Chatanika Radar observations of ionospheric convection velocities spanning auroral latitudes (58°–75°Λ), and contours of electrostatic potential derived from them, are described for a period of moderately strong geomagnetic activity in the midnight sector. Digitized H component magnetograms and contours of magnetic perturbation, ΔH, derived from them are presented in order to compare the convection pattern and radar-deduced currents with the observed ground response to the auroral electrojets. In spite of midnight sector substorm activity, the convection pattern maintains a two cell character. The latitude of greatest negative potential in the evening sector, determined from the radar data, accurately denotes the position of the Harang discontinuity in the ionospheric currents.
Premidnight, substorm enhancements in the westward electrojet current are accompanied by precipitation-enhanced Hall conductivity but little change in the convection electric field. At lower latitudes, the region of strong sunward convection (and northward electric field) is displaced equatorward during the substorm. As a result, the electric field increases as the eastward current increases at previously subauroral latitudes during the substorm. After midnight, enhancements in the electric field magnitude and the Hall conductivity alternate in supporting the substorm enhanced currents.