The level of the mid-latitude geomagnetic northward component on quiet days in northern summer is observed to decrease or to increase by about 5 &ggr;, depending on whether the interplanetary magnetic field (IMF) is directed predominantly toward or away from the sum. This effect produces about a 4¿ equatorward or poleward shift of the Sq current focus location. A simple method to determine the IMF polarities based on the effect is suggested.