We investigate the evolution of outwardly propagating simple MHD waves in a model of the expanding solar wind using MHD simulations. In order to understand the different evolution of slow, Alfvén and fast modes, the question of wave-action conservation is re-examined theoretically. Using the fluctuation averaged Lagrangian, we discuss the conservation of total wave action and Equi-partition of wave energy for MHD waves. Results show that, even though the wave action for a simple monochromatic wave is subject to loss under resonance/degeneracy condition - conditions that can occur in the expanding solar wind in the regions where plasma ß crosses one, the total wave action possessed by all modes remains conserved, representing a wave action exchange between different degrees of freedom. The Expanding Box simulations demonstrate the results of the theoretical modeling, and reveal further details about mode-mixing, Alfvén resonance and wave steepening. All of these may help to understand the evolution of fluctuations from the inner heliosphere out to Earth orbit and beyond.
