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The Sq variations recorded by the ground stations contain total geomagnetic effects produced by the two current systems. The Sq currents deduced from the data of ground geomagnetic observatories are called Sq equivalent current systems. So far, there have been many studies on the seasonal variations of Sq equivalent current systems. Matsushita and Maeda (1965) found the equivalent Sq current system had a remarkable dependence on season, hemisphere, and longitudinal zone. Takeda (2002) found the Sq external equivalent currents field was stronger in local summer than in local winter, and the mean value of Sq intensity in both hemispheres showed a semiannual variation. Besides, the equivalent current vortex centers also have an annual variation in local time (LT). And the vortex center in summer hemisphere tends to be located at an earlier LT than in winter hemisphere (Takeda, 1990; Van Sabben, 1964). The seasonal variation is due to the interhemispheric field aligned currents (IHFACs) which flows from summer hemisphere to winter hemisphere in dawn sector and in the opposite direction in dusk sector (