Gyrokinetic theory of parametric decays of kinetic Alfvén waves

¹ Institute for Fusion Theory and Simulation, Zhejiang University - Hangzhou 310027, PRC
² Department of Physics and Astronomy, University of California - Irvine, CA 92697-4575, USA
³ Associazione Euratom-ENEA sulla Fusione, C.R. Frascati - C.P. 65, 00044 Frascati, Italy, EU

^a liuchen@uci.edu

Received: 11 August 2011
Accepted: 9 September 2011

Abstract

The fundamental parametric decay processes of kinetic Alfvén waves (KAW) have been reexamined by employing the nonlinear gyrokinetic equations. Dispersion relations, valid for arbitrary k_⊥ρ_i, are derived for parametric decays to KAW and ion sound waves. Here, k_⊥ and ρ_i are, respectively, the wave number perpendicular to the magnetic field and the ion Larmor radius. It is found that, contrary to the small k_⊥ρ_i drift-kinetic results, nonlinear ion Compton scatterings also contribute significantly to the nonlinear ion Landau damping. Furthermore, for k_⊥ρ_i>|ω₀/Ω_i|^1/2, with ω₀ and Ω_i being, respectively, the KAW and ion cyclotron frequencies, the decay processes are significantly enhanced over and qualitatively different from the ideal-magnetohydrodynamic (MHD) results. These findings are relevant for collisionless plasma transports, as well as non-local wave energy transports. In particular, they question the applicability of ideal-MHD–based theories for the prediction of saturated Alfvén wave spectra and the corresponding fluctuation-induced transports in space and laboratory plasmas, suggesting that gyrokinetic theories are necessary for realistic comparisons with experimental measurements and observations.