Why the September 2017 geomagnetic storm was stronger at certain locations than the famous Halloween event

In this study, we compare two significant geomagnetic storms of the 21st century: the well-known Halloween geomagnetic storm of 2003 (Kp index 9) and a somewhat milder storm of September 2017 (Kp 8). Both events caused exceptionally high values of geomagnetically induced currents (GIC) and earned a place among the top ten with respect to the measured GIC in the Finnish natural gas pipeline.. We analyze solar wind and geomagnetic data as well as modeled geoelectric fields during these two events to better understand the drivers behind these strong GIC. We discover certain geographic locations that experienced stronger magnetic field time derivatives during the 2017 storm. This is interesting because in terms of magnetic indices, the 2017 storm was a weaker event. We use equivalent currents to get a view of the ionospheric and induced currents in the Fennoscandian region. We find that the interplay between different structures of ionospheric currents and the three-dimensional ground conductivity leads to a complex behaviour of the geoelectric field. This study improves knowledge in space weather preparedness by identifying location-specific risks for geoelectric hazards, which can create severe problems in the high-voltage power grid.