Gyrokinetic theory of parametric decays of kinetic Alfvén waves
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⊥ρi, are derived for parametric decays to KAW and ion sound waves. Here, k⊥ and ρi are, respectively, the wave number perpendicular to the magnetic field and the ion Larmor radius. It is found that, contrary to the small k⊥ρi drift-kinetic results, nonlinear ion Compton scatterings also contribute significantly to the nonlinear ion Landau damping. Furthermore, for k⊥ρi>|ω0/Ωi|1/2, with ω0 and Ωi 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.