ON INDUCTION EFFECTS AT EISCAT AND IMAGE MAGNETOMETER STATIONS

When ground-based magnetometer data are used for ionospheric/magnetosp heric studies, it is important to know the external (ionospheric/magne tospheric) and internal (telluric) contributions to the observed total variation. To study them, we calculate geomagnetic induction vectors, and apply the Siebert-Kertz separation method using data from the EIS CAT and IMAGE magnetometer stations in Fennoscandia. Induction vectors in the period range 20-2560 s show the expected, and dominating, ocea n effect near the Arctic Ocean. At Masi, a small-scale inland anomaly is detected in addition to the ocean effect. At Hankasalmi a man-made anomaly due to the nearby STARE radar is present. The source effect on induction vectors at high latitudes is briefly investigated. By separ ating the magnetic variations of several bay events at IMAGE stations into external and internal parts, we study the importance of the large -scale induction in the recordings. Depending very much on the particu lar event, the internal horizontal part is 10-30 per cent of the total field in the auroral electrojet region, and its relative contribution increases in more distant regions. The internal contribution of the v ertical field at inland stations is 10-40 per cent (having an opposite sign to the total field), or 10-30 per cent (having the same sign as the total field), depending on the location with respect to the ionosp heric currents and to the ocean. Results are also discussed using a th in-sheet model.