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It is confirmed that even when disturbance variations are taken into account, the spatial scale of day-to-day variability of S4 is local, that is, of the order of one or two thousand kilometres, rather than globally correlated.

S U M M A R YIn 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 S q ( 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 S q ( H ) at stations like Hartland which are poleward of the Sq focus.

Other workers have recently shown that the apparent day-to-day variability of S, is appreciably reduced when days with bay-like disturbances are removed from the analysis.In this paper, it is shown that when these days are removed, there is only a small effect on the scale o f the spatial coherence of Sq-ranges and therefore the regional scale of the correlation must be a real feature of S, range variability rather than an effect due to disturbance.

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