Kinetic effects of Alfven wave nonlinearity. I. Ponderomotive density fluctuations

The Vlasov theory is used to study kinetic corrections to fluid descriptions of Alfven wave nonlinearity. The method is to obtain an expression for the second-order perturbed distribution function produced by a nonlinear Alfven wave. From this distribution function a kinetically correct expression is obtained for δn, the plasma density perturbation associated with an envelope-modulated Alfven wave. This kinetic theory result differs substantially from the fluid expression when the plasma β approx > 1, and the electron and ion temperatures are approximately equal. This result is of interest because density fluctuations are an observationally accessible indicator of wave nonlinearity in solar system Alfven waves. It also will assist in the determination of properties of Alfven waves in the interstellar medium. Finally, this analysis also yields a kinetically correct expression for u, the magnetic field-aligned component of the plasma fluid velocity. For parallel-propagating wave packets, this field-aligned flow is a major or even predominant contributor to the wave nonlinearity. Therefore these results provide a starting point for a general discussion of kinetic effects on Alfven wave nonlinearity