RELATION OF GEOMAGNETICALLY INDUCED CURRENTS AND LOCAL GEOMAGNETIC-VARIATIONS

Geomagnetically induced currents (GICs) were recorded in June 1991 to May 1992 in a 400 kV transmission line in southern Finland. Geomagneti c field variations were simultaneously measured at the nearby Nurmijar vi Geophysical Observatory. Two models are presented here to calculate GICs from the time derivative of the magnetic field. A plane wave mod el with a homogeneous earth is applied by letting the earth's conducti vity be a fitting parameter. Another approach derives experimental imp ulse responses between GIC and time derivatives. Both methods yield,ap proximately the same accuracy. When GICs larger than 10 A, 20 A and 30 A are considered the average misfits are ca. 40 %, 30 %, and 20 %, re spectively.