A comparison of the geomagnetic H-field at Huancayo with the vertical F-region] electron drifts at Jicamarca and the Interplanetary Magnetic Field measured by the HEOS 1 satellite has shown that, during the geomagnetic storm of 29-30 December 1970, changes in the north-south component of the IMF (Bz) cause well correlated changes in the equatorial electrojet current over a time scale ranging from a few minutes to a few hours. A northward excursion of the IMF produces a reduction of the electrojet current. This is explained as being the result of an additional westward electric field in the equatorial ionosphere caused by the joint action of the solar wind velocity and Bz.