The auroral oval is a ring of luminosity enclosing geomagnetic field lines connected to the solar wind. Occasionally the ring has a bar across it, which seems to imply a bifurcation of the open region. Here results confirm that this theta-auroras actually does represent this odd bifurcated configuration, and they demonstrate how it happens. It has hitherto been assumed that theta-auroras occur when the interplanetary magnetic field is directed northward, because that is true of most very-high-latitude arcs. In fact, theta-auroras occur exclusively during the dynamic reconfiguration that follows when the interplanetary magnetic field turns southward after a prolonged northward interval.