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 generato r of the auroral current circuit, is probably located in the outer equatori al magnetosphere on closed magnetic field lines. Alfven waves have often be en suggested as carriers for field-aligned currents, momentum, and electrom agnetic 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 mo del for the interaction of Alfven waves with the plasma in the generator re gion: the tau generator. We show that existing concepts like current and vo ltage generators correspond to extreme values of the intrinsic parameter ta u. A frequency dependent reflection coefficient can be found which has cons iderable effect on the evolution of magnetospheric oscillations. Furthermor e, we discuss the physical significance of the parameter tau and relate it to some. measure for the effective height of the generator region.