In the ring coupling model the criterion for substorm onset is the narrowing of the auroral oval to some critical limit. The auroral oval is taken to be the annulus bounded by Rings 1 and 2, corresponding to the locations of the Iijima-Potemra Regions 1 and 2 Birkeland currents. It is shown that this criterion is met following both southward and northward turnings of the interplanetary magnetic field (IMF). The behavior of the rings in response to typical variations in the north-south component of the IMF produces a variety of observed substorm sequence patterns. The energy source for substorms in the model is the solar wind energy stored in the magnetotail, whereas the energy for geomagnetic storms comes directly from the solar wind. The model is consistent with the half-wave rectifier response of the magnetosphere to the IMF.