Role of ionospheric and ground contributions in driving GIC: Northern Europe at the peak of the May 2024 superstorm

Abstract. 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.