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The Great Geomagnetic Storm of May 1921
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It began on May 12, 1921 when giant sunspot AR1842, crossing the sun during the declining phase of Solar Cycle 15, began to flare. One explosion after another hurled coronal mass ejections (CMEs) directly toward Earth. For the next 3 days, CMEs rocked Earth’s magnetic field. Scientists around the world were surprised when their magnetometers suddenly went offscale, pens in strip chart recorders pegged uselessly to the top of the paper.
The Great Storm of May 1921: An Exemplar of a Dangerous Space Weather Event
From: Advancing Earth and Space Science
By: Mike Hapgood
June 23, 2019