Sources of hazardous geoelectric fields in the United States during magnetic storms

Geomagnetic perturbations related to various phenomena in the near-Earth space environment can induce electric fields within the electrically conducting Earth. The geoelectric field is an important link between magnetospheric/ionospheric phenomena and geomagnetically induced currents in grounded electricity transmission networks. In evaluations of contiguous United Sates hazards, most previous studies have been focused on either 1-minute resolution geoelectric field measurements or geoelectric field time series derived from convoluting 1-minute geomagnetic field data with surface impedance tensors. To investigate sources of hazardous geoelectric fields during magnetic storms, including geoelectric fields induced by ultra-low frequency (ULF: 1 mHz to 1 Hz) waves, we use directly measured 1-second geoelectric field data from magnetotelluric survey stations that are distributed across the contiguous United States. Temporally-localized perturbations in measured geoelectric fields with a prominence of at least 0.5 V/km are detected during magnetic storms with a Dst minimum of at least -100 nT from 2008 to 2019. Most of these perturbations cannot have been resolved with 1-minute data since they correspond to phenomena that vary on smaller timescales and higher frequencies. The sources of geomagnetic perturbations inducing these extreme geoelectric fields can be categorized as interplanetary shocks, substorms, and ULF waves. We compare the geoelectric fields associated with the three sources and characterize their features. Extreme geoelectric fields related to these sources can have amplitudes of 1-2 V/km, comparable to the thresholds commonly used to identify hazardous events.