The instantaneous relationship between polar cap and oval auroras at times of northward interplanetary magnetic field

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Abstract
Optical images of the polar cap region at both 5577 and 3914 Å obtained from 1400 km above the earth have been used to study the relationship between polar cap and oval aurora during periods when the interplanetary magnetic field is strongly northward, i.e., B2 > 3.5 nT. When this rather rare condition occurs, the distinction between the two types of aurora is no longer as clear as depicted on the basis of statistical definitions of the auroral oval. Diffuse, weak emission can fill in the region between the auroral oval and discrete auroral features in the polar cap. The polar cap discrete features can appear very similar to auroral oval arcs in intensity, intensity ratio, and structure. Even more striking are the situations where discrete polar cap features merge with oval auroras. From this study it is concluded that under conditions of large positive B2 the region of closed magnetic field lines can expand poleward to occupy much of the high latitude region.

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