An extreme geomagnetic storm comparable to the Carrington event in 1859 could have a significant impact on modern infrastructure, such as power grids. A previously published simulation by Blake et al. (2021, Space Weather, https://doi.org/10.1029/2020SW002585) reconstructed the magnetic field observations at Colaba, India, during the Carrington storm and provided estimates of the magnetic field variations around the world. We use these results as an input to a first-principle modelling method to estimate the geoelectric field in Fennoscandia based on a 3-dimensional ground conductivity model. We compare the results with the Oct 2003 Halloween storm, which is one of the strongest events in the past 100 years, and of which spatially dense magnetometer recordings are available in Northern Europe. Comparison of the maximum modelled geoelectric field values in Fennoscandia indicates that a Carrington-class storm could generate electric fields 1.4–20.4 times larger than the Halloween storm did, with the Carrington to Halloween ratio having a mean of 6.7 and standard deviation of 2.7.

The geoelectric field drives geomagnetically induced currents (GICs) in technological conductor networks, which can affect the performance of critical ground infrastructure such as electric power transmission grids. The three-dimensional (3-D) electric field at the Earth's surface consists of an external divergence-free (DF) part due to temporally and spatially varying ionospheric and magnetospheric currents, an internal DF part due to temporally and spatially varying telluric currents, and a curl-free (CF) part due to charge accumulation at ground conductivity gradients. We have developed a new method for estimating these contributions. The external and internal parts of the DF electric field are calculated from the time derivative of the external and internal parts of the observed ground magnetic field, respectively, using DF two-dimensional (2-D) spherical elementary current systems (SECSs). The horizontal surface CF electric field is calculated from the known surface DF electric field using coefficients that linearly relate the DF electric field to the CF electric field. The coefficients were obtained from the 3-D induction model PGIEM2G (Polynomial Galerkin Integral Equation Modelling in ElectroMagnetic Geophysics). The calculations are carried out in the time domain, and only two consecutive time steps of the observed magnetic field are needed to compute the surface electric field. The external part of the DF electric field is valid at and below the ionosphere, the internal part at and above the Earth's surface, and the CF part at the Earth's surface. A dense magnetometer network is a requirement for reliable results. The external and internal parts of the DF electric field are generally oppositely directed and have comparable amplitudes, both on the ground and in the ionosphere, indicating that both contributions are significant for the total DF electric field. The largest peaks of the total DF electric field tend to occur when either the external or internal contribution is temporarily suppressed. At a given location, a DF electric field with a given amplitude can result in a total surface electric field amplitude with a difference of orders of magnitude depending on the direction of the DF electric field with respect to the locally dominant conductivity gradient structure. The electric field calculation is computationally light, facilitating operational implementation of a near-real-time 3-D surface electric field monitoring and derivation of long electric field time series.

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.

Abstract In this study, we perform 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 International Monitor for Auroral Geomagnetic Effects (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.

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.

Abstract We propose a novel approach to model the ground electric field (GEF) induced by laterally‐nonuniform ionospheric sources in 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 Mäntsälä natural gas pipeline recording point and again observed remarkable agreement between modeled and measured GIC.

—The paper presents the results of analysis of the geoelectric (telluric) field variability during the Earth’s magnetic field disturbances, caused by extreme space weather events. The area of the study is the territory of the Yenisei-Khatanga Regional Trough (YKRT) situated in the auroral zone, where geomagnetic disturbances are characterized by a high level of intensity. The economic development of the YKRT as a large oil and gas-bearing area in the Russian Arctic increases the relevance of the study of possible negative impacts of space weather on future infrastructure facilities. The most serious threat to conductive industrial structures in the polar region will be posed by geomagnetically induced currents (GIC) driven by geoelectrical responses to rapid geomagnetic field changes. The analysis of the variability of telluric fields and calculations of their extreme values in the YKRT area were made using a unique magnetotelluric impedance tensor database collected by Nord West Ltd. as a result of the regional phase of the geophysical study of the trough and adjacent areas. The geoelectric field spatial-frequency distributions on the Earth’s surface were calculated on the basis of the impedance estimates and harmonic approximations of the external geomagnetic excitation. The obtained maps were correlated with geological data to find areas characterized by maximal distortions of the telluric field. Extreme amplitudes of geoelectrical responses at a series of representative locations in the YKRT were evaluated on the time series of telluric field variations, synthesized through the impedance dependences on frequency and magnetic field time series recorded during geomagnetic storms and substorms at the nearest stationary monitoring sites. The resulting estimates of amplitudes and directions of geoelectric fields during space weather disturbances can be used to account for possible destructive effects of GIC in design of pipelines, power transmission lines and railways.

