It has been suggested recently that the traditional single-fluid MHD description of ULF pulsations is wrong and that a two-fluid model (which includes electron inertia) is required Bellan, 1994. If this claim is correct, it suggests most previous studies are inadequate and of questionable value. We have examined the appropriate equations and find that two-fluid effects are not important for typical ULF pulsations. Specifically, the time taken to develop spatial scales similar to the electron inertia length far exceeds the lifetime of the waves. We argue that the singular normal modes of single-fluid MHD are important properties of the magnetospheric system that should be studied. They may be employed to calculate the evolution of waves and are also a useful mathematical limit of the more realistic dissipative normal modes.
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