May 1921 geomagnetic storm

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May 1921 geomagnetic storm
Magnetosphere rendition.jpg
Artist's depiction of solar wind striking Earth's magnetosphere (size and distance not to scale)
TypeGeomagnetic storm
Formed13 May 1921 (1921-05-13)
Dissipated15 May 1921 (1921-05-15)
DamageSubstantial damage to overhead and underwater telegraph equipment; electrical fires
Power outagesLocalized electric grid interruptions
Areas affectedWorldwide

The three-day May 1921 geomagnetic storm, also known as the New York Railroad Storm, was caused by the impact of an extraordinarily powerful coronal mass ejection on Earth's magnetosphere. It occurred on 13–15 May as part of solar 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 the March 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 near Grand Central Terminal which made it known as the "New York Railroad Storm".[1] Contemporary scientists estimated the size of the sunspot (AR1842)[1] which began on May 10th—and caused the storm—as 151,000 by 34,000 km (94,000 by 21,000 miles).[3][4]

The storm was extensively reported in New York City, which was a center of telegraph activity as a railroad hub.[5] Auroras ("northern lights") appeared throughout the eastern United 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. Radio propagation was enhanced during the storm due to ionosphere involvement, however, enabling unusually good long-distance reception. Electric lights were not noticeably affected.[6]

Undersea telegraph cables were affected by the storm. Damage to telegraph systems was also reported in Europe[7] and the Southern Hemisphere.[8]

2021 study estimated Dstm of -533 nT.[9]

See also[edit]



  1. ^ a b c Phillips, Tony (12 May 2020). "The Great Geomagnetic Storm of May 1921". Retrieved 1 June 2020.
  2. ^ Dr Tony Phillips (January 21, 2009). "Severe Space Weather - Social and Economic Impacts". NASA. Retrieved December 18, 2012.
  3. ^ "Borealis Cause, Sun Spots, Will Diminish Today" (PDF). Chicago Daily Tribune. May 16, 1921. p. 4. Archived from the original (PDF) on August 12, 2014. Retrieved December 19, 2012.
  4. ^ "Sun Spots Vanishing" (PDF). The Los Angeles Times. May 16, 1921. pp. 1 & 2. Archived from the original (PDF) on August 12, 2014. Retrieved December 19, 2012.
  5. ^ "May 13, 1921 – The New York Railroad Storm". Archived from the original on 28 September 2006. Retrieved 31 May 2020.
  6. ^ "Sunspot Aurora Paralyses Wires" (PDF). New York Times. May 15, 1921. pp. 1 & 3. Archived from the original (PDF) on October 22, 2013. Retrieved December 19, 2012.
  7. ^ "Cables Damaged by Sunspot Aurora" (PDF). New York Times. May 17, 1921. pp. 1 & 4. Archived from the original (PDF) on August 12, 2014. Retrieved December 19, 2012.
  8. ^ "Aurora Borealis". Hawera & Normanby Star. May 16, 1921. p. 8. Retrieved December 18, 2012.
  9. ^ Love, Jeffrey J. (2021). "Extreme‐event magnetic storm probabilities derived from rank statistics of historical Dst intensities for solar cycles 14‐24". Space Weather. 19 (4). Bibcode:2021SpWea..1902579L. doi:10.1029/2020SW002579.


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