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.

The most detrimental geomagnetically induced currents (GICs) documented to date have all taken place during geomagnetic storms. Yet, the probability of GICs throughout geomagnetic storms driven by different solar wind transients, such as high-speed streams/stream interaction regions (HSS/SIR) or interplanetary coronal mass ejection (ICME) sheaths and magnetic clouds (MC), is poorly understood. We present an algorithm to detect geomagnetic storms and storm phases, resulting in a catalog of 755 geomagnetic storms from January 1996 to June 2023 with the solar wind drivers. Using these storms and the IMAGE magnetometer network, we study the temporal and spatial evolution of spikes in the external dH\textsubscript{ext}/dt greater than 0.5 nT/s during geomagnetic storms driven by HSS/SIR, sheaths and MCs. Spikes occur more often toward the end of the storm main phase for HSS/SIR and MC-driven storms, while sheaths have spikes throughout the entire main phase. During the main phase most spikes occur in the morning sector around 05 magnetic local time (MLT) and the extent in MLT is narrowest for MCs and widest for sheaths. However, spikes in the pre-midnight sector during the main and recovery phases are most prominent for HSS/SIR-driven storms. During the storm sudden commencement (SSC), three MLT hotspots exist, the post-midnight at 04 MLT, pre-noon at 09 MLT and afternoon at 15 MLT. The pre-noon hotspot has the highest probability of spikes and the widest extent in magnetic latitude.

Ground-based technological systems, such as power grids, can be affected by geomagnetically induced currents (GIC) during geomagnetic storms and magnetospheric substorms. This motivates the necessity to numerically simulate and, ultimately, forecast GIC. The prerequisite for the GIC modeling in the region of interest is the simulation of the ground geoelectric field (GEF) in the same region. The modeling of the GEF in its turn requires spatio-temporal specification of the source which generates the GEF, as well as an adequate regional model of the Earth’s electrical conductivity. In this paper we compare results of the GEF (and ground magnetic field) simulations using three different source models. Two models represent the source as a laterally varying sheet current flowing above the Earth. The first model is constructed using the results of a physics-based 3-D magnetohydrodynamic (MHD) simulation of near-Earth space, the second one uses ground-based magnetometers’ data and the Spherical Elementary Current Systems (SECS) method. The third model is based on a “plane wave” approximation which assumes that the source is locally laterally uniform. Fennoscandia is chosen as a study region and the simulations are performed for the 7-8 September 2017 geomagnetic storm. We conclude that ground magnetic field perturbations are reproduced more accurately using the source constructed via the SECS method compared to the source obtained on the basis of MHD simulation outputs. We also show that the difference between the GEF modeled using laterally nonuniform source and plane wave approximation is substantial in Fennoscandia.

Using the list of the omega structures based on the The Magnetometers - Ionospheric Radars- Allsky Cameras Large Experiment (MIRACLE) network (Partamies et al. 2017), we obtained a number of important statistical characteristics describing the surface magnetic field. Based on 438 events, typical magnetic variation associated with the passage of the single omega were obtained. The typical variation, obtained using superposed epoch analysis, is associated with a local bending of the western electrojet and statistically confirms the distribution of equivalent ionospheric currents obtained in earlier observations of single omegas. It was found that during low and moderate geomagnetic activity, appearance of the omega structures in the dark morning MLT sector results in twice higher than average dB/dt on the ground surface. Also, the velocity, direction of movement , and area of omega structures were calculated. It is shown that faster and bigger omegas produce larger time derivatives of the ground magnetic field. Furthermore, we demonstrate that in the 03-08 MLT sector, superposed magnetic variations for the arbitrary events of very high time derivatives |dB/dt|>10nT/s, reveal magnetic signatures similar to omegas. Our findings, together with the results described in Apatenkov et al., 2020, emphasize the important role of omega structures in the formation of large geomagnetically induced currents.

Geomagnetically induced currents or GICs are signatures of a rapidly time-varying magnetic field (dB/dt) and occur mainly during substorms and storms. When, where and why exactly GICs may occur, is still vague. Thus, we investigated storms for the last 40 years (from 1980 with a storm-list created by W.T. Walach) and analyzed the negative and positive dB/dt spikes (threshold of 500 nT/min) in the north and east component using a worldwide coverage (SuperMAG). Our analysis confirmed the existence of two dB/dt spikes “hotspots” located in the pre-midnight and in the morning MLT sector, independently of the geographic location of the stations. The associated physical ionospheric phenomena are most probably substorm current wedge (SCW) onsets and westward travelling surges (WTS) in the evening sector, and wave- or vortex-like current flows in Omega bands in the morning sector. Additionally, we observed a spatio-temporal evolution of the negative northern dB/dt spikes. The spikes initially occur in the pre-midnight sector, and then develop in time towards the morning sector. This spatio-temporal sequence is correlated with bursts in the AE index, and can be repeated several times throughout a storm. Finally, we investigated the intensity (Dst and AE) of the storms compared to the number of dB/dt spikes, but we did not find any correlation. This result implies that moderate storm with many spikes can be as (or more) dangerous for ground-based infrastructures than a major storm with fewer dB/dt spikes. Our findings may help to improve the GICs forecast to accurately predict dB/dt spikes.