The mechanism of Swing Absorption of fast magnetosonic waves in inhomogeneous media

The recently suggested swing interaction between fast magnetosonic and Alfv'en waves (2002) is generalized to inhomogeneous media. We show that the fast magnetosonic waves propagating across an applied non-uniform magnetic field can parametrically amplify the Alfv'en waves propagating along the field through the periodical variation of the Alfv'en speed. The resonant Alfv'en waves have half the frequency and the perpendicular velocity polarization of the fast waves. The wavelengths of the resonant waves have different values across the magnetic field, due to the inhomogeneity in the Alfv'en speed. Therefore, if the medium is bounded along the magnetic field, then the harmonics of the Alfv'en waves, which satisfy the condition for onset of a standing pattern, have stronger growth rates. In these regions the fast magnetosonic waves can be strongly ‘absorbed’, their energy going in transversal Alfv'en waves. We refer to this phenomenon as ‘Swing Absorption’. This mechanism can be of importance in various astrophysical situations.