On hydromagnetic wave interactions in collisionless, high-β plasmas

We describe the interaction of parallel-propagating Alfven waves with ion-acoustic waves and other Alfven waves, in magnetized, high-beta collisionless plasmas. This is accomplished through a combination of analytical theory and numerical fluid simulations of the Chew-Goldberger-Low (CGL) magnetohydrodynamic (MHD) equations closed by Landau-fluid heat fluxes. An asymptotic ordering is employed to simplify the CGL-MHD equations and derive solutions for the deformation of an Alfven wave that results from its interaction with the pressure anisotropy generated either by an ion-acoustic wave or another, larger-amplitude Alfven wave. The difference in time scales of acoustic and Alfvenic fluctuations at high-beta means that interactions that are local in wavenumber space yield little modification to either mode within the time it takes the acoustic wave to Landau damp away. Instead, order-unity changes in the amplitude of Alfvenic fluctuations can result after interacting with frequency-matched acoustic waves. Additionally, we show that the propagation speed of an Alfven-wave packet in an otherwise homogeneous background is a function of its self-generated pressure anisotropy. This allows for the eventual interaction of separate co-propagating Alfven-wave packets of differing amplitudes. The results of the CGL-MHD simulations agree well with these predictions, suggesting that theoretical models relying on the interaction of these modes should be reconsidered in certain astrophysical environments. Applications of these results to weak Alfvenic turbulence and to the interaction between the compressive and Alfvenic cascades in strong, collisionless turbulence are also discussed.