Nowcasting and validating Earth's electric‐field response to extreme space‐weather events using magnetotelluric data: application to the September 2017 geomagnetic storm and comparison to observed and modelled fields in Scotland

In the UK, geomagnetically induced currents (GICs) are calculated from thin‐sheet electrical conductivity models. In the absence of conductivity models, time derivatives of magnetic fields are sometimes used as proxies for GIC‐related electric fields. An alternative approach, favored in the US, is to calculate storm‐time electric fields from time‐independent impedance tensors computed from an array of magnetotelluric (MT) sites and storm‐time magnetic fields recorded at geomagnetic observatories or assumed from line‐current models. A paucity of direct measurements of storm‐time electric fields has restricted validation of these different techniques for nowcasting electric fields and GICs. Here, we present unique storm‐time electric‐field data from 7 MT sites in Scotland that recorded before, during and after the September 2017 magnetic storm. By Fourier transforming electric‐field spectra computed using different techniques back to the time domain, we are able to make direct comparisons with these measured storm‐time electric‐field time series. This enables us to test the validity of different approaches to nowcasting electric fields. Our preferred technique involves frequency‐domain multiplication of magnetic‐field spectra from a regional site with a local impedance tensor that has been corrected for horizontal magnetic‐field gradients present between the local site and the regional site using perturbation tensors derived from geomagnetic depth sounding (GDS). Scatter plots of scaling factors between measured and nowcasted electric fields demonstrate the importance of coupling between the polarization of the storm‐time magnetic source field and Earth's direction‐dependent deep electrical conductivity structure