S U M M A R YSecular changes in the solar geomagnetic variation at 11 observatories with a worldwide distribution have been studied. The secular changes at UK observatories were found to be large. Numerical calculations, details of which are described in this paper, agree in predicting the large changes at UK observatories. The numerical calculations also predict a large secular change in Sq at Hermanus in all elements; this would be expected as the main field at this station shows a large secular change. The secular change in Sq observed a t Hermanus is, however, found to be fairly small compared with that observed at the other stations.

The effect of the IMF on the position of the Sq focus in summer has been discussed by Matsushita who found an apparent poleward motion of the focus on days when the IMF was directed away from the Sun compared to those when the IMF was directed towards the Sun. However, his analysis took no account of the well known phase variability in Sq(H). It is shown here that if quiet days are separated into 'normal quiet days', NQDs (defined for a station on the poleward side of the focus as days when the minimum in H occurs between 0830 and 1330 LT) and 'abnormal quiet days', AQDs (minimum in H occurring outside the NQD range), then the apparent poleward motion of the focus only occurs on AQDs and even then only when the IMF is directed away from the Sun. This effect is found to occur in both summer and winter and is probably of magnetospheric origin.

In a previous paper (Butcher & Brown 1981) we discussed the occurrence of so-called 'abnormal quiet days' (AQD), defined as magnetically quiet days when the time of the daily minimum in the horizontal component H at a mid-latitude station like Hartland occurred outside the 'normal' time range 0830--1330 LT. It was shown that for the period 1963-1965 an AQD was caused by the influence of a negative (southward) bay-like perturbation field lasting about 4 hr and having a small amplitude of about 6 nT. Examination of the effects at a number of northern hemisphere stations situated close to the 0" longitude meridian showed that such perturbations occur on both sides of the S4 focus, but it is only on the poleward side of the focus that they may cause an AQD by forming a minimum in H on a particular day deeper than the normal diurnal minimum caused by the S4 current.These perturbations can occur at any time. including, of course, the interval 0830-1330 LT when they may be expected to affect the amplitude and/or the phase of the normal S4(H) variation. In this paper we consider these influences. The perturbations may be identified in the magnetograms of stations at adjacent longitudes, since they tend to occur at the same UT over a fairly wide geographical extent. For the 0" meridian chain it has been found that the events are particularly well defined at Meanook, 7% hr west of Greenwich. From the correlation between these events and parameters of the interplanetary magnetic field it is shown that they are of the same type as the perturbations associated with the occurrence of AQDs. It is suggested that (i) much of the reported variability of the latitude of the inferred Sq focus arises from the influence of these perturbations on the amplitude of the normal S4(H) variation, which is oppositely directed on either side of the focus, and (ii) much of the apparent skewing of the equivalent Sq current systems arises from the influence of these perturbations on the phase of Sq(H), which will be asymmetrical on either side of the focus.

It is well known that the time of occurrence of the minimum in the horizontal component of the Earth's magnetic field (Hmin) on quiet days at a mid-latitude station on the poleward side of the Sq focus shows considerable variability from day to day. This variability has previously been discussed in terms of the incidence of so-called 'abnormal quiet days' (AQD). arbitrarily defined for the station Hartland as quiet days when Hmin occurred outside an interval of +2% hr centred on the most common time of 1 I30 LT. AQDs have some interesting properties, which have been documented, but their precise nature and cause have not been elucidated. In this paper we report the results of a study of AQDs as identified at Hartland using a chain of Northern hemisphere stations situated approximately along the 0" longitude meridian and extending on both sides of the Sq focus. It is found that there are two effects on AQDs: (i) a northward component field varying in LT is superposed at all latitudes throughout the day, so reducing the amplitude of the normal Sq(H) variation at stations poleward of the focus and increasing it on the equatorward side, (ii) a southward perturbation field, of most probable magnitude 6.0 nT for the period studied, is imposed for about 4 lir at a fixed UT at all latitudes. so constituting an 'AQD event' which can lead to the occurrence of Hmin at an anomalous local time for a station poleward of the focus. It is shown that the AQD event may be of large geographical extent and that it is related to the interplanetary magnetic field. All the main properties of AQD occurrences are explained. and it is suggested that much of the dayto-day variability in the amplitude and phase of the normal Sq(H) variation probably also arises from the occurrence of AQD events at times close to the diurnal turning points.

Declination data have been analysed on NQDs and AQDs for the years 1963-64-65. Such an analysis indicates that there is a larger southward current in the morning and a larger northward current in the afternoon on NQDs than AQDs in both the northern and southern hemispheres. The strength of this current is found t o be IMF-dependent. Taken in conjunction with the previous analyses of the H-data it is concluded that on AQDs the Sq(H) amplitude is affected by a reduction in the current of a large current vortex which extends over both hemispheres and is part of the normal Sq(H) current system. Such a current is driven by diurnal winds, variations in the strength of the wind thus contributing to the day-to-day variability of the Sq(H) amplitudes. AQDs therefore represent days of small amplitude in the general day-to-day variability. The IMF-dependence arises due t o complicated current systems that flow on AQDs and which are associated with high latitude phenomena and generally have their focus at latitudes 2 60°, similar to those found by Takeda & Araki.