The influence of space factors on technological systems in the Arctic (power transmission lines, oil/gas pipelines) has become critically important. To examine in depth these effects, an archive of digital 1 min data from Soviet/Russian magnetic stations deployed along the Arctic coast was created, starting from 1983 to the present. All data from various sources were converted to daily files in standard IAGA-2002 format and supplemented with quick-look magnetograms. Some of these data are included already in the existing world magnetic field databases, but not all. Examples of disturbances known to excite intense geomagnetically induced currents in power transmission lines were presented: irregular Pi3 pulsations and magnetic perturbation events. The database was augmented with the global 3D model of the Earth’s conductivity structure. The given example showed how the combined usage of the geomagnetic field database and the conductivity model enables one to synthesize the geoelectric field response to geomagnetic variations, and to assess the distortions of the pipeline-soil potential. To determine regions most susceptible to geomagnetic hazard, a map with normalized telluric fields was created for a uniform sinusoidally varying magnetic disturbance. This map showed that the largest electrotelluric potentials and field are induced in regions with a high resistivity (e.g., Kola Peninsula and Ural Mountains). This database can be also a useful support for space missions in the magnetosphere. The database is publicly available on the anonymous FTP site.

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.

An archive of digital 1-min data from Soviet/Russian Arctic magnetic stations has been created, starting from 1983 to the present. The archive includes data from stations deployed along the Arctic coast by various USSR/Russia institutes. All data are divided into daily files, converted into a standard IAGA2002 format, and provided with graphs for quick-look browsing. Some of the data are not included in the existing world data portals (SuperMAG, INTERMAGNET). We give examples of using the database for the Arctic: study of irregular disturbances and waves of the Pc5/Pi3 range exciting intense geomagnetically induced currents; distortion of the pipe-to-soil potential during magnetic storms; ground support for radar observations of the ionosphere. To assess the regions most susceptible to geomagnetic hazard, we calculated a map with normalized telluric fields for a uniform magnetic disturbance with a unit amplitude and periods 100–1000 s. This map shows that the geological structure significantly affects the magnitude of the geoelectric fields generated by magnetic disturbances. The database is made publicly available on the anonymous FTP site [ftp://door.gcras.ru/ftp_anonymous/ARCTICA_Rus].

We present a methodology that allows researchers to simulate in real time the spatiotemporal dynamics of the ground electric field (GEF) in a given 3‐D conductivity model of the Earth based on continuously augmented data on the spatiotemporal evolution of the inducing source. The formalism relies on the factorization of the source by spatial modes (SM) and time series of respective expansion coefficients and exploits precomputed GEF kernels generated by corresponding SM. To validate the formalism, we invoke a high‐resolution 3‐D conductivity model of Fennoscandia and consider a realistic source built using the Spherical Elementary Current Systems (SECS) method as applied to magnetic field data from the International Monitor for Auroral Geomagnetic Effect network of observations. The factorization of the SECS‐recovered source is then performed using the principal component analysis. Eventually, we show that the GEF computation at a given time instant on a 512 × 512 grid requires less than 0.025 s provided that GEF kernels due to pre‐selected SM are computed in advance. 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. This opens a practical opportunity for GEF (and eventually geomagnetically induced currents) nowcasting and forecasting.

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 spatiotemporal 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 September 7–8, 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.

