Jeffrey J. Love

Space weather poses an important but under‐quantified threat to society. While severe geomagnetic storms are recognized as potential global catastrophes, their socio‐economic impacts remain poorly quantified. We present a novel physics‐engineering‐economic framework that links geophysical drivers to power grid geoelectric fields, transformer vulnerability, and macroeconomic consequences. Using the United States as an example, we estimate daily economic losses for a 250‐year geomagnetic storm from transformer thermal heating of 1.81 billion USD (95 percent confidence interval: 1.65 to 1.96 billion USD), disrupting power for approximately 5.1 million people and 135,000 businesses. These estimates are conservative lower bounds, reflecting only transformer thermal heating effects and excluding voltage collapse, cascading failures, and restoration costs. The true societal risk could be substantially higher. Nonetheless, this contribution provides the first nationwide end‐to‐end coupling from space physics to potential macroeconomic loss, with quantified uncertainties. Our results demonstrate that coupled socio‐economic modeling of space weather is feasible and essential, and the framework is scalable and transferable, offering a template for assessing space weather risk to critical infrastructure in other countries.

Using solar wind and ground magnetometer data for solar cycles 23–25, extreme‐value power‐law models are developed that quantify relationships between near‐Earth solar wind speed and Sun‐to‐Earth transit times of interplanetary coronal mass ejections (ICMEs). The models of near‐Earth solar wind speed, conditional on ICME transit time, are (a) approximately stable for transit times as short as 15 hr, (b) sublinear, consistent with hydrodynamic drag acting on ICMEs during transit, with linear (ballistic) models not supported by the data, (c) representative of most of the information content of the data, and (d) insensitive to the solar cycle and interplanetary preconditioning. Applying the models to the September 1859 Carrington event ( hours), we estimate an ICME‐average speed of km/s and a 1‐hr ICME‐maximum speed of km/s. These estimates indicate that Carrington‐class magnetic storms do not require exceptionally extreme solar wind speeds, with values lower than some previous estimates.

Using a list of sudden‐commencement storms, the ring‐current index, and 1‐h near‐Earth solar‐wind measurements from solar cycles 20–25, we develop extreme‐value statistical models relating storm intensity to the storm main‐phase maximum duskward interplanetary electric field . The conditional relationship is demonstrably sublinear—linear models are confidently rejected—indicating saturation of magnetospheric response under extreme solar‐wind forcing. An event like that of July 2012 ( mV/m), if Earth‐directed, would be associated with a median storm intensity of nT. Storms comparable to March 1989 ( nT) correspond to electric fields of mV/m, while Carrington‐class storms ( nT) correspond to mV/m—substantially lower than several previous estimates. These results indicate that solar‐wind conditions capable of driving extremely intense magnetic storms are less exceptional, and potentially more frequent, than previously thought.

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 induce 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.

The United States Magnetotelluric Array (USMTArray) data set, collected in the years 2006–2024, consists of more than 1,700 long‐period magnetotelluric stations covering the entirety of the contiguous United States on a quasi‐regular 70 km grid. Funding across multiple federal agencies was critical to sustaining this effort to its completion. Important components of the project included active guidance and participation from the MT community, the open and timely availability of all data, and the application of consistent instrumentation and robust data processing. Together with parallel advancement in the development of publicly available three‐dimensional (3D) inversion codes, the USMTArray has revitalized the US magnetotelluric community and increased the visibility of magnetotellurics within the Earth‐science community. Taken as a whole, these data are visualized as the National Impedance Map, which, together with a 3D synthesis conductivity model of the nation, reveals the electrical architecture of the contiguous US. USMTArray data are used by researchers worldwide for fundamental and applied studies, including investigations of continental architecture and evolution, estimation of hazards to critical infrastructure due to geomagnetic storms, and assessment of the nation's undiscovered geothermal and mineral resources. We here review the history and development of the project, discuss the challenges and successes in its execution, present the National Impedance Map and synthesis conductivity model, and highlight the breadth of research stemming from this rich data set.

A map is presented of median 1‐min‐resolution peak geoelectric‐field strength across the United States as would be induced by magnetic storms as intense as the 2 September 1859 Carrington storm. The map is constructed from two data sets: Magnetometer time series from 22 ground‐based observatories recording 40 magnetic storms, and surface impedance tensors derived from magnetotelluric measurements acquired at 1616 survey sites across the contiguous United States. Carrington‐class storm geoelectric fields are likely to be very strong in the United States East and Midwest; V/km at many places. In Virginia, strengths would likely range from 30.30 V/km, with a 68% confidence interval of [19.44,47.20] V/km, to as low as 0.05 [0.03,0.07] V/km. Comparison of model geopotentials with those measured on 30 long lines, indicates errors of about 18%. A Carrington‐class storm would likely induce geoelectric fields with strengths 55% greater than for the 13–14 March 1989 storm.

