Japanese
scientists studying tree rings data found something strange: 1,200 years
ago an extremely intense burst of high-energy radiation of unknown origin
hit planet Earth.
The radiation burst, which seems to have hit between ad 774 and
ad 775, was detected by looking at the amounts of the radioactive isotope
carbon-14 in tree rings that formed during the ad 775 growing season
in the Northern Hemisphere. The increase in 14C levels is so clear that
the scientists, led by Fusa Miyake, a cosmic-ray physicist from Nagoya
University in Japan, conclude that the atmospheric level of 14C must
have jumped by 1.2% over the course of no longer than a year, about
20 times more than the normal rate of variation.
But what happened, exactly?
The only known events that can produce a 14C spike are floods of
gamma-rays from supernova explosions or proton storms from giant solar
flares. But neither seems likely, Miyake says, because each should have
been large enough to have had other effects that would have been observed
at the time.
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