Methodology for time-domain estimation of storm time geoelectric fields using the 3-D magnetotelluric response tensors

Geoelectric fields at the Earth's surface caused by magnetic storms constitute a hazard to the operation of electric power grids and related infrastructure. The ability to estimate these geoelectric fields in close to real time and provide local predictions would better equip the industry to mitigate negative impacts on their operations. Here we report progress toward this goal: development of robust algorithms that convolve a magnetic storm time series with a frequency domain impedance for a realistic three-dimensional (3-D) Earth, to estimate the local, storm time geoelectric field. Both frequency domain and time domain approaches are presented and validated against storm time geoelectric field data measured in Japan. The methods are then compared in the context of a real-time application. Plain Language Summary Magnetic storms can pose significant risks to critical technology infrastructure such as the electric power grids. Real-time estimation of geoelectric fields can give the grid operators an opportunity for prompt intervention that would mitigate the long-term consequences. We develop and validate a method to efficiently and accurately (to within 85-90%) nowcast storm time geoelectric fields in close to real time in places where both geomagnetic field data are collected or estimated, and a magnetotelluric impedance has been previously measured. This method can be applied in all regions where magnetotelluric data have been collected, including in the United States, where USArray magnetotelluric data survey is ongoing and by 2018 will cover about two thirds of the country. In this work, we focus on the algorithm development only.