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An analysis is made of geophysical records of the 24 March 1940, magnetic storm and related reports of interference on long‐line communication and power systems across the contiguous United States and, to a lesser extent, Canada. Most long‐line system interference occurred during local daytime, after the second of two storm sudden commencements and during the early part of the storm's main phase. The high degree of system interference experienced during this storm is inferred to have been due to unusually large‐amplitude and unusually rapid geomagnetic field variation, possibly driven by interacting interplanetary coronal‐mass ejections. Geomagnetic field variation, in turn, induced geoelectric fields in the electrically conducting solid Earth, establishing large potential differences (voltages) between grounding points at communication depots and transformer substations connected by long transmission lines. It is shown that March 1940 storm‐time communication‐ and power‐system interference was primarily experienced over regions of high electromagnetic surface impedance, mainly in the upper Midwest and eastern United States. Potential differences measured on several grounded long lines during the storm exceeded 1‐min resolution voltages that would have been induced by the March 1989 storm. In some places, voltages exceeded American electric‐power‐industry benchmarks. It is concluded that the March 1940 magnetic storm was unusually effective at inducing geoelectric fields. Although modern communication systems are now much less dependent on long electrically conducting transmission lines, modern electric‐power‐transmission systems are more dependent on such lines, and they, thus, might experience interference with the future occurrence of a storm as effective as that of March 1940. Plain Language Summary: On 24 March 1940, a pair of concentrated, and possibly interacting, bursts of solar wind forced an intense magnetic storm on Earth. Geomagnetic field variation during the storm induced high‐amplitude geoelectric fields in the solid Earth's conducting interior. These geoelectric fields drove uncontrolled currents in grounded long‐wire communication‐ and electricity‐power‐transmission systems in the United States and Canada, causing significant operational interference in those systems. This interference was primarily experienced in the upper Midwest and the eastern United States, and many incidents of interference were reported in the popular press. Voltages monitored on several lines were greater than studies estimate would have occurred during the great storm of March 1989. In terms of its impact on communication and power systems, the March 1940 magnetic storm was one of the most significant ever experienced by the United States. Modern communication systems are less dependent on long electrically conducting transmission lines. On the other hand, modern electric‐power‐transmission systems are more dependent on such lines, and they, thus, might experience interference with the future occurrence of a storm like that of March 1940. Key Points: Extreme geomagnetic field variation realized during the March 1940 storm might have resulted from the interaction of ICMEsLong‐line interference in the U.S. occurred during local daytime and in the upper Midwest and East, where surface impedance is highVoltages measured on grounded long lines during the storm exceeded 1‐min voltages that would have been induced by the March 1989 storm [ABSTRACT FROM AUTHOR]

A study is made of the relations between geomagnetic and geoelectric field variation, Earth‐surface impedance, and operational interference ("anomalies") experienced on electric‐power systems across the contiguous United States during the 13–14 March, 1989, magnetic storm. For this, a 1‐min‐resolution sequence of geomagnetic field maps is constructed from magnetometer time series acquired at ground‐based observatories. Induced geoelectric field maps are calculated by convolving the geomagnetic maps with magnetotelluric impedance tensors. During the storm, anomalies were concentrated where the lithosphere is electrically resistive, and when and where geoelectric field amplitudes were high. This was particularly true in the Mid‐Atlantic, Northeast, and the upper Midwest. Few anomalies were experienced in other parts of the Midwest and across much of the West, where the lithosphere is more conductive, and when and where geoelectric field amplitudes were low. Peak 1‐min‐resolution geoelectric field amplitude ranged from 21.66 V/km in Maine and 19.02 V/km in Virginia to <0.02 V/km in Idaho. Latitude‐dependent organization of geoelectric hazards by auroral‐zone electrojet currents is detectable, but it is much weaker than geographic organization due to surface impedance. Hazardous geoelectric fields were induced during different storm phases, at different local times, and, by inference, by a variety of ionospheric currents. Compared to geoelectric field amplitudes realized across the United States during March 1989, hazard maps used by utility companies to estimate systems exposure have much less geographic detail and a much smaller maximum‐to‐minimum range in geoelectric field amplitude. Future research would benefit from denser geomagnetic monitoring, additional magnetotelluric surveying, and access to power‐system