On ponderomotive effects induced by Alfven waves in inhomogeneous 2.5D MHD plasmas

Abstract: Where spatial gradients in the amplitude of an Alfven wave are non-zero, a nonlinear magnetic-pressure gradient acts upon the medium (the ponderomotive force). We investigate the nature of such a force in inhomogeneous 2.5D MHD plasmas by analysing source terms in the nonlinear wave equations for the general case of inhomogeneous B and {\rho}, and consider supporting nonlinear numerical simulations. Our equations indicate there are two distinct classes of ponderomotive effect induced by Alfven waves in general 2.5D MHD, each with both a longitudinal and transverse manifestation; i) Geometric Effects: Gradients in the pulse geometry relative to the background magnetic field cause the wave to sustain cospatial disturbances, the longitudinal and transverse daughter disturbances - where we report on the transverse disturbance for the first time. ii) Grad (c_A) Effects: Where a pulse propagates through an inhomogeneous region (where the non-zero gradients in the Alfven-speed profile are non-zero), the nonlinear magnetic-pressure gradient acts to accelerate the plasma. Transverse gradients (phase mixing regions) excite independently propagating fast magnetoacoustic waves (generalising the result of Nakariakov et al. 1997, Solar Physics, 175, 93) and longitudinal gradients perturb along the field (thus creating static disturbances in {\beta} = 0, and slow waves in {\beta} = 0). We additionally demonstrate that mode conversion due the nonlinear Lorentz force is a one-way process, and does not act as a mechanism to nonlinearly generate Alfven waves due to propagating magnetoacoustic waves. We conclude that these ponderomotive effects are induced by an Alfven wave propagating in any MHD medium, and have the potential to have significant consequences on the dynamics of energy transport and aspects of dissipation provided the system is sufficiently nonlinear and inhomogeneous.