Phase mixing and phase motion of Alfven waves on tail-like and dipole-likemagnetic field lines

The time-dependent phase structure of Alfven waves on open and closed field lines is studied. In accord with previous observations we find that Alfven waves on near-Earth closed field lines exhibit a poleward phase motion unl ess they are close to the plasmapause, in which case the motion may be equa torward. Alfven waves generated on tail-like closed or open field lines thr eading the plasma sheet boundary layer have received much less attention bu t may be shown to have an equatorward phase motion [Liu et al., 1995]. Phas e mixing in the magnetotail proves to have a much richer behavior than that on near-Earth (dipole-like) closed field lines as not only the Alfven freq uency varies across the background field lines but the field-aligned wavenu mber varies too. The two contributions tend to cancel each other partially for typical tail equilibria. Observations are given of a double oval config uration showing long-period pulsations on the poleward portion of this oval . Equatorward phase motion is observed and supports the theory presented he re. These observations illustrate that Pc5 pulsation activity can be much r icher than previously thought and can occur at locations not in the dipole- like region, as is usually supposed. The concepts presented in this paper p rovide a powerful framework with which to interpret observations related to auroral arcs, substorms, and magnetospheric equilibria.