Measurements have been made of electric and magnetic fields, plasma drifts, and electron precipitation within a surge at the westward, leading edge of the auroral ''bulge'' at the peak of the substorm expansion phase. The trajectory of the DE 2 satellite over the auroral emissions is determined from nearly simultaneous observations with the imager on the DE 1 satellite at a higher altitude. The electric field and plasma drift measurements have enabled us to deduce the basic configuration of the ionospheric electric potential, or plasma convection, around the surge. The electric potential shows that the bulge is associated with a protrusion of the dawn convection cell into the dusk cell, poleward of the ''Harang discontinuity.'' This protrusion contains a westward electric field that strongly enhances the westward electrojet current by the creation of a ''Cowling channel.'' This westward electric field, and the associated Cowling current, appear to terminate within the surge, which contains an intense, upward field-aligned current. The magnetic field measurements show that the region containing this field-aligned current. is shaped more like a cylinder rather than a long sheet. The total current is found to exceed one-half million amperes. ¿ American Geophysical Union 1994