Annual, semi-annual and solar cycle variations in Sq, and ring current effects

The difference of H at two magnetic observatories on the same meridian, but having latitudes equatorwards and polewards of the latitude of the Sq focus, yields a measure of Sq intensity largely free from disturbance. This is because the Sq variations are of opposite sign at the two observatories whereas the disturbance variations, having the same sign and roughly equal magnitudes, almost cancel each other in the difference. As a consequence, data from all days may be used to study Sq, rather than from quiet days alone, and this results in improved time resolution so that Sq is easily examined from month to month and the seasonal variation of Sq is clearly delincated.