Developing a UK new ground electric field model for SWIMMR N4 (SAGE) : interim report

This interim report describes progress made to date in the SWIMMR N4 (SAGE) project regarding the update of geoelectric field modelling in Britain during magnetic storms. We describe research efforts to understand how the present thin-sheet method of computing the geoelectric field from magnetic variation data compares to the measured field at the three UK observatories. We then examine how measured magnetotelluric (MT) impedances can be used to improve the modelled geoelectric field during space weather events. Next, we describe the fieldwork campaign to collect new high quality magnetotelluric data in Britain. As of August 2022, BGS have collected magnetic and electric field measurements at 32 sites across England, southern Scotland, and Wales. Around 18 remain to be completed by March 2023. We present results from all the sites collected thus far. The measurements have been used to compute MT impedances which demonstrate large variability attributed to the underlying geology across Britain. We use the new MT data to re-evaluate the geoelectric field during the September 2017 storm and find large differences, for example, in central Yorkshire the electric field estimates are about one-tenth the magnitude observed in Lincolnshire around 100 km distant. In the future the MT measurements will be included in a new 3D model of the conductivity of Britain for space weather hazard purposes.