The March 1940 Superstorm

What happened

A cluster of at least five major solar flares in late March 1940 — including an estimated X35-class event — produced one of the largest storm sudden commencements recorded since 1868, with a sudden magnetic impulse exceeding 425 nT at Tucson. Long-distance telephone and telegraph lines across the United States and Canada experienced severe interference, and the storm's amplitude and rapidity overwhelmed the recording range of contemporary magnetometers across North America, leaving parts of the event permanently under-documented.