Abstract
The theta aurora is an approximately Sun-aligned transpolar arc that often extends the entire length of the polar cap, linking the dayside and nightside auroral zones. It is a remarkable manifestation of polar cap auroral activity that appears during periods of northward interplanetary magnetic field (IMF). In this paper we describe a magnetic configuration of the open magnetosphere that is conducive to the formation of the theta aurora when the IMF has a significant northward component. We present a magnetic field topology and polar cap configuration that is derived from a quantitative model of the open magnetosphere that incorporates Crooker's antiparallel merging hypothesis. Under this hypothesis, when the IMF has a northward component, the dayside merging line bifurcates, leaving a large fraction of the subsolar magnetopause untouched by the merging process. The polar cap, defined by tracing magnetic field lines that connect from the solar wind to the Earth, is similarly bifurcated, leaving a sun-aligned stagnation region that is not magnetically connected to the solar wind and may plausibly be associated with the sun-aligned “bar” of the theta aurora. The model provides testable predictions with regard to the position of this “convection gap” in both northern and southern hemispheres as functions of IMF direction.
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