Long term monitoring of the geoelectric field in the UK – 2012–2024

Abstract. During severe geomagnetic storms, rapid changes in the Earth's magnetic field can induce a significant geoelectric field in the conductive subsurface. This field can cause large potential differences in grounded systems that have long conductors between earthing points, such as power lines, and cause currents to flow between them. These currents are known as Geomagnetically Induced Currents (GIC). To predict the effects on ground-based infrastructure from geomagnetic storms during space weather events, estimates of the subsurface electrical resistivity are required. These can be constructed using transfer functions between the magnetic and electric field, calculated from measurements made at short-term monitoring installations, recording for days to weeks. However, longer-term monitoring of years to decades is valuable too, as this provides a large and rich set of data encompassing both quiet periods and storms that can be used to enhance and ground-truth geoelectric field and GIC estimates. There are a limited number of permanent monitoring systems around the world, and until 2012 there were none in the UK aside from historical measurements made in the 19th century. The British Geological Survey (BGS) installed three geoelectric field monitoring sites in 2012 and 2013 co-located with our INTERMAGNET observatories at Hartland Point, Eskdalemuir and Lerwick to provide new data sets. We describe in detail how the systems were installed, their history, the electronics used to condition and digitize the signal, and how the data are processed and supplied in near real time to users. During more than a decade of measurements, we encountered several operational issues requiring mitigation and developed improvements as we gained experience of the systems.