Intense and long-duration geomagnetically induced current events: solar cycle and geomagnetic dependences

We study the intense geomagnetically induced current (GIC) > 10 A events at Finland. A total of 48 GIC events were identified from 1999 to 2017. Peak intensity of the events varied from ~10 A to ~57 A, with an average (median) intensity of ~21±10 A (~19 A) for all events. The duration varied from ~5 h to ~38 h, with an average (median) duration of ~17±9 h (~14 h). The intense GIC events were characterized by time-integrated current flows or total charges varying from ~18×10³ Coulombs to ~334×10³ C, with an average (median) value of ~(86±63)×10³ C (~73×10³ C) for all events. The occurrence of the strongest events was centered around local midnight, giving a good clue as to the source of the disturbances. The GICs had a highly asymmetric solar cycle dependence with the peak occurrence at solar maximum, and no events at solar minimum. Almost all events occurred during geomagnetic storm main phases. The GIC events occurred in association with clusters of intense auroral substorms during the storms. The substorms had an average (median) duration of ~20±10 h (~17 h), peak SME and SML intensities of ~2793±908 nT (~2566 nT) and -2238±843 nT (~-2099 nT), and integrated SME and SML indices of ~789±438 nT-d (~666 nT) and ~-524±309 nT-d (~-443 nT), respectively. It is concluded that clusters of intense substorms are the causes of these intense and long-duration GIC events. However, the question of whether the GICs are due to field-aligned currents, auroral (precipitation) motions, or precipitation generated enhanced ionospheric conductivity is still unknown.