ARE 2-FLUID EFFECTS RELEVANT TO ULF PULSATIONS

It has been suggested recently that the traditional single-fluid MHD d escription of ULF pulsations is wrong and that a two-fluid model (whic h includes electron inertia) is required [Bellan, 1994]. If this claim is correct, it suggests most previous studies are inadequate and of q uestionable 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 argu e that the singular normal modes of single-fluid MHD are important pro perties 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.