For multiple days in 1770, a swath of sky over Japan, the Korean Peninsula, and the eastern coast of China looked as if it had been set ablaze, illuminated in a scorching red light. No one knew what caused it, and we only knew it happened thanks to a few scant recordings that survived the intervening centuries. It wasn’t until modern astronomy gave us a better understanding of aurora events centuries later that we learned what prompted it: A magnetic storm caused by solar activity likely struck Earth’s atmosphere, creating a crimson spectacle few people have seen since. Now, new documents reveal that there is a lot more to the story than just a red hue to the sky. A team of Japanese researchers unearthed a trove of 111 historical documents in East Asia that show that the red auroral display actually lasted not two days as we thought, but nine, from Sept, 10–19, 1770. The storm may have been the longest geomagnetic storm on human record, and the region of sky it covered was twice as large as historians initially thought.
Red auroras like the one in 1770 are caused by solar particles hitting oxygen particles in the atmosphere

Aurora are caused by charged particles hitting the planet’s upper atmosphere. When the sun spews out a flare or has a violent belch of some sort, it shoots off charged particles like electrons toward the rest of space. When these particles hit the oxygen and nitrogen in Earth’s atmosphere, they charge up the gases themselves. As those excited gases return to their normal states, they emit that excess energy in the form of gorgeous celestial lights. One of the most famous and regular instances of aurora is, of course, aurora borealis—the northern lights—which captivates the upper latitudes of the Northern Hemisphere.
Aurora can come in different colors and shapes. Scientists are still fuzzy on what creates any specific shapes and sizes for the lights, but it’s thought that they move in harmony with Earth’s magnetic field. The color generated depends on which gases were initially hit—oxygen emits a red light (as well as greenish-yellow), so it would seem oxygen was behind the 1770 storm. That also means it was powerful enough to pierce into some lower portions of the atmosphere where oxygen is more abundant, which explains how it was able to give the sky such an apocalyptic makeover.

The documents discovered were all written in September and October of 1770. They include government records and personal diary entries, which in sum suggest that the fiery aurora ranged from Japan to the Chinese mainland.

The team also compared sunspot drawings made by astronomers at the time with drawings recorded during the solar storm of 1859. That storm, which lasted two nights, was caused by the Carrington flare and is something of a standard by which astronomers assess other solar events. Up until now, it’s been considered the most extreme solar storm on the record. But the sunspots drawn during the 1770 event were twice as big as those recorded during the Carrington event.

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