Solar storm: Evidence found of huge eruption from Sun
Scientists have found evidence of a huge blast of radiation from the Sun that hit Earth more than 2,000 years ago.
The result has important implications for the present, because solar storms can disrupt modern technology.
The team found evidence in Greenland ice cores that the Earth was bombarded with solar proton particles in 660BC.
The event was about 10 times more powerful than any since modern instrumental records began.
The Sun periodically releases huge blasts of charged particles and other radiation that can travel towards Earth.
The particular kind of solar emission recorded in the Greenland ice is known as a solar proton event (SPE). In the modern era, when these high-energy particles collide with Earth, they can knock out electronics in satellites we rely on for communications and services such as GPS.
The radiation may also pose a health risk for astronauts. And passengers and crew on commercial aircraft that fly at high altitudes and close to the poles, such as on transatlantic routes, could receive increased radiation doses - though this depends on many variables.
Other types of solar radiation events can trigger aurorae in the high atmosphere and shut down electrical grids.
"There are high-energy solar energetic particle events, or solar proton events. These are the high energy particles directly hitting Earth and producing the particles we measure," co-author Raimund Muscheler, from Lund University in Sweden, told BBC News.
"Connected to this are also the lower energy particles that come usually within 1-4 days to Earth. These produce the geomagnetic storms."
The two types of particle events may not always coincide, however.
Modern instrumental monitoring data extends back about 60 years. So finding an event in 660BC that's an order of magnitude greater than anything seen in modern times suggests we haven't appreciated how powerful such events can be.
There wouldn't have been any appreciable signs of the event to people alive at the time. But if there were any associated geomagnetic storms, it might have triggered aurorae at lower latitudes than is usual.
660BC was the date, according to legend, when Japan's first emperor - Jimmu - acceded to the throne. It was the time of the Iron Age in Europe and the Middle East - before the rise of the Roman Empire.
The researchers found evidence for the event in the form of radioactive isotopes (particular forms of an element) present in the Greenland ice. These were beryllium-10 and chlorine-36, which are regarded as being of cosmic origin.
Researchers have also identified two other large events from the past, which left evidence in both Greenland ice cores and tree rings. The signature researchers look for in tree rings is the isotope carbon-14.
One of these, which occurred between 774 and 775AD, was comparable in its magnitude to the one in 660BC.
"Our event is about the same size as [the event in 774/775]. There is some uncertainty, but they look very similar," said Dr Muscheler.
However, the event in 660BC does not have such a clear carbon-14 signature in tree ring data.
Scientists are now working to understand how common the extreme events are, something that could help us plan for big solar storms in future.
The research has been published in the journal Proceedings of the National Academy of Sciences (PNAS).
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