Development of the polar electrojet during polar magnetic substorms

Evidence is presented indicating that the structure of polar magnetic substorms is determined by the intensification of the current at the northern and/or southern borders of a small pre-existing westward electrojet. The substorm onset is characterized by the growth of the current at the southern border of the electrojet and further develops through quasi-periodic intensifications of the current at the northern border of the electrojet. The southern part of the electrojet varies only sporadically in intensity, and its strength and general stability make it the center of activity throughout the course of the substorm. The intensification of current at the northern border of the electrojet that occurs at roughly 15-min intervals is thought to be connected with the westward-traveling surge observed during auroral substorms and suggests a quasi-periodic input of energy from the magnetotail.