The 1859 Carrington Event, the most intense geomagnetic storm in recorded history (peaking September 1–2, 1859), was driven by a massive coronal mass ejection (CME) from the Sun, causing widespread telegraph disruptions, brilliant auroras visible near the equator, and white-light solar flares observed by Richard Carrington.

Tree rings detect solar events via spikes or anomalies in radiocarbon (Δ¹⁴C). High-energy solar protons from solar energetic particle (SEP) events collide with atmospheric nitrogen, producing excess ¹⁴C that mixes into CO₂ and gets fixed into tree cellulose during photosynthesis. This creates sharp, annual spikes for extreme “Miyake events” (e.g., AD 774, 993), but subtler signals for moderate storms like Carrington.

Breakthrough in 2024: Researchers led by Joonas Uusitalo (University of Helsinki) detected the first tree-ring signature of the Carrington Event in subarctic Scots pines (Pinus sylvestris) from Lapland, Finland (high latitudes: Inari, Värriö, Hetta; ~68–69°N). They analyzed 5–8 replicate annual samples per year from 1853–1871 across three trees, measuring Δ¹⁴C via accelerator mass spectrometry (AMS) at three labs (Helsinki, ETH Zurich, Yamagata).

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From Watts Up With That?

By Anthony Watts

A research group coordinated by the University of Helsinki was able to measure a spike in radiocarbon concentration of trees in Lapland that occurred after the Carrington flare. This discovery helps to prepare for dangerous solar storms.

The Carrington Event of 1859 is one of the largest recorded solar storms in the last two centuries. It was seen as white light flares on a giant sunspot group, fires at telegraph stations and disturbances in geomagnetic measurements, as well as aurorae even in tropical regions. 

In a joint study carried out by the University of Helsinki, Natural Resources Institute Finland and the University of Oulu, a sign of an increase in radiocarbon concentrations following the Carrington storm was detected for the first time in tree rings. Previously, radiocarbon traces have only been detected from far more intense solar storms.

Discovery through a cosmic marker

Encounters between strong magnetised clouds of charged particles released from the Sun, known as solar plasma flows, and Earth’s geomagnetic field result in geomagnetic storms. The geomagnetic field directs the solar storm particles into the atmosphere primarily through the Polar regions. The most visible consequence of the phenomenon are aurorae. 

In the upper atmosphere, sufficiently high-energy particles can, through nuclear reactions, also produce radiocarbon (14C), a radioactive isotope of carbon. Over the course of months and years, radiocarbon ends up in the lower atmosphere as part of atmospheric carbon dioxide, and eventually in plants through photosynthesis. The process of photosynthesis preserves the information contained in carbon dioxide in the annual rings of trees.

To obtain the information held by radiocarbon, samples are extracted by carving from the wood material grown over individual years. The samples are processed to cellulose and the cellulose into pure carbon by burning and chemical reduction. The fraction of radiocarbon in pure carbon is measured using a particle accelerator. 

“Radiocarbon is like a cosmic marker describing phenomena associated with Earth, the solar system and outer space,” says Markku Oinonen, Director of the University of Helsinki’s Laboratory of Chronology, who headed the study.

Mapping solar storms

A solar storms corresponding to the Carrington event in modern times would disrupt electrical and mobile networks and cause major problems for satellite and navigation systems, leading to problems in, for example, air traffic. This is why accurate knowledge of solar behaviour benefits society.

Solar storms smaller and more common than the Carrington storms can be studied with measuring devices and satellites nowadays, while larger ones can be investigated, for example, by measuring radiocarbon concentration in tree rings. 

So far, it has not been possible to study specifically medium-sized storms such as the Carrington event, which have not occurred in modern times, using conventional radiocarbon techniques. This recent study opens up a potential new way of investigating the frequency of Carrington-sized storms, which may help to better prepare for future threats.

Increasingly accurate information on the carbon cycle

The results were interpreted using a numerical model of radiocarbon production and transport developed by researchers at the University of Oulu.

