Generation of broadband noise in the magnetotail by the beam acoustic instability

Abstract

Analytic properties of the ion beam acoustic instability are evaluated for reasonable magnetotail conditions and are found to be consistent with observations of broadband turbulence not only in the plasma sheet boundary layer but also in the lobes and central plasma sheet regions. Numerical calculations confirm many of the analytic results. Specifically, the dependence of the ion beam acoustic mode on particle distribution models is investigated. Without cold ions present the acoustic mode is stable. With cold streaming ions present the acoustic mode is unstable, but the growth rate is significantly enhanced when warm streaming boundary layer ions are simultaneously present. These results suggest that the interaction between the warm and cold ion streams is responsible for the peak in the electrostatic wave intensities observed within the plasma sheet boundary layer. In addition, the beam acoustic instability, for magnetotail models, is found to be due to resonant, not nonresonant, interactions. Relative streaming effects and dependence on ion species are considered and found to play significant roles in the behavior of beam acoustic growth as a function of wave normal angle and wave number.