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

Abstract

The recently suggested swing interaction between fast magnetosonic and Alfvén 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 Alfvén waves propagating along the field through the periodical variation of the Alfvén speed. The resonant Alfvén 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 Alfvén speed. Therefore, if the medium is bounded along the magnetic field, then the harmonics of the Alfvén 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 Alfvén waves. We refer to this phenomenon as 'Swing Absorption'. This mechanism can be of importance in various astrophysical situations.