Generation of nonlinear Alfvén and magnetosonic waves by beam–plasma interaction

Abstract

One-dimensional (1-D) and two-dimensional (2-D) hybrid simulations are carried out to study the interaction between a background plasma and an ion beam, whose velocity is parallel to the ambient magnetic field B0. It is found that the beam–plasma interaction and the associated wave evolution can be divided into four phases. The simulation results in phase 1 in the early stage of wave evolution are consistent with the linear theory. Right-hand nonresonant instabilities are present and dominant in cases with a relatively strong ion beam (e.g., the ratio of beam ion density to background ion density >0.06 for beam velocity =10VA, where VA is the Alfvén speed), while right-hand resonant instabilities are present in the weak beam cases. During phases 2 and 3, the waves grow to form nonlinear structure, and are then saturated. A detailed analysis shows that the wave evolution in these phases is through secondary instabilities associated with parametric decay or the wave modulation. In addition, it is shown for the first time from the self-consistent simulation that in the final phase, nonlinear shear Alfvén waves with right-hand polarization in the magnetic field are generated. The magnetohydrodynamic (MHD) wave conditions of the Alfvén mode are satisfied. These Alfvén waves propagate mainly with k⋅B0>0, and the dispersion relation ω=kVA cos α is satisfied, where α is the angle between the wave vector k and B0. On the other hand, fast magnetosonic/whistler waves and slow mode waves are formed in the final phase of weak beam cases. In the 2-D simulations, field-aligned filaments (with k∼k⊥) in the density and magnetic field can be present due to the 2-D effects, in addition to the Alfvén, fast, and slow modes. The heating rate of background ions and its dependence on the wave propagation direction are also examined.