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KINETIC ALFVÉN WAVE GENERATION by LARGE-SCALE PHASE MIXING

Abstract:

One view of the solar wind turbulence is that the observed highly anisotropic fluctuations at spatial scales near theroton inertial length dp may be considered as kinetic Alfvén waves (KAWs). In theresentaper, we show howhase mixing of large-scalearallel-propagating Alfvén waves is an efficient mechanism for theroduction of KAWs at wavelengths close to dp and at a largeropagation angle with respect to the magnetic field. Magnetohydrodynamic (MHD), Hall magnetohydrodynamic (HMHD), and hybrid Vlasov-Maxwell (HVM) simulations modeling theropagation of Alfvén waves in inhomogeneouslasmas areerformed. In the linear regime, the role of dispersive effects is singled out by comparing MHD and HMHD results. Fluctuationsroduced byhase mixing are identified as KAWs through a comparison ofolarization of magnetic fluctuations and wave-group velocity with analytical linearbkp_redictions. In the nonlinear regime, a comparison of HMHD and HVM simulations allows us tooint out the role of kinetic effects in shaping theroton-distribution function. We observe the generation of temperature anisotropy with respect to the local magnetic field and theroduction of field-aligned beams. The regions where theroton-distribution function highly departs from thermal equilibrium are located inside the shear layers, where the KAWs are excited, this suggesting that the distortions of theroton distribution are driven by a resonant interaction ofrotons with KAW fluctuations. Our results are relevant in configurations where magnetic-field inhomogeneities areresent, as, for example, in the solar corona, where theresence of Alfvén waves has been ascertained.