Andrew P. Dimmock

Long-term measurements of geomagnetically induced currents (GICs) in power grids are rare, particularly in Sweden. This means that knowledge of GIC levels and impacts is usually based on modeling the geoelectric field and estimating GICs, which can have varying accuracy, making the diagnosis of GIC impacts difficult. However, following the power grid disturbance during the Gannon storm in May 2024, the Swedish power grid operator, Svenska Kraftnät, started measuring GIC in the most GIC-affected transformer in the Swedish power grid, located in Karlshamn, southern Sweden. Elevated levels of GIC (relative to quiet periods) during a recent G4 geomagnetic storm on 19-22 January 2026 were captured by this measurement. In this study, we use the GIC measurement from January to evaluate different ways of modeling GIC at this specific location. Using the best-performing models and previously unpublished harmonic recordings, we revisit the power grid impact from the Gannon storm in May 2024 to provide valuable new insights into the circumstances around the disturbance. Lastly, we use the same model to investigate the levels of GIC over 11 years and compare them to simulations of rare and strong space weather impacts. This study shows that GIC measurements from one event can significantly improve the accuracy of modeled GICs during other events for which measurements are unavailable, given that the power grid configuration does not change significantly. Improving modeling of GICs in the power grid is an important step to understanding and mitigating one of the most

We examine the geoelectric field and geomagnetically induced current (GIC) in Northern Europe during the May 2024 superstorm using a recently developed method: The divergence-free part of the geoelectric field (E_DF), associated with rapid magnetic field variations, is estimated from ground-based magnetic field observations using spherical elementary current systems. The curl-free part of the geoelectric field (E_CF), associated with charge accumulation, is estimated from E_DF using coefficients that depend on ground conductivity and linearly relate E_CF to E_DF in the time domain. We apply the method to both regional 10 s International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations and global 1 min SuperMAG observations by adopting a global triangular grid adapted to local magnetometer density. We compare the resulting GIC in the Finnish benchmark power grid and conclude that, in the absence of ground conductivity information and higher cadence data, E_DF estimated from 1 min magnetic field observations alone can provide a reasonable proxy for GIC activity in a power grid compared to 10 s geoelectric field. However, polarization of the geoelectric field due to lateral variations in ground conductivity can produce intense GIC at substations connected to transmission lines traversing regions of enhanced geoelectric field. GIC at such substations may be poorly described by E_DF alone.

We examine the geoelectric field and geomagnetically induced current (GIC) in Northern Europe during the May 2024 superstorm using a recently developed method: The divergence-free part of the geoelectric field (E_DF), associated with rapid magnetic field variations, is estimated from ground-based magnetic field observations using spherical elementary current systems. The curl-free part of the geoelectric field (E_CF), associated with charge accumulation, is estimated from E_DF using coefficients that depend on ground conductivity and linearly relate E_CF to E_DF in the time domain. We apply the method to both regional 10 s International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations and global 1 min SuperMAG observations by adopting a global triangular grid adapted to local magnetometer density. We compare the resulting GIC in the Finnish benchmark power grid and conclude that, in the absence of ground conductivity information and higher cadence data, E_DF estimated from 1 min magnetic field observations alone can provide a reasonable proxy for GIC activity in a power grid compared to 10 s geoelectric field. However, polarization of the geoelectric field due to lateral variations in ground conductivity can produce intense GIC at substations connected to transmission lines traversing regions of enhanced geoelectric field. GIC at such substations may be poorly described by E_DF alone.

Geomagnetically Induced Currents (GICs) are a key space weather hazard to ground‐based infrastructure, and can cause mis‐operation or even equipment failure. The solar wind interacts with the Earth's geomagnetic field, causing the magnetic field at ground level to vary with time. This variability causes GICs in grounded, conducting networks via coupling with the subsurface geology. Most studies of the hazard focus on short, high amplitude GICs, but long intervals of lower amplitude GIC can also be problematic, representing a rare but known pathway to equipment damage and power failure. We evaluate an interval in June 2015 where the power network in New Zealand experienced long‐duration, steadily increasing GICs across the South Island. These GICs were up to 20A and lasted approximately 90 min during an otherwise moderate storm interval, while New Zealand was in the dawn sector. We investigate the magnetospheric cause, attributing them to a possible diversion of the ring current into the ionosphere. We test the ability of modeling and proxies to capture this hazard, finding that it can be recreated, but is dependent upon the regional geology, and requires detailed knowledge of the network configuration. We model two hypothetical examples, (a) where the magnetic perturbation is larger and (b) where a different mid‐latitude location is exposed, finding that larger currents are possible and that the vulnerability is not unique to New Zealand. Our case study highlights the limitations of geomagnetic indices, which we show to be vulnerable to influence from such rare mid‐latitude phenomena.

