To learn about the geometry and sources of the ionospheric current systems which generate strong geomagnetically induced currents, we categorize differential equivalent cur-rent systems (DEC) for events with strong dB/dt by decomposing them into the contributions of electrojet-type and vortex-type elementary systems. By solving the inverse problem we obtain amplitudes and locations of these elementary current systems. One-minute differences of the geomagnetic field values at the IMAGE magnetometer network in 1996-2000 are analysed to study the spatial distributions of large dB/dt events. The relative contributions of the two components are evaluated. In particular, we found that the majority of the strongest dB/dt events (100-1000nT/min) appear to be produced by the vortex-type cur-rent structures and most of them occur in the morning LT hours, probably caused by the Ps6 pulsation events associated with auroral omega structures. For strong dB/dt events the solar wind parameters are shifted toward strong (tens nT) southward IMF. enhanced velocity and dynamic pressure, in order for the main phase of the magnetic storms to occur. Although these events appear mostly during magnetic storms when the auroral oval greatly expands, the area of large dB/dt stays in the middle part of the auroral zone; therefore, it is connected to the processes taking part in the middle of the magneto-sphere rather than in its innermost region populated by the ring current.