May 1921 geomagnetic storm
May 1921 geomagnetic storm: a brief, easy-to-understand version
The three-day May 1921 geomagnetic storm, also known as the New York Railroad Storm, happened May 13–15 as a powerful coronal mass ejection hit Earth’s magnetic field during solar cycle 15. It was the strongest geomagnetic storm of the 20th century.
Strong ground currents damaged telegraph equipment and sparked fires around the world, including one near Grand Central Terminal in New York. Telegraph service in the United States slowed and essentially stopped around midnight May 14 due to blown fuses and damaged lines. Undersea telegraph cables were affected, and damage was reported in Europe and the Southern Hemisphere. Electric lights were not noticeably affected, but radio reception could improve because the ionosphere was energized. Auroras were seen across the eastern United States at night.
Scientists estimated the sunspot driving the storm (AR1842) was about 151,000 by 34,000 kilometers in size.
In space weather terms, the storm was very strong. The disturbance storm time index (Dst) peaked near −907 nT (with an uncertainty of about ±132 nT). For comparison, the famous 1859 Carrington Event is estimated to be between −800 nT and −1750 nT, and the March 1989 storm was about −589 nT.
The May 1921 storm shows how a powerful solar burst can disrupt telegraph networks and light up skies far from the poles.
This page was last edited on 2 February 2026, at 21:05 (CET).