Developing a New Ground Electric Field Model for Geomagnetically Induced Currents in Britain Based on Long‐Period Magnetotelluric Data

Abstract Large variations in ground electric field in mid‐latitude countries like the UK during a geomagnetic storm drive so‐called geomagnetically induced current, a major geohazard to ground‐based technological infrastructure like electrical transformers at high voltage substations, gas pipelines and railway signaling. In order to assess these effects, the ground electric field response over a large bandwidth of signal periods from seconds to hours needs to be known. In the UK, this was previously done using a thin sheet model of electrical conductivity based on airborne electromagnetic and laboratory measurements. More accurately, the geoelectric fields can be modeled from magnetotelluric (MT) measurements. Here we describe the recent collection of MT data at 53 sites across Britain. The recorded timeseries data of geomagnetic and geoelectric fields were processed into MT impedance tensors at each site. We complemented the data set with MT legacy data to a total of 70 sites. Using the spherical elementary current system approach to interpolate ground magnetic field variations in the UK at each site, we convolve these with the MT impedance tensors to compute the modeled geoelectric field across Britain for three major geomagnetic storms of the past decades: March 1989, October 2003, and September 2017. During the larger geomagnetic storms of 1989 and 2003 we observe that the modeled amplitude of geoelectric fields is not solely determined by latitude but also locally by the geology. The largest electric fields are surprisingly found in central Britain, reaching 12 V/km during the March 1989 storm.