Electromagnetic instabilities driven by cool heavy ion beams

Abstract

The linear and nonlinear behavior of the right‐hand resonant and nonresonant electromagnetic ion beam instabilities driven by cool, β ∼ 1 ion beams is investigated as a function of beam mass and density. When the heavy ion beam is the less dense component, the maximum growth rate of the resonant mode and the threshold velocity of the nonresonant mode decrease with beam mass, while the maximum growth rate of the nonresonant mode and the threshold velocity of the resonant mode are nearly independent of the mass of the beam. When the heavy ion beam is the more dense component, a reversal of roles occurs, with the properties of the instabilities being determined by the sparser core ions. The nonlinear properties obtained from simulation show a similar behavior. At low beam densities the magnetic field fluctuation level increases with beam mass; at higher beam density it depends on the core mass. Very large magnetic fluctuations (δB/B0 ∼ 3), and smaller but still sizeable density fluctuations that do not correlate with the magnetic fluctuations, are observed and shown to agree well with those derived from theory. Applications to comets and Venus are suggested.