The Expanding Box Model in ECHO: Application to the Parametric Decay of Alfvén Waves in the Fast Solar Wind

The nonlinear evolution and the decay instability of monochromatic Alfvén waves in the fast solar wind is studied through MHD numerical simulations taking into account the effects of the radial expansion of the background plasma. This is achieved by means of the expanding box model, a local approach which allows to maintain Cartesian coordinates and periodical boundary conditions. In this contribution we discuss the implementation of the model in the ECHO code and we present preliminary results of the decay instability in the presence of radial expansion effects.