A model for the reflection of Alfven waves at the source region of the Birkeland current system: The tau generator

The source region of the Birkeland current system, also called the generator of the auroral current circuit, is probably located in the outer equatorial magnetosphere on closed magnetic field lines. Alfven waves have often been suggested as carriers for field-aligned currents, momentum, and electromagnetic energy which is converted to particle kinetic energy and ohmic beat at the polar end of the auroral flux tubes. In this report we present a model for the interaction of Alfven waves with the plasma in the generator region: the tau generator. We show that existing concepts like current and voltage generators correspond to extreme values of the intrinsic parameter tau. A frequency dependent reflection coefficient can be found which has considerable effect on the evolution of magnetospheric oscillations. Furthermore, we discuss the physical significance of the parameter tau and relate it to some. measure for the effective height of the generator region.