Nonlinear evolution of electromagnetic ion beam instabilities

Abstract

An ion beam streaming along a magnetic field is known to be mainly unstable to two field-aligned, low-frequency electromagnetic instabilities: the ion/ion right-hand resonant instability and ion/ion nonresonant instability. A comparative study of the two instabilities by means of linear analysis and simulation is carried out to investigate and contrast their properties. Linear analysis demonstrates that the nonresonant instability becomes resonant if the density of the ion beam is sufficiently high. When the relative drift speed between the ion beam and the ambient ions is 10 υ_A, where υ_A is the Alfvén speed, the nonresonant instability undergoes a transition to a resonant interaction at n_b/n_o≃0.065, n_b(n_o) being the beam (total ion) density. Hybrid simulations carried out for the densities of n_b/n_o = 0.02, 0.1, and 0.25 show both the resonant and nonresonant instabilities result in the formation of nonlinear pulses called pulsations but with distinct features. For example, the pulsations generated by the resonant instability have positive correlation with the ion density, while those generated by the nonresonant instability are likely to have relatively weak negative correlation. Furthermore, the waves generated by the nonresonant instability are subject to a parametric decay instability and tend to form a state of condensate where the turbulence becomes nearly monochromatic. These features can be utilized to differentiate between pulsations produced by the two instabilities in space and, in particular, short large amplitude magnetic structures.