The Parametric Instability of Alfven Waves: Effects of Temperature Anisotropy

We study the stability of large-amplitude, circularly polarized Alfven 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 Alfven wave parametric decay in magnetohydrodynamics ( MHD). Anisotropy allows instability over a much wider range of values of parallel plasma beta (beta(parallel to)) when xi= p(0 perpendicular to)/p(0 parallel to) > 1. When the pressure anisotropy exceeds a critical value, xi >= xi* with xi*similar or equal to 2.7, there is a new regime in which the parametric instability is no longer quenched at high beta(parallel to), and in the limit beta(parallel to) >> 1, the growth rate becomes independent of beta(parallel to). In the opposite case of xi < xi*, 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 (xi, beta(parallel to)) parameter space and discuss possible implications for Alfvenic turbulence in the solar wind.