Abstract. For more than two decades numerical models of the Earth's magnetosphere have been used successfully to study magnetospheric dynamic features such as the excitation of ULF pulsations and the mechanism of ®eld line resonance. However, numerical formulations simplify important properties of the real system. For instance the Alfve n continuum becomes discrete because of a ®nite grid size. This discretization can be a possible source of numerical artifacts. Therefore a careful interpretation of any observed features is required. Examples of such artifacts are presented using results from a three dimensional dipole model of the magnetosphere, including an inhomogeneous distribution of the Alfve n velocity.