society’s vulnerability to space weather. The workshop concluded with a discussion of “the way forward,” in which the participants identified un- or underexplored topics relevant to the question of space weather impacts, highlighted various weaknesses in the existing space weather services infrastructure, and suggested improvements that would yield the greatest benefits in space weather risk management.

The key themes, ideas, and insights that emerged during the workshop’s 1½ days of informative presentations and lively discussions are summarized in this report, which was prepared by the members of the ad hoc NRC Committee on the Societal and Economic Impacts of Severe Space Weather Events: A Workshop tasked with organizing the workshop (Appendix D). To set the stage for the chapters that follow, we begin with a description of the magnetic superstorms of August-September 1859, by some measures the most severe space weather event on record. Known as the Carrington event, the 1859 storms were referred to throughout the workshop as an example of the kind of extreme space weather event that, if it were to occur today, could have profound societal and economic consequences, with cascading effects throughout the complex and interrelated infrastructures of modern society.

The Great Magnetic Storms of August-September 1859 (the Carrington Event)

Shortly after midnight on September 2, 1859, campers in the Rocky Mountains were awakened by an “auroral light, so bright that one could easily read common print.” The campers’ account, published in The Rocky Mountain News, continues, “Some of the party insisted that it was daylight and began the preparation of breakfast.”1 Eighteen hundred miles to the east, Henry C. Perkins, a respected physician in Newburyport, Massachusetts, observed “a perfect dome of alternate red and green streamers” over New England. To the citizens of Havana, Cuba, the sky that night “appeared stained with blood and in a state of general conflagration” (Figure 1.1). Dramatic auroral displays had been seen five nights before as well, on the night of August 28/29, when (again in the words of Dr. Perkins) “the whole celestial vault was glowing with streamers, crimson, yellow, and white, gathered into waving brilliant folds.”2 In New York City, thousands gathered on sidewalks and rooftops to watch “the heavens … arrayed in a drapery more gorgeous than they have been for years.” The aurora that New Yorkers witnessed that Sunday night, The New York Times assured its readers, “will be referred to hereafter among the events which occur but once or twice in a lifetime.”3

From August 28 through September 4, auroral displays of extraordinary brilliance were observed throughout North and South America, Europe, Asia, and Australia, and were seen as far south as Hawaii, the Caribbean, and Central America in the Northern Hemisphere and in the Southern Hemisphere as far north as Santiago, Chile (Figure 1.2).4 Even after daybreak, when the aurora was no longer visible, its presence continued to be felt through the effect of the auroral currents. Magnetic observatories recorded disturbances in Earth’s field so extreme that magnetometer traces were driven off scale, and telegraph networks around the world—the “Victorian Internet”5—experienced major disruptions and outages. “The electricity which attended this beautiful phenomenon took possession of the magnetic wires throughout the country,” the Philadelphia Evening Bulletin reported, “and there were numerous side displays in the telegraph offices where fantastical and unreadable messages came through the instruments, and where the atmospheric fireworks assumed shape and substance in brilliant sparks.”6 In several locations, operators disconnected their systems from the batteries and sent messages using only the current induced by the aurora.7

The auroras were the visible manifestation of two intense magnetic storms that occurred near the peak of the sunspot cycle. On September 1, the day before the onset of the second storm, Richard Carrington, a British amateur astronomer, observed an outburst of “two patches of intensely bright and white light”8 from a large and complex group of sunspots near the center of the Sun’s disk. The outburst lasted 5 minutes and was also observed, independently, by Richard Hodgson from his home observatory near London. Carrington noted that the solar outburst—a white-light flare—was followed the next day by a magnetic storm, but he cautioned against inferring a causal connection between the two events. “One swallow,” he is reported to have said, “does not make a summer.”9

Space Weather: “The Mysterious Connection Between the Solar Spots and Terrestrial Magnetism”

The dazzling auroral displays, magnetic disturbances, and disruptions of the telegraph network that occurred between August 28 and September 4, 1859, were recognized by contemporary observers—at least the scientifically

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