Conservation of Total Wave Action in the Expanding Solar Wind

Abstract The conservation of wave action in moving plasmas has been well known for over half a century. However, is not conserved when multiple modes propagate and coexist close to the degeneration condition (where sound speed equals Alfvén speed, i.e., plasma β ∼ 1). Here, we show that violation due mode conversion, total summed interacting still conserved. Though result general, focus on MHD waves identify three distinctive mode-conversion mechanisms, degeneracy, linear resonance, provide an intuitive physical picture processes. We use one-dimensional simulations with Expanding Box Model simulate nonlinear evolution monochromatic expanding solar wind. simulation results validate theory; therefore remains interesting diagnostic studies turbulence