From Scientific American:
As night was falling across the Americas on Sunday, August 28, 1859, the phantom shapes of the auroras could already be seen overhead. From Maine to the tip of Florida, vivid curtains of light took the skies. Startled Cubans saw the auroras directly overhead; ships’ logs near the equator described crimson lights reaching halfway to the zenith. Many people thought their cities had caught fire. Scientific instruments around the world, patiently recording minute changes in Earth’s magnetism, suddenly shot off scale, and spurious electric currents surged into the world’s telegraph systems. In Baltimore telegraph operators labored from 8 p.m. until 10 a.m. the next day to transmit a mere 400-word press report.
Just before noon the following Thursday, September 1, English astronomer Richard C. Carrington was sketching a curious group of sunspots—curious on account of the dark areas’ enormous size. At 11:18 a.m. he witnessed an intense white light flash from two locations within the sunspot group. He called out in vain to anyone in the observatory to come see the brief five-minute spectacle, but solitary astronomers seldom have an audience to share their excitement. Seventeen hours later in the Americas a second wave of auroras turned night to day as far south as Panama. People could read the newspaper by their crimson and green light. Gold miners in the Rocky Mountains woke up and ate breakfast at 1 a.m., thinking the sun had risen on a cloudy day. Telegraph systems became unusable across Europe and North America.
The news media of the day looked for researchers able to explain the phenomena, but at the time scientists scarcely understood auroral displays at all. Were they meteoritic matter from space, reflected light from polar icebergs or a high-altitude version of lightning? It was the Great Aurora of 1859 itself that ushered in a new paradigm. The October 15 issue of Scientific American noted that ‘‘a connection between the northern lights and forces of electricity and magnetism is now fully established.” Work since then has established that auroral displays ultimately originate in violent events on the sun, which fire off huge clouds of plasma and momentarily disrupt our planet’s magnetic field.
More here.