impact data. Plain Language Summary: Electric fields induced in the Earth during magnetic storms can drive uncontrolled currents in electric‐power systems, interfering with their operation. Geomagnetically induced currents realized during the magnetic storm of March 1989 caused a blackout in Québec, Canada, and, in the Mid‐Atlantic and Northeast United States, they caused operational interference for electric‐power companies and damaged a high‐voltage transformer. In support of projects for estimating geoelectric hazards and improving power‐system resilience, maps are made of March 1989 magnetic‐storm geoelectric hazards and corresponding impacts on United States power systems. Results are based on modeling geomagnetic monitoring data, geoelectromagnetic survey data, and a compilation of published reports of power‐system interference. During the storm, electric‐power system interference was concentrated where the lithosphere is relatively electrically resistive, and when and where the geoelectric field was of high amplitude. This was particularly true in the Mid‐Atlantic and Northeast, near many of America's largest cities, and in the upper Midwest. Retrospective analyses, such as this one for the March 1989 storm, show where utility companies might concentrate their efforts to mitigate the impacts of future magnetic superstorms. Key Points: Electric‐power system interference was concentrated where surface impedance is high, and when and where geoelectric field amplitudes were highHigh geoelectric hazards and numerous power‐system anomalies were realized in the Eastern United States, near many large citiesPower‐system impact data provide important, if partial, validation of retrospectively constructed geoelectric field maps [ABSTRACT FROM AUTHOR]

At present, the most reliable information for inferring storm‐time ground electric fields along electrical transmission lines comes from coarsely sampled, national‐scale magnetotelluric (MT) data sets, such as that provided by the EarthScope USArray program. An underlying assumption in the use of such data is that they adequately sample the spatial heterogeneity of the surface relationship between geomagnetic and geoelectric fields. Here, we assess the degree to which the density of MT data sampling affects geoelectric hazard assessments. For electrical transmission networks in each of four focus regions across the contiguous United States, we perform two parallel band‐limited (101–103 s) hazard analyses: one using only USArray‐style (∼70‐km station spacing) MT data, and one incorporating denser (≪70‐km station spacing) MT data. We find that the use of USArray‐style MT sampling alone provides a useful first‐order estimate of integrated geoelectric fields along electrical transmission lines. However, we also find that the use of higher density MT data can in some areas lead to order‐of‐magnitude differences in line‐averaged electric field estimates at the level of individual transmission lines and can also yield significant differences in subregional hazard patterns. As we demonstrate using variogram plots, these differences reflect short‐spatial‐scale variability in Earth conductivity, which in turn reflects regional lithotectonic structure and history. We also provide a cautionary example in the use of electrical conductivity models to predict dense MT data; although valuable for hazard applications, models may only be able to reproduce surface geoelectric fields as captured by the MT data from which they were derived. Plain Language Summary: Intense geomagnetic disturbances can drive large electrical currents in grounded technological infrastructure with deleterious effects. In the case of national power grids, a 1‐in‐100‐year event could cause widespread failure of long‐distance electrical distribution networks. The magnitude of these electrical currents and their associated electric fields depends crucially on how the Earth conducts electricity beneath transmission lines; as the Earth's electrical conductivity structure can spatially vary dramatically, the degree of this geoelectric hazard varies geographically. Many recent studies have utilized magnetotelluric (MT) measurements that are made several tens of kilometers apart to examine spatial patterns in hazard levels. Because the Earth's electrical conductivity structure can vary substantially at shorter length scales, we examine the degree to which higher density MT data coverage changes geoelectric hazard estimates. We find that coarse‐scale MT data, as used in previous studies, provide a valuable and generally valid estimate of regional hazard patterns. However, we also find that denser data can yield large differences (up to an order of magnitude) in hazard estimates at the level of individual electrical transmission lines. Therefore, coarse data coverage is valuable for first‐order hazard estimates, but denser data coverage can provide useful refinements to those estimates. Key Points: National‐scale magnetotelluric surveys (e.g., ∼70 km station spacing) provide a useful first‐order estimate of geoelectric hazardsHigher density magnetotelluric data coverage can yield local order‐of‐magnitude differences in hazard estimates in some settingsLithotectonic structure, which dictates lithospheric electrical conductivity, controls the spatial scale of geoelectric field variability [ABSTRACT FROM AUTHOR]