A Miyake event is a powerful burst of cosmic rays. The origin and cause of these increases in cosmic ray activity is currently unknown. The outbreaks are marked in particular by the increase in the carbon isotope C14 in tree rings. The events take place roughly every 1000 years on average.

Carbon-14 and some other isotopes, such as beryllium-10, are created in the atmosphere when cosmic rays reach Earth. That carbon-14 is absorbed by the trees and etched into the tree rings. By studying the amount of carbon-14 in a given ring, we can learn something about the Sun's activity at the time the ring formed.

Cosmic rays are charged particles, so they can be deflected by magnetic fields, including the Sun's if they originated outside the Solar System, but the Sun also emits cosmic rays. When there is an explosive event like a solar flare, the Sun spews out more cosmic rays and the amount of carbon-14 that is produced increases.

Since the Japanese team published the "Proceedings of the Royal Society A" , the tree ring signatures have come to be known as Miyake events, after the first author, the Japanese astronomer Fusa Miyake. Although the 774 CE event was the strongest, there’s also been events found in 993 CE, 660 BCE, 5259 BCE, 5410 BCE, and 7176 BCE.

The researchers can go this far back partially due to ancient trees – some Japanese cedar trees can be 2000 years old, while the Great Basin Bristlecone Pine is over 5,000 years old.

More Info: en.wikipedia.org