Magnetic storms induce geoelectric fields at Earth's surface that can interfere with grounded long-line systems. The September 1859 storm disrupted global telegraph operations, the March 1989 storm caused a blackout in Canada and interfered with electric-power-transmission systems in the United States, and other storms have had related impacts. The geographic and temporal dependence of geoelectric fields are functions of both geomagnetic variation and local surface impedance, which differ considerably across different geological regions. These dependencies can be mapped across the contiguous United States by combining magnetotelluric impedance tensors with ground magnetometer time series. This review illustrates such mapping for the 1989 storm and shows that power-system interference was experienced where surface impedance is high, and when and where geoelectric fields were intense. Statistical analyses indicate that storms comparable to that of March 1989 occur roughly once every four solar cycles. Ongoing developments in numerical modeling and real-time monitoring are anticipated to enable prediction of geoelectric hazards.

• ▪  Magnetic storms can induced electric fields in the solid Earth that interfere with electric-power-transmission systems.
• ▪  Geoelectric hazards depend on the storm-time geomagnetic disturbance and the electrical conductivity structure of Earth.
• ▪  Historically, impacts on telecommunication and power-transmission systems in the United States have been concentrated in the East and Midwest.
• ▪  The future occurrence of a magnetic superstorm could cause widespread disruption of electric-power-transmission systems.