In Lapland, trees bear witness to a major solar eruption | Polarjournal
The trees of Lapland have trapped in their rings the traces of the 1859 solar flare (also known as the Carrington event) that a research team has just uncovered. Photo: Joonas Uusitalo / University of Helsinki

Traces of a major solar eruption that happened in the 19th century have been found in the rings of trees in Lapland. This discovery could help us to better prepare our society for the consequences of a major solar storm, and to better understand the carbon cycle.

In 1859, the Sun was raging. This was the Carrington event, the biggest solar storm recorded in the last two centuries, and whose traces have been found in the very heart of Lapland’s trees. Or so say the results of a study published on March 5 in Geophysical Research Letters.

Researchers have found a peak concentration of radiocarbon (or carbon-14) in trees in northern Finland, directly attributable to the solar storm that occurred over a century ago. And this is a first, as the Carrington event has never been detected in tree rings. The results of this study could help us to better anticipate and prepare for the consequences of major solar storms. Such events are synonymous with disruptions of electrical and mobile networks, as well as problems for satellite and navigation systems.

But how can traces of a past solar storm be found in the trunks of trees? When solar flares occur, the Sun releases powerful magnetized clouds of charged particles (plasma). When these clouds meet the Earth’s geomagnetic field, they create geomagnetic storms. The geomagnetic field then directs the solar storm particles towards the atmosphere, in particular the polar regions. This is the phenomenon that gives rise to the aurora borealis and australis.

In their rings, trees retain traces of many past events, including solar storms. To study the information contained in the wood of Lapland trees, researchers took samples of wood and processed them to cellulose. The latter was then processed by burning and chemical reduction into carbon, so that the fraction of radiocarbon in pure carbon could be measured using a particle accelerator. Photo illustration: Wikicommons

In the upper atmosphere, particles can produce radiocarbon which, over the course of years or months, returns to the lower atmosphere in the form of atmospheric carbon dioxide. This carbon is then “absorbed” by plants via photosynthesis, and ends up in tree rings. “Radiocarbon is like a cosmic marker describing phenomena associated with Earth, the solar system and outer space,” notes Markku Oinonen , director of the study, in a press release issued by the University of Helsinki on March 28. The latter conducted the research in conjunction with the Finnish Natural Resources Centre and the University of Oulu, as well as other institutions such as ETH Zurich.

A particularly significant feature of this study was the way in which the carbon-14 content of trees in Lapland differed from that of trees in more southerly latitudes. A discovery that may help to better understand atmospheric dynamics and the carbon cycle prior to man-made fossil fuel emissions, enabling the development of increasingly detailed carbon cycle models, notes the press release.

Auroras are not just an impressive sight. They also allow us to see solar activity. The more intense the aurora, the greater the solar activity. Usually confined to the northern and southern polar regions, they can be observed well beyond the polar circles in the event of a major solar storm. Photo: Michael Wenger

At the time, the solar storm of 1859 not only caused auroras to be seen in the tropics, but also severely disrupted telegraph networks, sometimes even setting fire to telegraph stations.

Such events could well happen again. In fact, the sun observes an 11-year cycle in its activity. And the next peak is scheduled for 2025. In the event of a major storm, NASA estimates that we could have barely half an hour to protect power grids and satellite infrastructures in particular. In a world as technology-dependent and connected as ours, a storm as severe as Carrington could cause massive damage and financial losses in the trillions of dollars.

Link to study: 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

Mirjana Binggeli, PolarJournal

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