Study suggests ancient tree rings might not have chronicled solar flares after all

Back in 2012, a letter was published in the journal Nature that shook up the astronomy world. Researchers had found a spike of carbon-14 (¹⁴C) in Japanese cedar tree rings. The dates of the spike correlated to 774-775 CE, and the researchers suggested that the ¹⁴C could have been due to a huge solar flare.

Now, a new paper published in the Proceedings of the Royal Society A has used statistics and modelling to shed light on this event – and others like it – and the results show that it’s not as simple as it seems.  

“The leading theory is that these are huge solar flares, and so that’s what we set out to prove. We sort of haven’t proved it,” says Dr Benjamin Pope, an astrophysicist at the University of Queensland.

“It’s a little bit of a head scratcher.”

Since the Japanese team published the letter, the tree ring signatures have come to be known as Miyake events, after the first author, Fusa Miyake. Although the 774 CE event was the strongest, there’s also been events found in 993 CE, 660 BCE, 5259 BCE, 5410 BCE, and 7176 BCE.

The researchers can go this far back partially due to ancient trees – some Japanese cedar trees can be 2000 years old, while the Great Basin Bristlecone Pine is over 5,000 years old.

The Miyake events show an increase in ¹⁴C, which is a radioactive isotope of carbon. This type of carbon is immensely rare – there’s only a few kilograms of the stuff in the entire plant, and it naturally occurs through cosmic rays hitting the nitrogen in our atmosphere.

Although it hasn’t been confirmed what causes these huge radiocarbon spikes, the traditional theory was that it was caused by a huge solar flare.

---

Read more: The Sun is angrier than expected, but we don’t know why

---

Having such a large spike of solar flares would not have been good if it happened today. It would destroy satellites, internet cables long distance power lines and much much more.

“The effect on global infrastructure would be unimaginable,” says Pope, in a press release. 

“It would damage telecommunications, the power grid and satellites.”

This event would potentially be an order of magnitude larger than even the Carrington Event, which happened in 1859 and was the most intense geomagnetic storm in recorded history.

The team behind the new research created software to analyse every available piece of data on tree rings, producing the most comprehensive research on Miyake events to date.

They found that the events didn’t show a consistent relationship to the 11-year solar cycle, which is the cycle that the Sun’s magnetic field goes through. (Currently we’re heading towards the solar maximum which means more sunspots and solar flares.)

This lack of relationship to the solar cycle means that Miyake events probably aren’t due to a solar flare, as flares occur more during the solar maximum.

“It doesn’t seem that we know when they happen in the solar cycle,” says Pope.

“There’s one that occurs at maximum and other than that occurs mid cycle, another that occurs minimum. That’s not saying it’s not Sun, but you get about four times as many solar flares at the solar maximum as the solar minimum.”

We actually get more ¹⁴C deposited on Earth during solar minimums than solar maximums, as cosmic rays are mostly produced from outside the Solar System, and the Sun protects us more when it’s flaring up.

Pope suggests that this could mean that the Miyake events could have something to do with ‘grand solar minimums’ or ‘Dalton minimums’ – which are decades or centuries where solar activity is significantly lower than usual.

The other problem with the solar flare theory is that these events are lasting much longer than a solar flare would normally. We know from the Carrington Event that it lasted only a day or two, whereas the team’s research suggests that the Miyake event of 774 CE seemed to have lasted years.

“This is the biggest and most comprehensive study of data on these events, and all it raises is questions,” says Pope.

“Some of this might be sort of biology that we don’t understand, some of it might be geophysics that we don’t understand, and some of it might be astronomy that we don’t understand.” Unfortunately, as with many of these problems, scientists need more data. The team is looking at recently discovered Miyake events, as well as using data from ice cores to confirm the findings.