MAGNETOHYDRODYNAMIC WAVES LINEAR EVOLUTION IN PARALLEL SHEAR FLOWS - AMPLIFICATION AND MUTUAL TRANSFORMATIONS

Evolution of three-dimensional magnetohydrodynamic (MHD) waves [fast m agnetosonic (FMW), slow magnetosonic (SMW) and Alfven waves] is studie d in unbounded parallel flows with uniform shear of velocity and unifo rm magnetic field directed along the flow. The energy exchange between the MHD waves and background flow is explored. This process is notice ably different for each type of wave and is characterized by the unusu al (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 limi ted time interval for a wide range of systems (flow and waves) paramet ers, Significant transformation of Alfven waves into FMW may take plac e (depending on the parameters of the system) if the former has been i nitially generated in shear flow. It is possible to reveal these resul ts by employing the nonmodal linear approach which has been extensivel y used in the study of evolution of perturbations in shear flows since the beginning of the 1990s. The change in the understanding of flow t urbulence due to the coupling of the MHD modes is discussed. Namely, t he usual consideration of just Alfven wave turbulence in some astrophy sical flows is not always sufficient for a complete analysis-not only should Alfven waves be ''ingredients'' of turbulence, but magnetosonic waves, as well. In this case MHD turbulence should be of a ''mixed'' type. (C) 1997 American Institute of Physics.