Ice Cores Could Solve Thousand-Year-Old Cosmic Ray Mystery

An ice core from Greenland (Photo: Eden, Janine and Jim, Flickr)

New evidence confirms that an influx of cosmic rays struck the entire planet around 994 CE—and it might solve the mystery of the event’s origin.

Ice cores and tree rings around the world show mysterious increases in the concentrations of certain elements around 994 CE. The newest evidence originates in Antarctic ice, validating the prior observations and suggesting that the cosmic rays came from the Sun.

The Japanese scientists behind the new research analysed ice cores from Greenland as well as from two locations in Antarctica. They looked specifically at the concentration of beryllium-10, a radioactive but long-lived form of beryllium with one more neutron than the most common beryllium atoms. This element is typically produced when high-energy particles from space called cosmic rays strike certain atmospheric atoms, like oxygen. Ice cores can store a record of these atoms and their concentrations over time in layers, kind of like tree rings.

The researchers found a 50 per cent increase in the concentration of beryllium-10 in the Antarctic ice cores around 992 CE, according to the paper published recently in Geophysical Research Letters. They point out that there isn’t much data elsewhere on the beryllium-10 concentrations, likely because the signal is so small that it could be hard to pick out of background noise or other terrestrial processes that could form beryllium-10.

What does this all mean? Well, the beryllium-10 data supports previous observations, first found in Japanese tree rings, of an increase in certain radioactive elements formed by cosmic rays interacting with the atmosphere around 994 CE. But finding the same space-born element in both the northern hemisphere and southern hemisphere data implies that the excess of cosmic rays could have come from the Sun. The “Solar Energetic Particles” that would have caused these events could have originated in especially large solar flares, or in rare instances, releases of plasma from the Sun called coronal mass ejections.

In order to confirm that the beryllium didn’t come from something terrestrial, the scientists pointed out that most terrestrial variations in beryllium-10 concentration came with a corresponding change in sodium ion concentrations. But the 994 event didn’t seem to come with an increase in the sodium atom, according to the paper.

The researchers mention that the Greenland core’s beryllium peaks aren’t easily picked out of the background variation in 994 CE. They are hesitant to definitively state that the 50 per cent increase they see comes from the 994 event—just that the concentration and the timing look consistent. But they felt that the sum of the data pointed to an “extreme” solar event around the time.

It’s wild to think that the Earth itself might have been recording extreme solar events before humans had the capability to do so themselves.


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