The Parametric Instability of Alfvén Waves: Effects of Temperature Anisotropy

Abstract

We study the stability of large-amplitude, circularly polarized Alfvén waves in an anisotropic plasma described by the double-adiabatic/CGL closure, and in particular the effect of a background thermal pressure anisotropy on the well-known properties of Alfvén wave parametric decay in magnetohydrodynamics (MHD). Anisotropy allows instability over a much wider range of values of parallel plasma beta ( β ∥ ) when ξ  =  p 0⊥ / p 0∥  > 1. When the pressure anisotropy exceeds a critical value, ξ  ≥  ξ * with ξ * ≃ 2.7, there is a new regime in which the parametric instability is no longer quenched at high β ∥ , and in the limit β ∥  ≫ 1, the growth rate becomes independent of β ∥ . In the opposite case of ξ  <  ξ *, the instability is strongly suppressed with increasing parallel plasma beta, similarly to the MHD case. We analyze marginal stability conditions for parametric decay in the ( ξ , β ∥ ) parameter space and discuss possible implications for Alfvénic turbulence in the solar wind.