Computer simulations of electromagnetic cool ion beam instabilities

Abstract

Electromagnetic instabilities driven by a relatively cool ion beam are studied by one‐dimensional hybrid computer simulations. Both the beam and the instabilities propagate parallel or antiparallel to a uniform magnetic field. At relatively small beam‐core relative drift speeds, similar to conditions predicted for ion beams upstream of slow shocks in the magnetotail, the right‐hand resonant instability exhibits quasi‐linear behavior. In particular, instability saturation is due to a reduction in the beam‐core relative drift speed as well as to an increase in the perpendicular‐to‐parallel beam temperature. For tenuous beams in this quasi‐linear regime the fluctuating magnetic field amplitude at saturation scales as the beam drift kinetic energy. At higher drift speeds, including parameters similar to those upstream of the quasi‐perpendicular bow shock, the right‐hand resonant and nonresonant instabilities both lead to phase bunching of the ion beam. The relative phase between the fluctuating velocity vector of the beam and the fluctuating magnetic field is examined; computed values agree with both linear theory during the early growth phase and nonlinear arguments at saturation. In this nonlinear regime both instabilities saturate when approximately half the initial beam drift kinetic energy density is converted to fluctuating magnetic field energy density.