Magnetohydrodynamic waves amplification and mutual transformations in parallel shear flows

Evolution of three dimensional magnetohydrodynamic (MHD) waves (fast magnetosonic (FMW), slow magnetosonic (SMW) and Alfven 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) behaviour for linear amplification processes. Another novelty is demonstrated in wave linear evolution process - coupling of MHD waves and their mutual transformations are originated in a limited time interval for a wide range of system (flow and waves) parameters. Revelation of the results becomes possible owing to the nonmodal linear approach. The change of understanding of flow turbulence due to the coupling of the MHD modes is discussed. (author)