Variable solar activity: sometimes friend, sometimes foe

GCompared to the earth, our sun behaves like a big brother who cares about the protection of his little sister, but occasionally jerks her chin out of the blue. This is what happened in the years 993, 1052 and 1279: At that time our day star bombarded the earth for a short time with extraordinarily high-energy particles. What might have been noticed in the Middle Ages by the splendid northern lights would at least give the technologized society of our day at least a black eye.

This is suggested by a study published in the journal “Nature Geosciences” at the beginning of January. Nicolas Brehm from ETH Zurich and an international team of researchers had studied the concentration of radioactive carbon-14 (C-14) in tree rings from 969 and 1933. C-14, also known as “radiocarbon”, is formed when high-energy particles of cosmic radiation, especially hydrogen and helium nuclei, collide with nitrogen atoms in the atmosphere. Radiocarbon makes up less than one percent of all carbon on earth, but is incorporated into the cellulose of living plants during photosynthesis. In dead living beings, the C-14 content is reduced because of the radioactive decay according to a precisely known decay law. In this way, the age of historical artifacts can be determined. Brehm and his team now turned the principle around: They used the amount of C-14 in old tree trunks, the history of which was already known from the number of annual rings, to reconstruct the C-14 concentration in the earth’s atmosphere over the past 1000 years .

To do this, they did not measure the radioactive radiation emanating from the radiocarbon, as was customary in the past, but instead counted the carbon 14 atoms in the wood directly with an accelerator mass spectrometer. This process is faster and requires significantly less sample material: For a measurement, millimeter-fine drill cores that had been extracted, for example, from tree trunks installed in historical buildings, were sufficient. It is also so precise that the scientists were able to determine precisely the C-14 fluctuations from one year to the next for the first time.


The course of solar activity over the past 1000 years (blue, with error area in white), in the background is a typical eleven-year cycle of the sun.
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Image: ETH Zurich

According to the study, the C-14 concentration followed the variable solar activity over the past 400 years. Since the invention of the telescope, astronomers have been monitoring these using the number of sunspots and therefore know that the sun is particularly restless about every 11 years. As expected, the C-14 proportion in the corresponding annual rings was lower: the more active the sun, the better its magnetic field keeps the energy-rich cosmic particles off – like its “big brother” in the schoolyard. The fact that, thanks to the tree rings, the ups and downs of the sun can now be traced back significantly, will help astrophysicists to better understand the variable solar magnetic field.

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