Joint two‐dimensional observations of ground magnetic and ionospheric electric fields associated with auroral zone currents 1. Three‐dimensional current flows associated with a substorm‐intensified eastward electrojet

Two‐dimensional distributions of ground magnetic and ionospheric electric fields in the evening sector auroral oval have been simultaneously observed by the Scandinavian Magnetometer Array and the Scandinavian Twin Auroral Radar Experiment (Stare) radars, respectively, on February 15, 1977. They were associated with varying, substorm‐intensified, eastward electrojet current flows. Using both data sets, we have derived three‐dimensional models for the current systems of the western, middle, and eastern segment of the eastward electrojet. We conclude that the substorm‐intensified eastward electrojet was a nearly pure Hall current driven by northward electric fields. The observed eastward increase of the current in the western segment of the electrojet was due to a gradual enhancement of the Hall conductivity. Here, the electrojet was fed by a broad sheet of net downward field‐aligned current. During one period, the eastern‐terminating part of the eastward electrojet diverged up the field lines in a rather local area because of a strong longitudinal decrease in the northward‐directed electric field. On another occasion, it diverged northward within the ionosphere and joined the westward‐flowing current because of a rotation of the northward electric field with increasing latitude through west‐ to southward. These two observed mechanisms of current divergence in the region where eastward and westward electrojets coexist may shed some new light on the controversy over the existence of upward field‐aligned current flow in the Harang discontinuity.