Abstract Geomagnetic observatories around the world continuously measure time variations of the geomagnetic field. Long-period (> 3 hours) variations are traditionally used to constrain the electrical conductivity of the Earth’s mantle either in terms of one-dimensional (1-D) or three-dimensional (3-D) conductivity distributions. Recent studies have also shown that vertical transfer functions (tippers) estimated from short-period (< 3 hours) variations at island observatories can constrain the 1-D conductivity distribution of the oceanic lithosphere and upper mantle. This is feasible due to the bathymetry-dependent ocean induction effect (OIE), which originates from lateral conductivity contrasts between ocean and land and leads to non-zero tippers even for 1-D conductivity distributions below the ocean. Thus, proper analysis of island tippers requires accurate 3-D modeling of the OIE, for which so far was performed assuming constant sea water electric conductivity with depth. However, significant changes of electric conductivity with depth in the top hundreds of meters of the water column do occur. In this study we explore – using rigorous 3-D electromagnetic (EM) modeling – to what extent realistic, depth-dependent, oceanic conductivity affects island tippers. The modeling is performed for 10 island observatories around the world in the period range 10 −1 to 10 4 seconds, for which a perceptible OIE on tippers is expected. We then compare the predicted tippers with tippers estimated from the observatory data. We also investigate, again using 3-D EM modeling, the effect of seasonal variations of the oceanic conductivity and to which extent this could explain the observed systematic seasonal variation of tippers. We find good agreement between predicted and estimated tippers for all 10 island observatories. This confirms that tippers at island observatories are mainly influenced by the bathymetry-dependent OIE. Our model studies suggest that for most of the considered island observatories, the effect from depth-varying oceanic conductivity is tangible and exceeds the error floor of 0.025, which usually is assigned to tippers during their inversion. Contrarily, the effects from seasonally varying oceanic conductivity were found to be too small to be worth consideration.

6 Geomagnetic observatories around the world continuously measure time variations of the geomagnetic 7 field. Long-period (> 3 hours) variations are traditionally used to constrain the electrical conductivity 8 of the Earth’s mantle either in terms of one-dimensional (1-D) or three-dimensional (3-D) conductivity 9 distributions. Recent studies have also shown that vertical transfer functions (tippers) estimated from 10 short-period (< 3 hours) variations at island observatories can constrain the 1-D conductivity 11 distribution of the oceanic lithosphere and upper mantle. This is feasible due to the 12 bathymetry-dependent ocean induction effect (OIE), which originates from lateral conductivity 13 contrasts between ocean and land and leads to non-zero tippers even for 1-D conductivity distributions 14 below the ocean. Thus, proper analysis of island tippers requires accurate 3-D modeling of the OIE, for 15 which so far was performed assuming constant sea water electric conductivity with depth. However, 16 significant changes of electric conductivity with depth in the top hundreds of meters of the water 17 column do occur. In this study we explore – using rigorous 3-D electromagnetic (EM) modeling – to 18 what extent realistic, depth-dependent, oceanic conductivity affects island tippers. The modeling is 19 performed for 10 island observatories around the world in the period range 10−1 to 10 seconds, for 20 which a perceptible OIE on tippers is expected. We then compare the predicted tippers with tippers 21 estimated from the observatory data. We also investigate, again using 3-D EM modeling, the effect of 22 seasonal variations of the oceanic conductivity and to which extent this could explain the observed 23 systematic seasonal variation of tippers. We find good agreement between predicted and estimated 24 tippers for all 10 island observatories. This confirms that tippers at island observatories are mainly 25 influenced by the bathymetry-dependent OIE. Our model studies suggest that for most of the considered 26 island observatories, the effect from depth-varying oceanic conductivity is tangible and exceeds the 27 error floor of 0.025, which usually is assigned to tippers during their inversion. Contrarily, the effects 28 from seasonally varying oceanic conductivity were found to be too small to be worth consideration. 29

AbstractThe intensification of the fluctuating geomagnetic field during space weather events leads to generation of a strong electric field in the conducting earth, which drives geomagnetically induced currents (GICs) in grounded technological systems. GICs can severely affect the functioning of such infrastructure. The ability to realistically model the ground electric field (GEF) is important for understanding the space weather impact on technological systems. We present the results of three‐dimensional (3‐D) modeling of the GEF for the eastern United States during a geomagnetic storm of March 2015. The external source responsible for the storm is constructed using a 3‐D magnetohydrodynamic (MHD) simulation of near‐Earth space. We explore effects from conductivity contrasts for various conductivity models of the region, including a 3‐D model obtained from inversion of EarthScope magnetotelluric data. As expected, the GEF in the region is subject to a strong coastal effect. Remarkably, effects from landmass conductivity inhomogeneities are comparable to the coastal effect. These inhomogeneities significantly affect the integrated GEF. This result is of special importance since the computation of GICs relies on integrals of the GEF (voltages), but not on the GEF itself. Finally, we compare the GEF induced by a laterally varying (MHD) source with that calculated using the plane wave approximation and show that the difference is perceptible even in the regions that are commonly considered to be negligibly affected by lateral nonuniformity of the source. Overall, the difference increases toward the north of the model where effects from laterally variable high‐latitude external currents become substantial.