Field-aligned current flow associated with intrusion of the substorm-intensified westward electrojet into the evening sector

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Previous studies have indicated that during substorm activity the westward electrojet penetrates into the evening sector poleward of the eastward electrojet. By use of magnetometer data from the polar-orbiting Triad satellite and ground-based magnetometer data it is shown that a region of intense upward field-aligned current flow encompasses the boundary between the eastward electrojet and the substorm-associated westward electrojet and that there is downward current flow both to the north and to the south of the boundary between the two electrojets. This result is interpreted in terms of the transition in electric field polarity at the boundary between the auroral oval and the polar cap reported by Cauffman and Gurnett (1971), and it is suggested that the substorm-associated westward electrojet extension into the evening sector is due primarily to conductivity enhancements and to a lesser extent to electric field ehnancements. Evidence is shown that demonstrates the perturbation pattern observed at 800 km due to electrojet currents flowing in the ionosphere. It is suggested that double sheets of field-aligned currents may flow in the evening sector of the magnetotail during substorms.