The Parametric Instability of Alfvén Waves: Effects of Temperature Anisotropy

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 = p0 /p0 > 1. When the pressure anisotropy exceeds a critical value, ≥ * with * 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 < *, 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 ( , β ) parameter space and discuss possible implications for Alfvénic turbulence in the solar wind.