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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 "θ-aurora" actually does represent this odd bifurcated configuration, and they demonstrate how it happens. It has hitherto been assumed that θ-auroras occur when the interplanetary magnetic field is directed northward, because that is true of most very-high-latitude arcs. In fact, θ-auroras occur exclusively during the dynamic reconfiguration that follows when the interplanetary magnetic field turns southward after a prolonged northward interval.
Newell et al. (Fri,) studied this question.
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