The 7-8 September 2017 Storm: Geoelectric and Geomagnetic Response Over Fennoscandia and their Link to GIC.

Geomagnetically Induced Currents (GIC) are caused by a geoelectric field which is set up by rapid variations in the geomagnetic field that induce currents in the conductive ground. The geoelectric field can peak over a few hundred km due to localized ionospheric current enhancements and/or complex ground conductivity gradients (e.g. from mineral deposits or coastal effects), meaning GIC can manifest locally. Fennoscandian countries may be susceptible to such effects due to their high-latitudes as they experience the effects from intense Auroral electrojet currents during intervals of high geomagnetic activity. In some regions such as Sweden, very complex ground conductivity features could enhance their space-weather susceptibility.

We study the ground impact over Fennoscandia during the geomagnetic storm of 7-8 September 2017. During this period, we observed 30A peak GIC in the Finnish natural gas pipeline at the Mäntsälä compressor station, and prolonged GIC intervals exceeding 10A. The largest GIC (30A) were associated with westward electrojet current enhancements, but GIC exceeding 15A corresponding to variations in the eastward electrojet were also measured. Based on IMAGE ground magnetometer measurements, the geomagnetic response revealed many spatially and temporally localised features. Similarly, maps of ionospheric equivalent currents demonstrate complex small spatiotemporal structures and local rapid variations. Combining the maps of ionospheric equivalent currents with a three-dimensional ground conductivity model, we reconstruct the geoelectric field over Fennoscandia, and make a comparison between the modelled and measured GIC. In this presentation, we report our findings.