Velocity shear generated Alfvén waves in electron–positron plasmas

Abstract

Linear magnetohydrodynamic (MHD) modes in a cold, nonrelativistic electron–positron plasma shear flow are considered. The general set of differential equations, describing the evolution of perturbations in the framework of the nonmodal approach is derived. It is found, that under certain circumstances, the compressional and shear Alfvén perturbations may exhibit large transient growth fueled by the mean kinetic energy of the shear flow. The velocity shear also induces mode coupling, allowing the exchange of energy as well as the possibility of a strong mutual transformation of these modes into each other. The compressional Alfvén mode may extract the energy of the mean flow and transfer it to the shear Alfvén mode via this coupling. The relevance of these new physical effects to provide a better understanding of the laboratory e+e− plasmas is emphasized. It is speculated that the shear-induced effects in the electron–positron plasmas could also help solve some astrophysical puzzles (e.g., the generation of pulsar radio emission). Since most astrophysical plasmas are relativistic, it is shown that the major results of the study remain valid for weakly sheared relativistic plasmas.