Generation of nonlinear Alfven and magnetosonic waves by beam-plasma interaction

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 B-0. 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 =10V(A), where V-A is the Alfven 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 Alfven waves with right-hand polarization in the magnetic field are generated. The magnetohydrodynamic (MHD) wave conditions of the Alfven mode are satisfied. These Alfven waves propagate mainly with k.B-0>0, and the dispersion relation omega=kV(A) cos alpha is satisfied, where alpha is the angle between the wave vector k and B-0. 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 ksimilar tok(perpendicular to)) in the density and magnetic field can be present due to the 2-D effects, in addition to the Alfven, fast, and slow modes. The heating rate of background ions and its dependence on the wave propagation direction are also examined. (C) 2003 American Institute of Physics.