Geomagnetically induced currents in the UK: geomagnetic variations and surface electric fields

The geomagnetically induced current (GIC) risk to the power transmission grid in the United Kingdom is discussed with reference to an example of a geomagnetic storm during which GICs were suspected of causing abnormal transformer behaviour. A simple measure of the power of the magnetic field variation, the hourly standard deviation (HSD) in the north or east horizontal component, is used to determine the general risk to the UK power grid from rapid magnetic variations, according to season and local time. Monitoring and forecasting of HSD may be a useful means of gauging the likely risk to high-cost power engineering equipment. A simpli,ed but representative three-dimensional geological model of the UK landmass and surrounding seas is used to provide an indication of the surface electric field for various amplitudes and orientations of external magnetic field variations. It is found that the resistivity contrast between seawater and the onshore geology, particularly around the Scottish metamorphic terranes, produces enhanced electric ,elds at coastal sites. These are as much as 4 V/km for a 1 A/m (or 1257 nT) external field with 10 min period.