Scientific American: "New Studies Warn of Cataclysmic Solar Superstorms"

Article here. Although the 1859 Carrington Event is considered to be the largest geomagnetic storm recorded, new research shows that the May 1921 "New York Railroad Storm" might have been even stronger. From the article:

       A new analysis published in Space Weather a month before Love’s paper, however, shows the effects of the 1921 New York Railroad Storm were just as severe, if not more so. Although the latter event gets its name from disruption to trains in New York City following a fire in a control tower on May 15, study author Mike Hapgood of Rutherford Appleton Laboratory in England found that the association between those occurrences and the storm was weak. But looking at previously overlooked written records, Hapgood noted that three major fires had erupted on the same day. One, sparked by strong currents in telegraph wires at a railroad station in Brewster, N.Y., burned the station to the ground. The second was a fire that destroyed a telephone exchange in Karlstad, Sweden, while the third occurred in Ontario.

      The 1921 event unfolded in two phases, unleashing an opening burst of disruption before intensifying into a full-fledged superstorm. In Karlstad, for instance, night-shift operators of the telephone exchange initially reported that their equipment was malfunctioning and had begun emitting smoke. After the smoke cleared, in the hours before dawn, electrical cables in the exchange erupted in flames, eventually setting the entire structure ablaze. By sunrise, the interior had burned to ashes.

      Hapgood’s research shows just how impactful the storm of 1921 really was—and not just in the U.S. and Sweden. Records from Samoa, which is not far from the equator, show that auroral displays were visible to observers even in this low-latitude locale. “It’s an astonishing observation,” Hapgood says. Auroras were also recorded near Paris and in Arizona, while telegraph systems and telephone lines were disrupted in the U.K., New Zealand, Denmark, Japan, Brazil and Canada. “[This storm] has got a period of earlier activity that caused some problems,” Hapgood says, “and then the next night, all hell broke loose,” as what began as a more modest event from the sun grew in strength to become far more disruptive.