Magnetohydrodynamic waves linear evolution in parallel shear flows: Amplification and mutual transformations

Abstract

Evolution of three-dimensional magnetohydrodynamic (MHD) waves [fast magnetosonic (FMW), slow magnetosonic (SMW) and Alfvén waves] is studied in unbounded parallel flows with uniform shear of velocity and uniform magnetic field directed along the flow. The energy exchange between the MHD waves and background flow is explored. This process is noticeably different for each type of wave and is characterized by the unusual (algebraic) behavior of the linear amplification processes. Another novelty is shown in the wave linear evolution process — the coupling of MHD waves and their mutual transformations are originated in a limited time interval for a wide range of systems (flow and waves) parameters. Significant transformation of Alfvén waves into FMW may take place (depending on the parameters of the system) if the former has been initially generated in shear flow. It is possible to reveal these results by employing the nonmodal linear approach which has been extensively used in the study of evolution of perturbations in shear flows since the beginning of the 1990s. The change in the understanding of flow turbulence due to the coupling of the MHD modes is discussed. Namely, the usual consideration of just Alfvén wave turbulence in some astrophysical flows is not always sufficient for a complete analysis-not only should Alfvén waves be “ingredients’’ of turbulence, but magnetosonic waves, as well. In this case MHD turbulence should be of a “mixed’’ type.