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 field line resonance. However, numerical formulations simplify important properties of the real system. For instance the Alfvén continuum becomes discrete because of a finite 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 Alfvén velocity.
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Acknowledgements. This work was supported by a grant of computer time from the Regionales Rechenzentrum für Nieder-sachsen, Universität Hannover and financially through a grant by the German Science Foundation and the German Space Agency DARA. Additional support was provided by the Division of Atmospheric Sciences of the National Science Foundation under grant ATM-9314239. R.L. Lysak was supported by the National Science Foundation under grant ATM-9301043.