A millennium of Solar Activity tracked in Tree Rings

Solar activity since 969 has been reconstructed by scientists using measurements of radioactive carbon in tree rings.These results help scientists better understand the dynamics of the sun and allow for more accurate dating of organic materials using the C14 method, according to a statement from ETH Zurich, where the research was conducted.

What happens to the sun can only be observed indirectly. Sunspots, for example, reveal the degree of solar activity: the more sunspots that are visible on the surface of the sun, the more active our central star is inside. Although sunspots have been known since ancient times, they have only been documented in detail since the invention of the telescope about 400 years ago. Thanks to this, we now know that the number of spots varies in regular cycles of eleven years and that, in addition, there are prolonged periods of strong and weak solar activity, which is also reflected in the Earth’s climate.

However, until now it has been difficult to reconstruct how solar activity developed before the start of systematic records.

An international research team led by Hans-Arno Synal and Lukas Wacker of the Ion Ray Physics Laboratory at ETH, which included the Max Planck Institute for Solar System Research in Göttingen and Lund University in Sweden, has now tracked eleven-year solar cycles to 969 using measurements of radioactive carbon concentration in tree rings. At the same time, the researchers have created an important database for more accurate age determination using the C14 method. Their results were recently published in the scientific journal Nature Geoscience.

To reconstruct solar activity over a millennium with extremely good time resolution of just one year, the researchers used tree ring files from England and Switzerland.

In those tree rings, the ages of which can be determined precisely by counting the rings, there is a small fraction of radioactive carbon C14, and only one in every 1 billion atoms is radioactive. From the known half-life of the C14 isotope, around 5,700 years, the concentration of radioactive carbon present in the atmosphere when the growth ring formed can be deduced. Since radioactive carbon is produced mainly by cosmic particles, which in turn are kept away from the Earth to a greater or lesser extent by the sun’s magnetic field (the more active the sun, the better it protects Earth), it is possible deduce solar activity from a change in the concentration of C14 in the atmosphere.

However, accurate measurements of a change in that already very small concentration resemble finding a grain of dust on a needle in a huge haystack. “The only such measurements were made in the 1980s and 1990s,” Lukas Wacker says in a statement, “but only for the past 400 years and using the extremely labor-intensive counting method.” In that method, the radioactive decay events of C14 in a sample are counted directly using a Geiger counter, which requires a relatively large amount of material and, due to the long half-life of C14, even longer.

“Using modern accelerator mass spectrometry, we were now able to measure the C14 concentration to within 0.1 percent in just a few hours with tree ring samples that were a thousand times smaller,” adds doctoral student Nicolas Brehm, responsible of those analyzes. .

In accelerator mass spectrometry, the C14 and C12 atoms (the “normal” non-radioactive carbon; the C14, by contrast, contains two additional neutrons in its nucleus) in the tree material are first electrically charged and then accelerated by an electrical potential of several thousand volts, after which they are sent through a magnetic field.

In that magnetic field, the two isotopes of carbon, which have different masses, deviate by different degrees and can therefore be counted separately. To finally get the desired information on solar activity from that raw data, researchers must perform intricate statistical analysis and further process the results using computer models.

This procedure allowed the researchers to smoothly reconstruct solar activity from 969 to 1933. From that reconstruction, they were able to confirm the regularity of the eleven-year cycle, as well as the fact that the amplitude of that cycle (by how much activity increases solar and down) is also lower during long-duration solar minima. These insights are important to better understand the internal dynamics of the sun.

The measurement results also confirmed the solar energetic proton event of 993. In such an event, highly accelerated protons reaching Earth during a solar flare cause a slight overproduction of C14. In addition, the research team also found evidence of two other hitherto unknown events in 1052 and 1279. This could indicate that such events, which can seriously disrupt electronic circuits on Earth and on satellites, occur more frequently than it was thought.