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Effective area for the northern polar cap magnetic activity index

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.