbations. It is shown that these phenomena act as efficient transformers of the equilibrium flow energy into the energy of the perturbations for moderate and high shearing rate solar plasma flows. Efficient generation of GAW by EM takes place for shearing rates about an order of magnitude smaller than necessary for development of a shear instability. The latter fact could have important consequences for the problem of angular momentum redistribution within the Sun and solar-type stars.