In 774 AD, the Sun blasted Earth with the biggest storm in 10,000 years
In the year 774 AD, an enormously powerful blast of matter and energy from space slammed into Earth.
Nothing like it had been felt on this planet for 10,000 years. A mix of high-energy light and hugely accelerated subatomic particles, when this wave impacted Earth it changed our atmospheric chemistry enough to be measured centuries later.
Our pre-electronic societies went entirely unaffected by it. But were this sort of event to happen today, the results would be bad.
It was first discovered by an analysis of tree rings, of all things. Scientists found that the level of carbon-14, an isotope of carbon, was much higher in rings from that year than usual. Some years later, looking at air samples from ice cores, scientists saw that there were elevated levels of beryllium-10 and chlorine-36 as well.
The common factor in all these elements is that they are created when extremely high-energy subatomic particles hit Earth's air and ground. They slam into the nuclei of atoms and change them, creating these isotopes. The only way to get particles at energies like this is from space, where powerful magnetic fields in exploding stars, for example, can accelerate the particles to such high speeds. We call these isotopes cosmogenic, made from space.
What could have created the space storm in 774 AD? The obvious candidate for such a thing is a very powerful solar flare, an explosion on the Sun created when intense magnetic field lines tangle up and short circuit, releasing huge blasts of energy and particles. But the 774 event was so powerful that at first scientists were skeptical it could be from a flare. Once any other type of astronomical phenomenon was ruled out, though, a flare was all that was left.