Scientists Use Annual Growth Rings to Detect Strong Solar Flares

08:43, December 21, 2021

Powerful solar storms occurred in 7176 and 5259 BC

An international team of scientists has discovered traces of extremely strong solar flares in annual growth rings, which were "deposited" as a sharp rise in radiocarbon in tree rings. The researchers estimate that the solar storms occurred in 7176 and 5259 B.C. The study description and results are published in the Journal Nature Portfolio.

"A powerful solar storm leads to a sharp increase in the formation of some radioactive elements in the Earth's atmosphere: beryllium-10, chlorine-36, carbon-14. Radioactive carbon as part of carbon dioxide enters and is deposited in trees. By analyzing the content of radiocarbon in individual annual rings of trees, we can tell how its concentration changed in the atmosphere with the accuracy of one year," says Rashid Khantemirov, a member of the research group from Russia, a leading researcher of the Laboratories of Dendrochronology of the Institute of Ecology, Plants and Animals of the Ural Branch of RAS and the Laboratory of Natural Science Methods in Humanities at UrFU.

Traces of solar flares were found in oak from Ireland and Germany, pine from the United States, larches from the Alps and Russia (dendrochronologists from Dendrochronology of the Institute of Ecology, Plants and Animals of the Ural Branch of RAS and the Laboratory of Natural Science Methods in Humanities at UrFU annually collect samples of ancient trees during expeditions to Yamal). It was possible to determine the exact years due to the method of cross-dating. It is based on comparing the tree-ring pattern: each time interval has its own unique pattern of tree-ring variability.

Radiocarbon analysis of the tree rings was carried out at the Laboratory of Ion Beam Physics (LIP) at the Swiss Federal Institute of Technology (ETH Zurich).

"On average, about 7-8 kilograms of radiocarbon are produced in the Earth's atmosphere per year under the influence of cosmic rays. A solar flare produces additional kilograms of radioactive carbon. The most powerful flare known to date, the year 774, produced an additional 26 kilograms in a few days. Whereas the 7176 and 5259 B.C. flares we detected each produced 29 kilograms. However, a flare of the same power can produce different amounts of radiocarbon, depending on the strength of the Earth's magnetic field at the time of the solar storm. So, the protection of the Earth in 774 was stronger. Therefore, if we make a correction for the strength of the Earth's magnetic field, the flare of 774 was at least not weaker than the ones we detected," explains Rashid Khantemirov.

The results of the study will help to improve the accuracy of dating natural events and processes during the Holocene epoch, and most importantly, to assess the probability of extreme solar flares in our time.

"Today, unlike past civilizations, where there was no electricity, no Internet, and no spacecraft, solar flares of this magnitude would have catastrophic consequences both for the global economy and for the social order in general. The probability of such an episode is not as low as previously thought. Based on the observation of stars identical to the Sun, it was assumed that episodes of such force may occur once every 3-6 thousand years. Now we can estimate the probability of such an episode from data on the Sun itself. As of today, 2030 years have been studied with a 1 or 2 years accuracy. During this period, five powerful solar flares have been detected accurately. That is about one episode every 400 years," says the scientist.

Scientists are sure that the analysis of changes of radiocarbon in the atmosphere in the past with a 1 year accuracy not only gives a more accurate picture of changes of this isotope in the atmosphere, but can lead to the discovery of fundamentally new phenomena. Therefore, the next stage of the work is a sequential analysis of the whole Holocene epoch, i.e. the last 10-12 thousand years for several regions of the Earth.

"Already now, 1.6 thousand separated annual rings of larch trees from Yamal are in Japan in the laboratory of Fusa Miyake at Nagoya University. They will be analyzed in the next few months. Part of the analysis will be duplicated in Switzerland. So, perhaps in a few years we will have several millennial series of radiocarbon measurements with a 1 year accuracy for several regions of the Earth, where there are long tree-ring chronologies. The results of such studies will make it possible to reconstruct the history of solar activity, and not only extreme episodes in the life of the Sun," adds Rashid Khantemirov.

To note, dendrochronologists from Switzerland, Germany, Sweden, Great Britain, Austria, USA, Russia, France, and Finland took part in the study. In Russia, the study was supported by the Russian Science Foundation (Project No. 21-14-00330 "The Polar Forest Frontier on Yamal: Holocene History and the Present").

Reference

Since the 1940s (when instrumental observations of solar flares began), scientists have recorded the strongest flare since February 23, 1956. At that time, there was about 50 times the normal level of cosmic rays. A more powerful solar storm occurred in September 1859. The geomagnetic storm it caused resulted in the failure of the telegraph.

In 2012, Japanese researchers published a paper in which they reported the discovery of a spike in radiocarbon concentrations in the Earth's atmosphere in 774 A.D. Later on, this episode, defined as a solar superburst, was recorded in annual tree rings around the world. A little later, scientists identified two more flares, in 993 A.D. and 660 B.C. Three more candidate episodes, in 1279, 1052 A.D. and 5410 B.C., are awaiting confirmation (they are not yet adequately separated from the normal solar modulation).