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The rapid changes dB/dt of the Earth’s magnetic field due to substorms can excite large geomagnetically-induced currents (GICs) that can have harmful effects on technological systems. This paper presents an analysis of the characteristics of dB/dt enhancements observed in Fennoscandia using data from the IMAGE magnetometer chain along a geomagnetic longitude of 110o. The abrupt geomagnetic field variations may be associated with the substorm onsets, isolated magnetic impulsive events (MIEs), and quasi-periodic series of MIEs, known as Ps6 pulsations (periods 5-15 min). For a detailed examination of the latitudinal structure of dB/dt enhancements and their association with the auroral oval boundaries we applied the technique of magnetic keograms. This simple technique helps to visualize and characterize the fine structure of substorm, namely the location in time and latitude of dB/dt enhancements. The location of the auroral oval boundaries has been estimated with the OVATION-prime model. Among the different types of impulsive disturbances investigated auroral substorm onset provides the largest GICs (few tens of A). Isolated nightside MIEs are also effective in the excitation of GICs (>10 A). Ps6 pulsations are effective in the excitation of GICs with magnitude about 20 A and even higher. Monochromatic Pc5 pulsations are capable to induce noticeable GICs, up to ~13 A. Large localized disturbances on short timescales are the actual drivers of GICs, and they may be imagined like geomagnetic “strokes” during magnetospheric “thunderstorms”.
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