“The dynamic atmospheric carbon transport model was specifically developed for describing geographical differences in the distribution of radiocarbon in the atmosphere,” says Postdoctoral Researcher Kseniia Golubenko from the University of Oulu. 

What was significant in the recently published study was how the radiocarbon content of trees in Lapland differed from that of trees at lower latitudes. The first measurements were carried out at the Accelerator Laboratory of the University of Helsinki, while repeat measurements conducted in two other laboratories significantly reduced the previous uncertainties.

The discovery can help to better understand atmospheric dynamics and the carbon cycle from the time before human-generated fossil fuel emissions, enabling the development of increasingly detailed carbon cycle models.

“It’s possible that the excess of radiocarbon caused by the solar flare was primarily transported to the lower atmosphere through northern regions, contrary to the general understanding of its movement,” muses Doctoral Researcher Joonas Uusitalo from the Laboratory of Chronology.

Other sources of radiocarbon

“It’s also possible that the cyclic change in the production of radiocarbon in the upper atmosphere caused by the variation in solar activity has resulted in the local differences on the ground level seen in our findings,” Uusitalo adds.

According to Uusitalo, the dominant fraction of radiocarbon is produced by galactic cosmic rays coming from outside the solar system, even though exceptionally strong solar storms generate individual bursts of the isotope in the atmosphere. Cosmic rays, in turn, are weakened by solar wind, a continuous flux of particles originating in the Sun that fluctuates between stronger and weaker in 11-year cycles. 

The topic requires further research. Historical records show that significant geomagnetic storms also took place in 1730 and 1770, which is why their tracking is likely to be in focus next.

The recently published study was carried out as a collaborative project of the University of Helsinki’s Laboratory of Chronology and Department of Physics, and Natural Resources Institute Finland. Researchers from the University of Oulu, Nagoya University, Yamagata University and ETH Zurich also contributed to the study. The study received funding from the Research Council of Finland, the Finnish Cultural Foundation and the Emil Aaltonen Foundation.

Original article: Joonas Uusitalo, Kseniia Golubenko, Laura Arppe, Nicolas Brehm, Thomas Hackman, Hisashi Hayakawa, Samuli Helama, Kenichiro Mizohata, Fusa Miyake, Harri Mäkinen, Pekka Nöjd, Eija Tanskanen, Fuyuki Tokanai, Eugene Rozanov, Lukas Wacker, Ilya Usoskin, Markku Oinonen. Transient Offset in 14C After the Carrington Event Recorded by Polar Tree Rings. AGU, 2024.

DOI: 10.1029/2023GL106632

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Transient Offset in ¹⁴C After the Carrington Event Recorded by Polar Tree Rings

Researchers analyzed annual Δ¹⁴C from subarctic Scots pines (Pinus sylvestris) in Finnish Lapland (>68°N). Multiple replicates (5–8 per year) from three trees were measured via accelerator mass spectrometry across 1853–1871.

No immediate spike in the 1859 or 1860 ring.

A statistically significant transient offset (excess Δ¹⁴C) appeared in high-latitude trees compared to mid-latitude records, peaking during 1861–1863 (~3–6‰ higher, with averages around 3.5‰ in 1862).

The offset faded by ~1864–1865, lasting ~4 years.

Solar energetic particles (SEPs) from the Carrington Event entered the atmosphere preferentially over the poles due to Earth’s geomagnetic funneling. Excess ¹⁴C production occurred mainly in the polar stratosphere.

Faster stratosphere-troposphere exchange (STE) in the Arctic (due to polar vortex dynamics and weaker stratification) transported this ¹⁴C downward more rapidly. Trees at high latitudes incorporated it into cellulose during photosynthesis, creating a delayed but persistent local excess not diluted globally.

Atmospheric models (e.g., SOCOL-AERv2) partially reproduce this polar enhancement but underestimate its magnitude, suggesting gaps in understanding high-latitude carbon transport.

From The Geophysical Research Letters – Wiley Online Library

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