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 (${\beta }_{\parallel }$) when $\xi =p_{0\perp }/p_{0\parallel }>1$. When the pressure anisotropy exceeds a critical value, $\xi \ge {\xi }^{\ast }$ with ${\xi }^{\ast }\simeq 2.7$, there is a new regime in which the parametric instability is no longer quenched at high ${\beta }_{\parallel }$ and in the limit ${\beta }_{\parallel }\gg 1$ the growth rate becomes independent of ${\beta }_{\parallel }$. In the opposite case of $\xi <{\xi }^{\ast }$, 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 $(\xi ,{\beta }_{\parallel })$ parameter space, and discuss possible implications for Alfvénic turbulence in the solar wind.

arXiv comment9 pages, 9 figures. Under review

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