According to tree rings analysis, the sun went through an astonishing change 7,497 years ago
Dendrochronology, or tree-ring dating, has shown "highly abnormal solar activity" in 5480BC.
By analysing the level of a carbon isotope in tree rings from an ancient bristlecone pine, researchers have found that the sun exhibited a "unique pattern" of activity in that year.
By comparing the event with other similar but more recent phenomena, they believe that the dramatic event may have involved a change in the sun's magnetic activity, or a number of successive solar burst emissions.
We found a change in 14C that was more abrupt than any found previously
Prof. Timothy Jull, University of Arizona
An international team led by researchers at Nagoya University in Japan, along with American and Swiss colleagues, identified the new type of solar event and dated it to the year 5480 BC.
They did so by measuring carbon-14 (14C) levels in tree rings, which reflect the effects of cosmic radiation on the atmosphere at the time.
The research team has also put forward possible causes of the event.
Tree-ring dating has shown 'highly abnormal solar activity' in 5480BC
They said that when the activity of the sun changes, it has direct effects on the earth. For example, when the sun is relatively inactive, the amount of a type of carbon called carbon-14 increases in the earth's atmosphere.
Because carbon in the air is absorbed by trees, carbon-14 levels in tree rings actually reflect solar activity and unusual solar events in the past.
The researchers took advantage of such a phenomenon by analysing a specimen from a bristlecone pine tree, a species that can live for thousands of years, to look back deep into the history of the sun.
Scientists analysed the level of a carbon isotope in tree rings
Professor Timothy Jull, of the University of Arizona, said: "We measured the 14C levels in the pine sample at three different laboratories in Japan, the US, and Switzerland, to ensure the reliability of our results.
"We found a change in 14C that was more abrupt than any found previously, except for cosmic ray events in AD 775 and AD 994, and our use of annual data rather than data for each decade allowed us to pinpoint exactly when this occurred."
The researchers attempted to develop an explanation for the anomalous solar activity data by comparing the features of the 14C change with those of other solar events known to have occurred over the last 2,000 years.
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Fusa Miyake, of Nagoya University, said: "Although this newly discovered event is more dramatic than others found to date, comparisons of the 14C data among them can help us to work out what happened to the sun at this time.
"We think that a change in the magnetic activity of the sun along with a series of strong solar bursts, or a very weak sun, may have caused the unusual tree ring data."
Although the poor understanding of the mechanisms behind unusual solar activity has hampered efforts to definitively explain the findings, Professor Jull hopes that additional studies, such as telescopic findings of flares given off by other sun-like stars, could lead to an accurate explanation.