Trees hold evidence of one of Earth's most extreme solar storms

The Northern Lights have been visible much farther south than usual this year, filing social media with stunning images of colorful skies. Now, imagine an aurora so intense that it would instantly render smartphones useless as particles from the sun overwhelm electronic devices. 

Such extreme solar storms are incredibly rare – only six have left their traces on Earth in the past 14,500 years, and none have been observed since the height of the Assyrian Empire nearly 2,700 years ago.

A cosmic event from 664 B.C.

A research team led by Irina Panyushkina of the University of Arizona‘s Laboratory for Tree-Ring Research and Timothy Jull from the Department of Geosciences has identified the last time Earth experienced such a powerful burst of cosmic radiation. 

Their findings, published in the journal Communications Earth & Environment, reveal a spike in carbon-14 dating to the year 664 B.C., pinpointing the timing of this elusive extreme solar storm.

What are solar storms?

Solar storms occur when the Sun releases massive amounts of energy in the form of solar flares or coronal mass ejections (CMEs). These events send high-energy particles and magnetic fields hurtling toward Earth at incredible speeds.

When these charged particles interact with Earth’s magnetic field, they can create dramatic and far-reaching effects.

Tracing solar storms through carbon-14

“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,” Panyushkina said. 

By analyzing tree rings for carbon-14, a naturally occurring radioactive variant of carbon, the team detected this ancient solar storm. 

Determining the precise timing of such massive solar eruptions provides important data for scientists who study and develop models of the sun’s activity over time.

Signature of extreme solar storms

It wasn’t until 2012 that scientists recognized extreme solar storms known as Miyake events.

That year, Fusa Miyake, a Japanese physicist and collaborator with Panyushkina’s team, published a paper reporting the storms’ telltale signature: spikes in radioactive carbon isotopes in the growth rings of trees. 

Miyake events occur when the sun’s electromagnetic field weakens, allowing plasma from the sun’s surface to escape into space. 

Increased solar activity leads to protons bombarding Earth’s atmosphere, triggering chemical reactions that result in spikes of radioactive isotopes.

“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.”

Methodology: Tree rings and ice cores

The research team used surgical knives to dissect individual tree rings from ancient wood samples, including those from dead trees buried in riverbanks and timbers excavated during archaeological digs. 

Next, the researchers burned the cellulose – the main component of wood – to determine the radiocarbon content. 

To confirm whether solar storms caused the radiocarbon spikes, the researchers compared their tree ring data to spikes found in a different isotope, beryllium-10, locked in ice cores from glaciers and ice sheets. 

Like carbon-14, beryllium-10 forms in the atmosphere due to solar particles and becomes trapped in ice through rain and snow.

“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.

Can we predict extreme solar storms?

Although tree rings act as “archivists” recording Miyake events, researchers have not yet found evidence of a pattern in these occurrences. 

According to Panyushkina, 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 will ever be able to predict when such an event is going to happen.

“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,” said Panyushkina.

“The energy from this type of event not only changes the atmosphere’s radiocarbon content but also the atmosphere’s chemistry. We are trying to figure out how those short-lived and powerful events affect the Earth system as a whole.”

Image Credit: NASA/SDO and the AIA, EVE, and HMI science teams

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