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.

We propose a novel approach to model the ground electric field (GEF) induced by laterally-nonuniform ionospheric sources in the real-time. The approach exploits the multi-site transfer function concept, continuous magnetic field measurements at multiple sites in the region of interest, and spatial modes describing the ionospheric source. We compared the modeled GEFs with those measured at two locations in Fennoscandia and observed good agreement between modeled and measured GEF. Besides, we compared GEF-based geomagnetically induced current (GIC) with that measured at the M\“ants\”al\”a natural gas pipeline recording point and again observed remarkable agreement between modeled and measured GIC.

In this study, we perform rigorous three-dimensional (3-D) ground electric field (GEF) modeling in Fennoscandia for three days of the Halloween geomagnetic storm (29-31 October 2003) using magnetic field data from the IMAGE magnetometer network and a 3-D conductivity model of the region. To explore the influence of the inducing source model on 3-D GEF simulations, we consider three different approaches to source approximation. Within the first two approaches, the source varies laterally, whereas in the third method, the GEF is calculated by implementing the time-domain realization of the magnetotelluric intersite impedance method. We then compare GEF-based geomagnetically induced current (GIC) with observations at the Mäntsälä natural gas pipeline recording point. We conclude that a high correlation between modeled and recorded GIC is observed for all considered approaches. The highest correlation is achieved when performing a 3-D GEF simulation using a “conductivity-based” laterally nonuniform inducing source. Our results also highlight the strong dependence of the GEF on the earth’s conductivity distribution.

We present a methodology that allows real-time simulation of the geoelectric field (GEF) spatiotemporal evolution in a given 3-D conductivity model of the Earth based on continuously augmented inducing source data. The presented concept is validated using Fennoscandia as a test region. The choice of Fennoscandia is motivated by several reasons. First, it is a high latitude region, where the GEF is expected to be particularly large. Second, there exists a 3-D ground electrical conductivity model of the region. Third, the regional magnetometer network, IMAGE, allows us to build a realistic model of the source for a given geomagnetic disturbance. Taking the 7-8 September 2017 geomagnetic storm as a space weather event, we show that real-time high-resolution 3-D modeling of the GEF is feasible and requires only a few tens of seconds.