A compilation is presented of impacts (interference and damage) realized on long‐line telegraph and telephone systems across North America during the 13–16 May 1921 magnetic storm. Impacts occurred primarily during local nighttime, after the third of four sudden commencements, and during the storm's most‐prominent main phase. Impacts are attributed to rapid and high‐amplitude geomagnetic field variation generated by substorms. This induced potential differences and between the grounding points of communication networks that were sufficient to cause system interference and damage. In the United States, impacts were concentrated in the Midwest and in the East, regions characterized by high electromagnetic surface impedance. Given technological changes, modern telecommunication systems are less exposed to storms like that of May 1921, while power‐grid systems are now more exposed to them.

From a survey of published accounts of visual sightings of aurorae, a compilation is presented of the lowest identified geomagnetic latitude at which discrete aurorae were seen at local zenith during magnetic storms having intensities with maximum nT. The compilation includes data for the superstorms of 2 September 1859, 4 February 1872, and 15 May 1921. A statistical model is developed representing the equatorward boundary of discrete aurorae versus storm intensity. The model indicates that a once‐per‐century storm would likely induce discrete aurorae at zenith down to a geomagnetic latitude of . Insofar as aurorae can be taken as a proxy for electrojet currents, such a storm would expose many nighttime electric‐power systems in the contiguous United States or Europe to high levels of geomagnetic disturbance. A Carrington‐class storm would induce discrete aurorae down to . These exposures are much greater than those indicated in recent numerical simulations of extreme magnetic storms. Using the model to infer storm intensity from reports of low‐latitude aurorae, a storm on 28 August 1859 likely had maximum nT. That this storm occurred just a few days before the Carrington storm of 2 September (maximum nT) deserves attention. A storm that occurred on 17 September 1770 is estimated to have had maximum nT. A vision described by Ezekiel could have been inspired by aurorae from a storm with maximum nT.

A detailed analysis is made of horizontal‐component geomagnetic‐disturbance data acquired at the Colaba observatory in India recording the Carrington magnetic storm of September 1859. Prior to attaining its maximum absolute value, disturbance at Colaba increased with an e ‐folding timescale of 0.46 hr (28 min). Following its maximum, absolute disturbance at Colaba decreased as a trend having an e ‐folding timescale of 0.31 hr (19 min). Both of these timescales are much shorter than those characterizing the drift period of ring‐current ions. Furthermore, over one 28‐min interval when absolute disturbance was increasing, the data indicate an absolute rate of change of ≥2,436 nT/hr. If this is representative of disturbance generated by a symmetric magnetospheric ring current, then, assuming a standard and widely used parameterization, an interplanetary electric field of ≥451 mV/m is indicated. An idealized and extreme solar‐wind dynamic pressure could, conceivably, reduce this bound on the interplanetary electric field to ≥202 mV/m. If the parameterization for electric‐field extrapolation is accurate, but the field strengths obtained are deemed implausible, then it can be concluded that the Colaba disturbance data were significantly affected by partial‐ring, field‐aligned, or ionospheric currents. The same conclusion is supported by the shortness of the e ‐folding timescales characterizing the Colaba data. Several prominent studies of the Carrington event need to be reconsidered.

A study is made of the intensity of the Carrington magnetic storm of September 1859 as inferred from visual measurements of horizontal-component geomagnetic disturbance made at the Colaba observatory in India. Using data from modern observatories, a lognormal statistical model of storm intensity is developed, to characterize the maximum-negative value of the storm-time disturbance index (maximum –Dst) versus geomagnetic disturbance recorded at low-latitude observatories during magnetic storms. With this model and a recently published presentation of the Colaba data, the most likely maximum –Dst of the Carrington storm and its credibility interval are estimated. A related model is used to examine individual Colaba disturbance values reported for the Carrington storm. Results indicate that only about one in a million storms with maximum –Dst like the Carrington storm would result in local disturbance greater than that reported from Colaba. This indicates that either the Colaba data were affected by magnetospheric-ionospheric current systems in addition to the ring current, or there might be something wrong with the Colaba data. If the most extreme Colaba disturbance value is included in the analysis, then, of all hypothetical storms generating the hourly average disturbance recorded at Colaba during the Carrington storm, the median maximum –Dst = 964 nT, with a 68% credibility interval of [855,1087] nT. If the most extreme Colaba disturbance value is excluded from the analysis, then the median maximum –Dst = 866 nT, with a 68% credibility interval of [768,977] nT. The widths of these intervals indicate that estimates of the occurrence frequency of Carrington-class storms are very uncertain, as are related estimates of risk for modern technological systems.