SATELLITE MEASUREMENTS THROUGH THE CENTER OF A SUBSTORM SURGE

Measurements have been made of electric and magnetic fields, plasma dr ifts, and electron precipitation within a surge at the westward, leadi ng edge of the auroral ''bulge'' at the peak of the substorm expansion phase. The trajectory of the DE 2 satellite over the auroral emission s is determined from nearly simultaneous observations with the imager on the DE 1 satellite at a higher altitude. The electric field and pla sma drift measurements have enabled us to deduce the basic configurati on 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, pole ward of the ''Harang discontinuity.'' This protrusion contains a westw ard electric field that strongly enhances the westward electrojet curr ent by the creation of a ''Cowling channel.'' This westward electric f ield, and the associated Cowling current, appear to terminate within t he surge, which contains an intense, upward field-aligned current. The magnetic field measurements show that the region containing this fiel d-aligned current is shaped more like a cylinder rather than a long sh eet. The total current is found to exceed one-half million amperes.