The three-dayMay 1921 geomagnetic storm, also known as theNew York Railroad Storm, was caused by the impact of an extraordinarily powerfulcoronal mass ejection onEarth'smagnetosphere. It occurred on 13–15 May as part ofsolar cycle 15, and was the most intense geomagnetic storm of the 20th century.[1]
Since it occurred before the extensive interconnectivity of electrical systems and the general electrical dependence of infrastructure in the developed world, its effect was restricted; however, its ground currents were up to an order of magnitude greater than those of theMarch 1989 geomagnetic storm which interrupted electrical service to large parts of northeastern North America.[2]
The storm's electrical current sparked a number of fires worldwide, including one nearGrand Central Terminal which made it known as the "New York Railroad Storm".[1] Contemporary scientists estimated the size of thesunspot (AR1842)[1] which began on May 10—and caused the storm—as 151,000 by 34,000 km (94,000 by 21,000 miles).[3][4]
The storm was extensively reported inNew York City, which was a center of telegraph activity as a railroad hub.[5]Auroras ("northern lights") appeared throughout the easternUnited States, creating brightly lit night skies.Telegraph service in the U.S. first slowed and then virtually stopped at about midnight on 14 May due to blown fuses and damaged equipment.[6] Radio propagation was enhanced during the storm due to ionosphere involvement, however, enabling unusually good long-distance reception. Electric lights were not noticeably affected.[7]
Undersea telegraph cables were affected by the storm. Damage to telegraph systems was also reported in Europe[8] and theSouthern Hemisphere.[9]
In space weather, thedisturbance storm time index (Dst index) is a measure often used for determining the intensity of solar storms. A negative Dst index means thatEarth's magnetic field is weakened—particularly the case during solar storms—with a more negative Dst index indicating a stronger solar storm.
A paper in 2019 estimated that the May 1921 geomagnetic storm had a peak Dst of −907±132 nT.[10]
^"Sun Spots Vanishing"(PDF).The Los Angeles Times. May 16, 1921. pp. 1 & 2. Archived fromthe original(PDF) on August 12, 2014. RetrievedDecember 19, 2012.
Henry K. Bunn (May 15, 1921)."The Story The Week Has Told"(PDF).The Atlanta Constitution. p. 8. Archived fromthe original(PDF) on March 4, 2016. RetrievedDecember 19, 2012.
"Aurora is Disturber"(PDF).The Los Angeles Times. May 15, 1921. pp. 1 & 2. Archived fromthe original(PDF) on March 3, 2016. RetrievedDecember 19, 2012.
"The Aurora Borealis"(PDF).The Los Angeles Times. May 16, 1921. p. 14. Archived fromthe original(PDF) on March 3, 2016. RetrievedDecember 19, 2012.
