The sun is punctuated by cycles during which its activity varies between a maximum and a minimum. These variations have impacts on the weather and climate on Earth. The best-known cycle so far is the Schwabe, with a cadence of 11 years. A team recently looked into the subject and discovered traces of solar activity since the year 969. To do this, the researchers relied on the rings inside trees.
Concretely, they measured the radioactive carbon atoms (C14) in the heart of plants using Accelerator Mass Spectroscopy (AMS). Each ring corresponds to a year of growth and contains a tiny amount of C14 which varies according to solar activity. The study was conducted by Hans-Arno Synal and Lukas Wacker of the Ion Beam Physics Laboratory at ETH Zurich.
The results were presented in an article titled “Eleven Years of Solar Cycles in Last Millennium Revealed by Radiocarbon in Tree Rings” . They were originally published in Nature Geoscience .
A millennium of solar activities reconstructed
Astronomers began observing the Sun with telescopes 400 years ago. According to the data collected, the maximum solar activity is manifested by the presence of several spots and by an increase in eruptions. The average length of a cycle can vary between 8 and 14 years.
It is important to note that C14 does not originate from the Sun, but rather from cosmic rays beyond the solar system. In fact, it is the Sun's magnetic field that prevents them from reaching Earth. Thus, the low amounts of carbon in the heart of the trees means that the period of solar activity was very important.
“Accelerator Mass Spectrometry allowed us to measure the concentration of C14 in tree rings in just a few hours. It made possible the reconstruction of a thousand years of solar activity (969-1933). "
Nicolas Brehm, head of analyzes
Astronomical secrets hidden by ancient woods
The Earth has thousand-year-old trees like the pine tree in California, called Mathusalems, which are said to be around 5,000 years old. However, the researchers did not want to disturb the living trees. They preferred to examine old timbers like those used in St Alban's Abbey Church in Hertfordshire, UK. The construction of the chapel dates back to the 11th century. For the experiment, the team examined 13 timbers from 11 old buildings in the UK and Switzerland.
For the continuation of the research, she plans to use the same method with sub-fossil wood still rich in carbon dating from 14,000 years. This would help to reconstruct the solar activity coinciding with the end of the last ice age.