Magnetospheric pulsations: Model and observations of standing Alfven wave resonances

Observations of ultra low frequency waves in the Pc 3,4, and 5 period range (10 to 600 sec) are presented. The generation of the waves is examined and the structure and distribution of these waves in space provides evidence to support the notion that many of the waves are standing Alfven wave resonances of the earth's magnetic field lines. In addition, the results of a theoretical model of standing Alfven wave resonances in a realistic magnetospheric magnetic field geometry are presented. Using the basic MHD equations, an equation is derived for the amplitude and period of low frequency transverse standing waves in an arbitrary field geometry. The Olson-Pfitzer field model which includes ring, tail and boundary currents is used, and the results indicate that the predicted period of oscillation can be quite different from that predicted by previous models in dipole field geometries. One important use for the model is for determining densities along field lines in space from ground based observations.