On hydromagnetic wave interactions in collisionless, high-$β$ plasmas

Abstract: We describe the interaction of parallel-propagating Alfv\'en waves with ion-acoustic waves and other Alfv\'en 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 Alfv\'en wave that results from its interaction with the pressure anisotropy generated either by an ion-acoustic wave or another, larger-amplitude Alfv\'en wave. The difference in timescales of acoustic and Alfv\'enic 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 Alfv\'enic fluctuations can result after interacting with frequency-matched acoustic waves. Additionally, we show that the propagation speed of an Alfv\'en-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 Alfv\'en-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 Alfv\'enic turbulence and to the interaction between the compressive and Alfv\'enic cascades in strong, collisionless turbulence are also discussed.