Polar forests may have just solved a solar storm mystery

The strongest solar flare in recorded history burst into Earth’s atmosphere in 1859, bathing both hemispheres in brilliantly colorful aurorae as it wreaked worldwide havoc on telegraph systems. The celestial chaos was broadly witnessed, but lingering physical evidence of that storm, dubbed the Carrington event, has proven stubbornly elusive — until now, researchers report in the March 16 Geophysical Research Letters.

Ecologist Joonas Uusitalo of the University of Helsinki and his colleagues have identified the first known traces of the Carrington event: atoms of carbon-14 preserved in tree rings in Finland’s far north. Scientists previously hadn’t detected tree ring evidence of this event, although other trees have recorded more powerful solar flares that occurred before modern recordkeeping began, such as in 774 and 993.

Those storms were perhaps 10 times more intense than the one in 1859, Uusitalo says, so it makes sense that they’d leave a stronger signal. Also, he says, the trees in which scientists have previously hunted for clues to the Carrington event have all been located in the mid-latitudes — for example, in Japan, Europe or the United States. But “based on our earlier research, we had this idea that maybe the polar trees are more sensitive to [less powerful storms].”

So Uusitalo’s team examined rings from three trees at different sites within the Lapland region of Finland, above the Arctic Circle, as well as rings from three trees from the mid-latitudes. These rings all dated between 1853 to 1871. The team found a statistically significant increase in carbon-14 in the polar trees compared with those in the mid-latitudes during the year of the Carrington event. That suggests it is possible to use polar tree rings to detect moderate-sized solar storms.

The extra sensitivity of those polar trees may be related to how solar particles interact with Earth’s magnetic field, the researchers suggest. Solar flares are bursts of particles that swiftly stream from the sun toward Earth. When the particles encounter Earth’s magnetic field, they get deflected toward the poles; that disturbance of the magnetosphere produces aurorae — and can also wreak havoc on radio signals.

As the particles enter the stratosphere, they react with atmospheric molecules to produce carbon-14, normally produced by the interaction of cosmic rays with atmospheric nitrogen. Researchers have hypothesized that that extra burst of carbon-14 from the solar particles eventually makes its way to the Earth’s lowest atmospheric layer, the troposphere, where it is drawn into the tissue of living trees, preserving a record of the solar flare.

These carbon-14 spikes in tree rings are known as Miyake events, after physicist Fusa Miyake of Nagoya University in Japan, who first connected the observed spikes to solar storms. Miyake is a coauthor on the new study.

Scientists previously thought that the carbon-14 would mix quickly into the atmosphere, and by the time it reaches the surface, it would be evenly distributed among trees at different latitudes. But recent studies suggest that in the Arctic, there’s faster air exchange between the stratosphere and the troposphere than at lower latitudes, Uusitalo says. So trees closer to the poles could receive a slightly bigger infusion of the carbon-14 than those in the mid-latitudes, making them better sensors for relatively weaker storms.

Using polar trees could give researchers more insight into how common more moderate solar storms are, Uusitalo says. Historical archives suggest that there were also flares in 1582, 1730 and 1770 that, so far, haven’t shown traces in mid-latitude tree rings. His team now plans to look for them closer to the north pole.

The finding could be “hugely important” for scientists’ understanding of radiocarbon spikes in the tree ring record, says physicist Benjamin Pope of the University of Queensland in St. Lucia, Australia. “It has always been a problem for us that the biggest-ever flare observed from the sun during the modern scientific era — the Carrington event of 1859 — doesn’t even show up in the radiocarbon record,” he says.

Pope and his colleagues recently questioned whether solar flares were even responsible for the Miyake events, based in part on uncertainties in how well the spikes align with the solar cycle, as well as on the apparent lack of evidence that trees nearer the poles contain higher levels of the carbon-14 (SN: 11/7/22). If the new findings hold up, they lend a new line of support to the link between Miyake events and solar storms. Still, Pope notes, this study’s findings are based just on three trees in polar regions, and replicating those results with other high-latitude trees will be essential before drawing any conclusions. 

Uusitalo agrees, and adds that it will also be key to study tree rings that span longer periods of time, beyond a single 11-year solar cycle. That’s because the sun’s activity may affect carbon-14 production in the atmosphere in another way, he says: The solar wind can actually push cosmic rays away from Earth, periodically reducing the normal influx of rays that would react to form carbon-14 in the atmosphere. If that subtle cycle is also detectable in polar tree rings, the trees could offer a new way to examine the historical cyclicity of the sun and of atmospheric circulation. 

Either way, he says, “I want to emphasize the importance of [studying] high-latitude trees.” Because scientists tend to analyze trees closer to where they live, most measurements come from the mid-latitudes. But, as this study hints, the trees of the far north may guard many secrets about the intertwined history of Earth and the sun.