Unified PC indices:: Relation to isolated magnetic substorms

To study the relations of the polar cap magnetic activity ( characterized by unified PC indices) to magnetic disturbances in the auroral zone (AE, AL, and AU indices) the average behavior of isolated magnetic disturbances was examined. The statistical analysis of the sudden isolated magnetic substorms, occurring on the background of the quiescent magnetic field, showed that the PC index increases during the hour before the substorm onset, demonstrating the growth phase development. The PC index in the summer polar cap ( the summer PC) turned out to be growing much faster than that in the winter polar cap and reached, by the time of the sudden onset, a higher value than the winter PC index, irrespective of the disturbance intensity. After the sudden onset ( during the expansion phase) the summer PC index changes little ( or even decreases in cases of long powerful substorms), contrary to the winter PC index that changes in conformity with the course of the magnetic substorm. The results give evidence that the summer PC index, being mostly affected by the geoeffective interplanetary electric field, is not responding as strongly to auroral disturbances as the winter PC index, which relies more on the auroral conductivity. The physical meaning of the discrepancy in the behavior of the summer and winter PC indices is discussed.