Plasma heating via parametric beating of Alfvén waves, with heliospheric applications

Abstract

This paper advances a novel mechanism to explain the dissipation of the Alfvén waves that carry much of the energy in heliospheric and astrophysical turbulence, with specific applications to solar wind heating. The essential point is that the nonlinear beating of relatively low-frequency Alfvén waves, which are abundant in the heliosphere, drives a compressible magnetosonic response whose damping can dissipate significant energy. This mechanism involves both kinetic and magnetohydrodynamic (MHD) processes. The damping of the magnetosonic waves is a kinetic process. The nonlinear beating of Alfvén waves, which produces the magnetosonic waves, is best described by MHD theory. This mechanism complements and may compete with the well-known alternative mechanism in which the cascade of turbulent energy to small-scale, high-frequency Alfvén waves dissipates by ion-cyclotron damping. The MHD analysis in this paper reveals that the fast magnetosonic mode dominates the dissipation when the plasma beta is near unity, and that the timescale of dissipation in the heliosphere can vary from hours to a year depending upon the direction of the driven wave and the plasma parameters where it is driven. The damping of the driven magnetosonic waves may also contribute to the observed high-energy particle distributions.