Global scale of the day-to-day variability of Sq

SUMMARY This paper examines whether the day-to-day variability of the daily range of Sq(H) is a local phenomenon with a coherence range of 1000-2000 km or is a large-scale phenomenon with similar variability occurring in all low and middle latitudes. Daily ranges of Hare obtained for four stations on the same meridian in the northern and southern hemispheres, for all quiet days in 1962-71, and the differencing procedure described by Hibberd (1981) is used to help reduce the effects of disturbance. When the day-to-day changes in the daily range of H are small it is impossible to distinguish unambiguously between changes that represent true variability in Sq and changes resulting from residual disturbances of non-Sq origin. When the day-to-day changes in the daily ranges are large, the difference between the daily ranges for the northern hemisphere pair of stations and that for the southern hemisphere pair vary from day to day in a similar manner in the two hemispheres. It is concluded that the day-to-day variability of Sq occurs on a global scale.