The newly introduced index PC for magnetic activity in the cap has been examined to establish to which extent it can serve as an indicator of auroral electrojet activity. PC is derived from a single nearpole station, as a 15-min average index. We have derived it for two stations, one in the northern hemisphere (Thule) and one in the southern hemisphere (Vostok). The simplicity of the PC index enables us to make a large data base for statistical investigations. We have thus used 7 years of PC values for the two stations to analyze the relationship between PC and the auroral zone indices AE, AU, and AL statistically. We find a very high correlation between PC and AE during winter and equinox, the linear correlation coefficient being ≈0.8--0.9 for Thule and ≈0.7--0.8 for Vostok. During summer the correlation is less because the PC index is then disturbed by polar cap currents controlled by the northward and east-west components of the interplanetary magnetic field. We therefore stress the importance of having PC available from both the northern and southern hemisphere. From event studies we find that PC is sensitive both to DP 2 type electrojet activity and to substorm intensifications of the westward electrojet in the midnight or postmidnight sector but less sensitive to substorm intensifications of the westward electrojet in the premidnight sector. We conclude that PC can serve as a fast available indicator of DP 2 and DP 1 activity in the polar regions, excluding intrusions of the westward electrojet in the premidnight sector. ¿ American Geophysical Union 1990