Researchers have used tree-rings to learn more about how the sun behaved more than 2,000 years ago. 

By analyzing radiocarbon locked over millennia in tree-ring samples, scientists from the University of Arizona Laboratory for Tree-Ring Research and ETH Zurich in Switzerland were able to create the most detailed record yet of solar activity during the first millennium B.C.E., from 1000 B.C.E. to 2 B.C.E. 

The results, published in Nature Communications, could help scientists better understand the sun's influence over thousands of years, gain new perspectives on the timing of historical events, and better predict future solar outbursts and "magnetic storms" that might pose a threat to technology.

"Our findings are exciting because they give us an incredibly detailed look at how the sun behaved more than two millennia ago," said Nicolas Brehm, lead author of the study and a former postdoctoral researcher and current associate at the U of A Laboratory for Tree-Ring Research. 

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The sun is a swirling ball of electrically charged hot gas, which generates a powerful magnetic field shielding Earth from cosmic radiation. The strength of this shielding rises, peaks, then falls within an 11-year cycle. When cosmic rays hit nitrogen molecules in Earth's atmosphere, they knock protons from atomic nuclei, which turn the nitrogen molecules into carbon-14, a naturally occurring radioactive carbon isotope. During periods of high solar activity, less carbon-14 is produced from cosmic radiation due to magnetic shielding, but when solar flares – intense bursts of energy and radiation – erupt on the sun, high-energy particles can break through and result in sharp carbon -14 spikes.

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"As trees breathe in carbon dioxide, they absorb a certain ratio of carbon-14 along with the much more common stable isotopes carbon-12 and carbon-13," explained Charlotte Pearson, an associate professor and researcher with the Laboratory for Tree-Ring Research and co-author of the study. "Carbon-14 is then locked into the wood of the trees and will remain there. Once the growth season begins in the following year, the process will start over and form the next tree-ring."

Because tree-ring growth follows a set cycle, researchers can use radiocarbon in tree-rings to examine solar activity on a year-by-year scale. Understanding solar storms that occurred deep in Earth's history is especially vital as the cause of solar flare events remains unknown. Yet, data from the periods between solar events can also provide valuable insights into solar activity – particularly when it comes to stable behavior like the 11-year solar cycle, which consists of periodic highs and lows in the sun's activity.

Observing the 11-year solar cycle across the first millennium B.C.E. is particularly important because the time period includes the so-called "Hallstatt plateau" or "radiocarbon disaster." This is a period of about 800 years during which the radiocarbon dating method fails to pinpoint exact dates because carbon-14 production barely changes. Revealing a clear 11-year cycle across this period provides surprising insights into the sun's behavior, Pearson said.

"I was surprised to see just how stable the sun's 11-year cycle was during the first millennium B.C.E.," Brehm said. "It was a fascinating discovery."

"I'm really a big fan of the gaps in between the events, because for me these data are just as exciting as solar storms themselves," Pearson added. "We get to see what our sun has been doing across long stretches of time, which could hold the key to how, when and why these events happen."

The data also reveals unique patterns that can help tie down archaeological timelines. 

"The solar patterns double up in importance because they are reproducible from trees anywhere in the world," Pearson said. "This means that these patterns can now be used to date wooden objects from the Hallstatt period with an accuracy that was previously impossible." 

With the recent arrival of a new radiocarbon analysis instrument at the Laboratory for Tree-Ring Research, the team anticipates unraveling many more insights from tree-ring samples. The core of the new instrument is a compact accelerator mass spectrometer capable of capturing radiocarbon data from minuscule amounts of sampled wood. 

"The new instrument will allow us to fully take advantage of our massive archive of tree-ring samples and fill in many more blank spots in the radiocarbon record throughout the ages," Pearson said.