Space weather effects on the ground are produced by magnetic field changes associated with electric currents resulting from the solar wind/magnetosphere interaction. For most ground events the principal currents involved are the auroral electrojet systems. However, on August 4, 1972, there was a major magnetic disturbance and an outage of the L4 cable system in the mid-western US that was thought to be caused by currents on the magnetopause. New model calculations now show that the observed magnetic disturbance was too localized to have been caused by magnetopause currents. Contour plots of the disturbance are instead consistent with an ionospheric current as the source. Equivalent current plots derived from the observed magnetic field variations show that a rapid intensification of an eastward electrojet was responsible for the magnetic disturbance and the cable system outage. ¿ 1999 American Geophysical Union