For half a day in the middle of May 1921, the Sun put the young electrical world to the test. The storm that struck the Earth was the most intense of the entire twentieth century. It burned out telegraph and telephone systems on both sides of the Atlantic, it set a railroad signal tower on fire in New York City, and it dropped the aurora far enough south that people far from the poles looked up at a burning sky.
What happened
A powerful eruption on the Sun drove a storm into the Earth's magnetic field over May 14 and 15, 1921. As the field heaved, it induced strong currents in every long conductor humanity had strung across the land. In 1921 that meant the telegraph and telephone networks and the signalling systems of the railroads. Those currents are the whole danger of a great storm. They turn the wires we depend on into the path the storm travels through.
The fire in New York
The storm gave the event its name. At a railroad control tower serving the lines into New York City, the induced current overwhelmed the signalling equipment and started a fire that burned the building. Across the country and across Europe, telegraph offices reported equipment running hot, operators taking shocks, and circuits carrying current no one had switched on. The pattern was the Carrington pattern repeated sixty years later, now on a far denser web of wire.
The storm found the control tower, overran its signals, and set it alight. The lights overhead were beautiful. The current in the wires was not.
How far south it reached
The aurora was reported far from its usual northern home, well down into the mid-latitudes of the United States and across similar latitudes elsewhere. Modern reconstructions of the 1921 storm, built from the surviving magnetic records and the global spread of sightings, rank it among the most intense events on the instrumental record, in the same conversation as 1859 and 1989.
If it happened today
In 1921 the grid barely existed. The danger lived in telegraph wires and railroad signals. Today the same induced currents would pour into continental power grids, long pipelines, undersea cables and the electronics that run them. A storm of 1921's strength is no longer a story about a burned signal tower. It is a direct test of the transformers at the heart of the power system, the large and slow to replace machines a severe storm threatens most.
How we know
The storm is read today from the magnetograms of early twentieth century observatories and from the newspaper and operator accounts of the damage it caused. Those records let researchers estimate its intensity and reach, and they place the May 1921 event at the top of the twentieth century's list.
Would you have seen it from your sky?
In 1921 the storm reached far past the poles and found the wires below. The next great storm will arrive the same way, with little warning. Magna Earth keeps the auroral oval under watch over your exact coordinates and tells you the hour it crosses into your sky, so you are standing under it rather than reading about it the morning after.
Know the night it reaches you — from $14/mo