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. Alfv¿n waves have often been suggested as carriers for field-aligned currents, momentum, and electromagnetic energy which is converted to particle kinetic energy and ohmic heat at the polar end of the auroral flux tubes. In this report we present a model for the interaction of Alfv¿n 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 &tgr;. 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 &tgr; and relate it to some measure for the effective height of the generator region. ¿ 1999 American Geophysical Union