Strongest geomagnetic storm

At 11:18 a.m. on 1 September 1859, British astronomer Richard Carrington observed two blinding explosions occurring on the surface of the Sun. The following dawn, all over the world the skies were ablaze with aurorae caused by the charged particles from the eruptions interacting with Earth's magnetosphere, having taken just 18 hours to cross the space between the Sun and the Earth (the journey normally takes three or four days).

The study of ice cores suggest that events of this magnitude occur roughly ever 500 years. The magnetic disruptions caused by the "Carrington Event" affected telegraph systems all over Europe and North America.