Data from seven solar rotation periods, partitioned into several smaller periods, are used to study the response of auroral zone magnetic activity to the IMF polarity at low and high solar wind velocities. At low velocities magnetic activity is almost exclusively controlled by negative Bz values; the magnetosphere acts like a half wave rectifier. At high velocities, however, magnetic activity starts to increase already in the positive Bz range, at ~+5 nT. This result poses strong constraints on the generally accepted reconnection/merging models. The solar wind power necessary to drive the magnetic activity observed in the positive BZ range is estimated to amount to 4¿1017 ergs/s. ¿ American Geophysical Union 1989