Earth’s Old Trees Keep A Record of Powerful Solar Storms

Most of the time the Sun is pretty well-mannered, but occasionally it’s downright unruly. It sometimes throws extremely energetic tantrums. During these events, a solar flare or a shock wave from a coronal mass ejection (CME) accelerates protons to extremely high velocities. These are called Solar Particle Events or Solar Proton Events (SPEs).

However, the exact timing of these events can be difficult to ascertain. New research has determined the date of one of the most powerful SPEs to strike Earth during the Holocene.

No one alive today has witnessed the Sun’s extreme power. But ancient people did. In the last 14,500 years, there have been several solar storms and SPEs powerful enough to damage living things and create aurorae at middle latitudes, even at the equator. Understanding the timing of these ancient events is a key part of understanding the Sun.

Powerful outbursts from the Sun are becoming a more significant threat as we expand our presence in space. They can damage satellites and pose a radiation threat to astronauts. Even the Earth’s surface isn’t safe from the most powerful SPEs which can knock out technological infrastructure like power grids and communications networks.

“If they happened today, they would have cataclysmic effects on communication technology.”

Irina Panyushkina, University of Arizona

The Sun’s most powerful outbursts seem to occur during solar maximum, the period of greatest activity during the Sun’s 11-year cycle. But there’s some uncertainty, and since SPEs can be so damaging, there’s a need to understand them better, beginning with their timing.

Only six SPEs have left their mark on Earth in about the last 14,500 years. Historical accounts can open a window into the timing of ancient SPEs, but they’re plagued by inaccuracies and inconsistencies. Fortunately, these natural events leave a trace in the natural world.

These solar outbursts create what are called Miyaki Events after the Japanese physicist Fusa Miyake. Miyake discovered that they create a sharp rise in cosmogenic isotopes due to increased cosmic rays striking Earth’s upper atmosphere. The events create carbon-14 (¹⁴C), a radioactive isotope that is present in tree rings. The events also create other isotopes like Beryllium-10 (¹⁰Be)and Chlorine-36 (³⁶Cl) that are present in ice cores.

In new research published in Nature Communications Earth and Environment, researchers pinpointed the timing of the last SPE to strike Earth. It’s titled “The timing of the ca-660 BCE Miyake solar-proton event constrained to between 664 and 663 BCE.” The lead author is Irina Panyushkina from the University of Arizona’s Laboratory for Tree-Ring Research.

There have been several Miyake events depending on how they’re defined.

“Thanks to radiocarbon in tree-rings, we now know that six Miyake events happened over the last 14,500 years,” Panyushkina said. “If they happened today, they would have cataclysmic effects on communication technology.”

Carbon-14 continuously forms in Earth’s atmosphere because of cosmic radiation. In the atmosphere, it combines with oxygen to form CO². “After a few months, carbon-14 will have traveled from the stratosphere to the lower atmosphere, where it is taken up by trees and becomes part of the wood as they grow,” said lead author Panyushkina.

During a Miyake event, the amount of carbon-14 spikes, and that spike is reflected in tree rings. There have been several of these events, depending on how they’re defined, and several more awaiting more rigorous confirmation. There rate of occurrence is poorly understood, but the data we have shows that they occur every 400 to 2400 years. One of them occurred around 660 BCE, and that event is the subject of much research.

“The precise positioning of a SPE in real time is extremely important for the parameterization of solar activity and forecasts,” the authors write in their research. “Notably, one of the recently confirmed SPE events does not have an exact calendar date. Multiple radionuclide evidence of an extreme SPE (or ME) event ca. 2610 BP (before 1950) more commonly referenced as ca. 660 BCE, was confirmed with high-resolution ¹⁰BE records of three ice cores from Greenland in 2019.”

The circa 660 Miyake event is different from the others. “However, the ca. 660 BCE ME has an unusual structure that is different from the short-term rapid increases in radionuclide production observed at 774–775 CE and 993–994 CE. One proposed explanation is the possible occurrence of consecutive SEPs over up to three years,” the authors explain in their research. If Miyake events can occur in such rapid succession, we need to know about it, for obvious reasons.

In this new research, the team analyzed tree rings for ¹⁴C content to generate an accurate date for the ca-660 BCE Miyake event. They focused on larch trees in arctic-alpine biomes, one in the Altai mountains and the other in the Yamal Peninsula. In these regions, larch trees are more sensitive to atmospheric changes and have clearer 14C spikes.

Panyushkina and her co-researchers examined tree rings from ancient samples, including trees buried in mud and sediment and timbers excavated during archaeological digs and measured the Carbon-14 content. Next, they correlated their findings with other research into Beryllium-10 found in ice sheets and glaciers. Beryllium-10 is also created during Miyake events. It isn’t absorbed by trees, but is deposited in ice.

“If ice cores from both the North Pole and South Pole show a spike in the isotope beryllium-10 for a particular year corresponding to increased radiocarbon in tree-rings, we know there was a solar storm,” Panyushkina said.

This sounds like a nice tidy way to determine the dates of Miyake events, but it’s not so easy. Researchers have struggled to find a pattern. Tree rings are clearly marked by growing seasons, but ice cores are not. There’s also a lag time between the creation of Carbon-14 in the atmosphere and its presence in trees, and in ice. Different trees also absorb the carbon at different times and rates, and they also store and recycle the carbon, which can influence how they serve as recorders of atmospheric CO2. These and other challenges mean that conclusions don’t jump out of the data.

But this research still has value, even if it isn’t the silver bullet when it comes to predicting these powerful solar events. The issue with the 660 BCE event is its complexity. It seems to have several spikes and declines in a short period, suggesting more complex solar behaviour than a simple single-spike storm.

“Our new 14C data defined the two-pulse duration, considerable magnitude, and the precise date of what was previously described as the event ‘around 660 BCE’,” the authors write. “We showed that the double pulse of cosmic radiation during 664—663 BCE produced a nontypical pattern of ME cosmogenic isotope production recorded at multiple locations in northern Eurasia.”

“The impact appears as a 2–3 year rise of Carbon-14 concentrations tailed by a 2–3-year peak (or plateau) before the signal decays,” the authors write. The Carbon-14 production in 664 BCE was 3.5 and 4.8 times greater than the 11-yr average.

What does it all mean?

There’s a lot of complexity. Different trees absorb carbon differently, the stratosphere and troposphere mix differently at different times, and growing seasons can vary significantly. “Finally, the double pulse of the 664–663 BCE ME onset and the prolonged waning of the ¹⁴C spike signal implies possible uncertainties complicating the use of this spike signal for single-year dating of archeological timbers and occurrences,” the researchers explain in their conclusion.

However, one thing is clear in all of the data. The Sun has blasted Earth with extreme SPEs in the past that are much more powerful than anything in modern time. “Extreme proton events that are hundreds or thousands of times stronger than those of modern instrumental observations may recur on the timescale of hundreds of years,” the authors write in their conclusion.

Ultimately, the tree rings can shed light on how powerful these solar storms are, but they’re not exact when it comes to dating them.

“Tree-rings give us an idea of the magnitude of these massive storms, but we can’t detect any type of pattern, so it is unlikely we’ll ever be able to predict when such an event is going to happen,” Panyushkina said. “Still, we believe our paper will transform how we search and understand the carbon-14 spike signal of extreme solar proton events in tree rings.”