We examine the geoelectric field and geomagnetically induced current (GIC) in Northern Europe during the May 2024 superstorm using a recently developed method: The divergence-free part of the geoelectric field (E_DF), associated with rapid magnetic field variations, is estimated from ground-based magnetic field observations using spherical elementary current systems. The curl-free part of the geoelectric field (E_CF), associated with charge accumulation, is estimated from E_DF using coefficients that depend on ground conductivity and linearly relate E_CF to E_DF in the time domain. We apply the method to both regional 10 s International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations and global 1 min SuperMAG observations by adopting a global triangular grid adapted to local magnetometer density. We compare the resulting GIC in the Finnish benchmark power grid and conclude that, in the absence of ground conductivity information and higher cadence data, E_DF estimated from 1 min magnetic field observations alone can provide a reasonable proxy for GIC activity in a power grid compared to 10 s geoelectric field. However, polarization of the geoelectric field due to lateral variations in ground conductivity can produce intense GIC at substations connected to transmission lines traversing regions of enhanced geoelectric field. GIC at such substations may be poorly described by E_DF alone.

We examine the geoelectric field and geomagnetically induced current (GIC) in Northern Europe during the May 2024 superstorm using a recently developed method: The divergence-free part of the geoelectric field (E_DF), associated with rapid magnetic field variations, is estimated from ground-based magnetic field observations using spherical elementary current systems. The curl-free part of the geoelectric field (E_CF), associated with charge accumulation, is estimated from E_DF using coefficients that depend on ground conductivity and linearly relate E_CF to E_DF in the time domain. We apply the method to both regional 10 s International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations and global 1 min SuperMAG observations by adopting a global triangular grid adapted to local magnetometer density. We compare the resulting GIC in the Finnish benchmark power grid and conclude that, in the absence of ground conductivity information and higher cadence data, E_DF estimated from 1 min magnetic field observations alone can provide a reasonable proxy for GIC activity in a power grid compared to 10 s geoelectric field. However, polarization of the geoelectric field due to lateral variations in ground conductivity can produce intense GIC at substations connected to transmission lines traversing regions of enhanced geoelectric field. GIC at such substations may be poorly described by E_DF alone.

Sweden’s communication and power systems have been impacted by major space weather events in the past. For instance, the May 1921 storm caused a fire at the telegraph and telephone station in Karlstad, and the 2003 Halloween storm led to a blackout in Malmö. In this study, we present the first comprehensive assessment of the potential impacts of a 1-in-100-year extreme geoelectric field on the entire Swedish power grid. Using magnetic field observations from the 30 October 2003 event as a baseline, we constructed two extreme scenarios. In Case 1, we used the observed magnetic field across Fennoscandia. In Case 2, we assume an idealized ionospheric current system in which all stations share the same temporal magnetic field pattern. Then the estimated 3D geoelectric field was scaled using region-specific scaling factors derived from recent statistical analyses of geoelectric field extremes in Sweden. The scaled geoelectric field and power line voltages are computed using the recently developed RAISE model, which includes realistic ground conductivity and power line topology. Our results show that the largest-magnitude horizontal geoelectric fields, around 12 V km⁻¹, occur within the 55 and 58° MLAT band, particularly in regions with sharp lateral conductivity gradients. East–west-oriented power lines are especially vulnerable, as they are mainly located south of 60° MLAT, where the largest geoelectric fields are calculated. Overall, during the peak of a 1-in-100-year geomagnetic storm, more than 100 transmission lines are expected to experience voltages above 50 V multiple times over the course of the substorm. At the peak of the largest disturbance, triggered by a sudden weakening of the westward electrojet, around 100 lines are expected to exceed 100 V. These results provide critical insights into infrastructure vulnerability under extreme space weather conditions.

Collisionless shocks convert bulk flow energy into heat, electromagnetic fields, and non‐thermal particle populations. Recent studies suggest that downstream magnetic oscillations could play an important role in ion‐scale energy dissipation at low‐Mach‐number shocks; however, the specific shock and plasma parameters involved remain poorly understood. Interplanetary (IP) shocks, often characterized by low Mach numbers, provide an excellent opportunity for investigating these kinetic dissipation mechanisms. We demonstrate, using observations of an IP shock from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) missions, supported by test‐particle simulations, that gyrating protons generate the downstream magnetic oscillations. We found bursts of ion‐acoustic waves at the troughs and crests of the magnetic oscillations, suggesting their energy source is related to proton gyration. Comparing MMS and SolO observations, we conclude that the upstream flow speed to ion thermal speed ratio and magnetic compression ratio are key parameters controlling the ion kinetic behavior that produces downstream magnetic oscillations.

We examine the geoelectric field and geomagnetically induced current (GIC) in Northern Europe during the May 2024 superstorm using a recently developed method: The divergence-free part of the geoelectric field (E_DF), associated with rapid magnetic field variations, is estimated from ground-based magnetic field observations using spherical elementary current systems. The curl-free part of the geoelectric field (E_CF), associated with charge accumulation, is estimated from E_DF using coefficients that depend on ground conductivity and linearly relate E_CF to E_DF in the time domain. We apply the method to both regional 10 s International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations and global 1 min SuperMAG observations by adopting a global triangular grid adapted to local magnetometer density. We compare the resulting GIC in the Finnish benchmark power grid and conclude that, in the absence of ground conductivity information and higher cadence data, E_DF estimated from 1 min magnetic field observations alone can provide a reasonable proxy for GIC activity in a power grid compared to 10 s geoelectric field. However, polarization of the geoelectric field due to lateral variations in ground conductivity can produce intense GIC at substations connected to transmission lines traversing regions of enhanced geoelectric field. GIC at such substations may be poorly described by E_DF alone.