Carbon-14 concentrations in tree rings can be used as an indicator of cosmic-ray activity — 14C is produced by the interaction of atmospheric nitrogen with cosmic-ray neutrons. A high-resolution analysis of the 14C content of annual rings in two Japanese cedar trees now reveals evidence of an increase of about 12‰ in 14C content between AD 774 and AD 775, about 20 times larger than that expected as a result of ordinary solar modulation. The authors conclude that an extremely energetic event occurred around AD 775 in our space environment, the cause of which is currently unknown as there is no supporting evidence for either a supernova or a large solar flare at this time.
- A signature of cosmic-ray increase in AD 774–775 from tree rings in Japan (Letter p240, doi: 10.1038/nature11123)
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