Characteristics and Sources of Intense Geoelectric Fields in the United States during Geomagnetic Storms

Intense geoelectric fields during geomagnetic storms drive geomagnetically induced currents (GIC) in power grids and other infrastructure, yet there are limited direct measurements of these storm-time geoelectric fields. Moreover, most previous studies examining storm-time geoelectric fields focused on single events or small geographic regions, making it difficult to determine the typical source(s) of intense geoelectric fields. We report a comparative analysis of the characteristics and sources of intense geoelectric fields over multiple geomagnetic storms, using 1-s cadence geoelectric field measurements made at magnetotelluric (MT) survey sites distributed widely across the United States. Temporally localized intense perturbations in measured geoelectric fields with prominences of at least 500 mV/km were detected during geomagnetic storms with Dst minima (Dstmin) of less than −100 nT from 2006 to 2019. Various sources of intense geoelectric fields have been identified, including interplanetary shocks, interplanetary magnetic field turnings, substorms, and ultralow frequency (ULF) waves. Detailed case studies showing different driving mechanisms of intense geoelectric fields were also investigated using coordinated space and ground observations. The sources of intense geoelectric fields differ from storm to storm. This is likely because the generation of geoelectric fields depends on multiple factors including regional Earth conductivity, sources from the solar wind and magnetosphere-ionosphere system, as well as the location of observations relative to these sources.