Gyrokinetic theory of parametric decays of kinetic Alfven waves

The fundamental parametric decay processes of kinetic Alfven waves (KAW) have been reexamined by employing the nonlinear gyrokinetic equations. Dispersion relations, valid for arbitrary k(perpendicular to)rho(i), are derived for parametric decays to KAW and ion sound waves. Here, k(perpendicular to) and rho(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(perpendicular to)rho(i) drift-kinetic results, nonlinear ion Compton scatterings also contribute significantly to the nonlinear ion Landau damping. Furthermore, for k(perpendicular to)rho(i) > vertical bar omega(0)/Omega(i)vertical bar(1/2), with omega(0) and Omega(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 Alfven 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. Copyright (C) EPLA, 2011