One of the bigger stories we covered last year was the start of solar cycle 25, where our Sun has finally moved out of a solar minimum and begun to show surface activity such as sunspots once again. The Sun cycles between minimum and maximum every eleven years, and we know this fact due to 400 years of careful observations. Galileo didn’t go blind for nothing, after all.

However, it’s not that we didn’t know about sunspots before the invention of the telescope. It’s that they weren’t documented in detail. And since we can only observe the sun indirectly as things like sunspots are surface features that merely hint at the underlying activity, scientists have been trying to find a way to push that record of solar cycles back as far as possible.

Now, a new paper in Nature Geoscience explains how a team of researchers led by the Laboratory of Ion Beam Physics at ETH figured out how exactly to recreate nearly one thousand years’ worth of solar activity data. They did it by using a very terrestrial method — tree rings.

It turns out that we have tree ring archives in places like England and Switzerland that can date trees back nearly 14,000 years. As many of us learned as children, you can date a tree by counting the number of concentric rings in its trunk. With those rings, you can not only understand the age of the tree but also many of the conditions that it grew under, such as flood or drought or fire. And you can even sample the radioactive carbon-14 isotopes in the rings, which is what this team eventually did.

Carbon-14 is mainly produced by cosmic particles, and the magnetic field of the Sun is influential in keeping those particles away from the Earth. So when the Sun is more active, the magnetic field is stronger, and fewer isotopes of carbon-14 get through to the surface of the planet. By sampling each ring in the tree and determining the amount of carbon-14, we can now tell how active the Sun was relative to other years. The team produced a reconstruction of solar activity from the years 969 to 1933.

So did the eleven-year cycle with which we are familiar hold up under a millennium of data and calculations? Yes. Yes, it did. Additionally, they gained insight into the varying amplitude of the cycles in relation to the length of the minima. They confirmed the existence of a “solar energetic proton event” in 993 where a solar flare caused an overproduction of carbon-14. And they found two more such events in 1052 and 1279, which may help other researchers understand how often such events can occur. These are the kinds of events that can break our satellites and power grid, so it’s always good to know more about them.

Now the team plans to use the rest of that tree-ring data, the stuff that goes back 14,000 years, and try to reconstruct the solar activity all the way back to the last ice age. We wish them luck and look forward to sharing their findings with you here on The Daily Space.

More Information

ETH press release

“Eleven-year solar cycles over the last millennium revealed by radiocarbon in tree rings,” Nicolas Brehm et al., 2021 January 4, Nature Geoscience