Abstract
SUMMARY In order to explain the results of a previous study of the variations in declination at Hermanus on abnormal quiet days at Hartland, it has been proposed that a single current vortex system spanning both hemispheres links the variations at the two stations (Butcher 1987). Earlier work (Butcher & Brown 1981) suggested that an abnormal quiet day results from the combination of the Occurrence of a day of low range in Sq(H) and a small magnetospheric substorm or mini-bay which therefore forms the minimum in H. In the present paper, results are presented which show that the single current vortex acts not only on abnormal quiet days, but more generally on days of small amplitude of Sq(H) at stations like Hartland which are poleward of the Sq focus.
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