In this letter, we report for the first time a study of an effective area where the polar cap (PC) magnetic activity index stably preserves a value. The 1998 data from three Greenlandic magnetic stations Qaanaaq (formerly Thule), Savissivik, and Kullorsuaq are utilized for derivation of 15-min PC indices using normalization coefficients defined earlier for Thule. The results obtained show that Qaanaaq and Savissivik produce almost identical PCI through the year; however, the index from Kullorsuaq correlates poorly with the other two stations. Therefore, as defined only by the Thule coefficients, the effective area for PC index is estimated to be ~15¿ in diameter centered at the north corrected geomagnetic pole. ¿ 1999 American Geophysical Union