Fine structure of substorm and geomagnetically induced currents

The rapid changes of magnetic fields dB/dt associated with substorms can excite large geomagnetically-induced currents (GICs) that can have harmful effects on technological systems. This paper presents analysis of the characteristics of enhancements of dB/dt observed in Fennoscandia and North-West Russia in 2015 using data from the magnetometer array IMAGE, covering a range of magnetic latitudes from 68° to 78°. The abrupt magnetic field variations may be associated with substorm onsets, isolated magnetic impulsive events (MIEs) with ~10 min duration, and Ps6/Pc5 pulsations (periods 5-15 min). For a detailed examination of the latitudinal structure of magnetic variability enhancements and their association with auroral oval boundaries we applied the technique of magnetic keograms. This technique is simple enough for mass-processing and helps to visualize and characterize the fine structure of substorm, namely the time and latitudinal localization of dB/dt enhancements. The proposed technique of dB/dt keograms could be an efficient supplementary method revealing the latitudinal localization of magnetic variability and its time evolution following substorm expansion phase onset. A location of the auroral oval boundaries in a given local time sector has been estimated with the OVATION-prime model based on energetic particle measurements from the polar-orbiting Defense Meteorological Satellite Program satellites. Different types of impulsive disturbances provided enhancement of dB/dt with different magnitudes, but mostly exceeding the commonly used threshold of 5 nT/s for potential damage to the electrical grid. Auroral substorm onset provided the largest magnetic response on the ground and most intense GIC (few tens of A) when the poleward moving intensification of dB/dt crossed the latitude of power line. Polar substorms originating at much higher latitudes are less effective in GIC excitation, ~6 A only. Isolated nightside MIEs are also effective in excitation of GICs (>10 A), but they are relatively rare. Quasi-periodic series of MIEs, known as Ps6 / Pi3 pulsations, are effective in excitation of GICs with magnitude about 20 A and even higher. Monochromatic Pc5 pulsations are capable to induce noticeable GICs, up to ~13 A.