Localized Threats: How Ground Conductivity Shapes the Geoelectric Response

Geomagnetic storms can induce strong geoelectric fields in the ground. These fields drive geomagnetically induced currents in technological conductor systems, such as power grids. In this study, we analyze 4‐hr periods of two such major geomagnetic storms: the Halloween storm (29–31 October 2003) and the 7–8 September 2017 storm. We study the geoelectric fields induced by these storms in Fennoscandia, a region characterized by a highly heterogeneous ground conductivity structure. We perform three‐dimensional geoelectric field modeling using the Multi‐Site Transfer Function method. We see an order of magnitude difference at nearby stations in the peak geoelectric field amplitude . The amplitude peaks are strongly related to the orientation of the divergence‐free (DF) electric field. These enhancements can happen when the DF field is perpendicular to a local conductivity strike. The results also have implications for GIC proxy selection and space‐weather hazard assessment, highlighting the non‐coincidence of and peaks and the importance of 3D conductivity structure.