The Sun has a lot of rhythm and goes through different cycles of activity. The most well-known cycle might be the Schwabe cycle, which has an 11-year cadence. But what about cycles with much longer time scales? How can scientists understand them?
As it turns out, the Sun has left some hidden clues in tree rings.
About 400 years ago, astronomers started watching the Sun with their newly-invented telescopes. They noticed sunspots coming and going and began to record their appearance and dissipation. They had no idea what they signified.
Those observations have taught us a lot about the Sun’s activity. The more sunspots there are, the more there is going on inside the Sun. But there are other cycles of longer duration which have an effect on Earth and its climate. And a 400 yr record, though great in some respects, can’t tell us much about the longer-term cycles.
The 11-yr Schwabe cycle is itself part of these even longer cycles. A team of scientists wanted to reconstruct the Schwabe cycle back in time beyond 400 years to understand how it all fits together. To do so, they had to uncover clues left behind by the Sun inside trees. Those clues are in the form of radionuclides created by cosmic rays.
The team of researchers is led by Hans-Arno Synal and Lukas Wacker of the Laboratory of Ion Beam Physics at ETH Zurich. They traced the Schwabe cycle back as far as the year 969 by measuring radioactive carbon concentrations in tree rings. They published their results in a paper titled “Eleven-year solar cycles over the last millennium revealed by radiocarbon in tree rings.” It’s published in the journal Nature Geoscience.
The great thing about trees is they grow in an annual cycle. So each year, as they grow another ring, it’s a snapshot of the Sun’s output for that year. Piecing all those rings together gives an accurate picture of solar activity. In this study, the scientists looked at tree-ring archives from England and Switzerland.
Each ring contains a tiny amount of radioactive carbon—as little as one atom of Carbon 14 per 1000 billion atoms. Since scientists know that C14’s half-life is about 5700 years, they can calculate the concentration of C14 atoms in the atmosphere when each ring was grown.
Here’s where it gets even more fascinating: the radioactive carbon in the tree rings doesn’t come from the Sun. It comes from cosmic rays that reach Earth from way outside our Solar System. But the Sun’s magnetic field helps keeps those cosmic rays from reaching the Earth. The more powerful the Sun’s magnetic field is, the fewer C14 isotopes reach Earth to be taken up by tree growth. So lower amounts of C14 in tree rings correlate with periods of greater solar activity.
But measuring these minuscule amounts of c14 isotopes in the tree rings is not easy, and neither is detecting differences from year-to-year. “The only measurements of that kind were made in the ’80s and ’90s,” says Lukas Wacker, “but only for the last 400 years and using the extremely laborious counting method.” The counting method used a Geiger counter to measure the decay event of each isotope. That method takes a lot of material and a lot of time.
The team came up with another method: accelerator mass spectrometry. This type of spectrometry was developed in the mid-twentieth century and is especially useful at detecting radioisotopes with long lives, like C14.
“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”, said Ph.D. student Nicolas Brehm in a press release, who was responsible for those analyses.
The tree ring samples contain two types of carbon. Alongside the radioactive C14 isotope is C12, the most abundant of the two types of stable carbon isotope. An accelerator mass spectrometer accelerates both those isotopes before being sent through a magnetic field. The field directs one type of carbon one way and the other isotope in another direction due to their different masses. The results of that measurement are then analyzed statistically.
As a result, the team of scientists was able to reconstruct the record of the Sun’s activity all the way from the year 969 to 1933. Their reconstruction confirmed the Sun’s 11-year Schwabe cycle all the way back to 969 AD. It also showed that the amplitude of that cycle, or how much the solar activity goes up and down, is smaller during long-lasting solar minima.
Their reconstruction also confirmed something else. In 993, there was a pronounced solar proton event that created a peak in atmospheric C14. These events happen when protons emitted by the Sun are accelerated enough to penetrate the Earth’s magnetic field and cause ionization in the atmosphere. There’s been debate around the 993 event, but this work confirms its existence.
In fact, the results went further than confirming the event in the year 993. The researchers also found evidence of two more proton events: one in 1052 and one in 1279. This is the first time those events have been detected, and it might indicate that they happen more frequently than thought. This is very interesting since these events can pose a hazard to electronics on Earth and satellites.
Earth has some very long-lived trees. One of them, a bristlecone pine tree in California named Methuselah, is thought to be about 5,000 years old. But for this study, there was no need to disturb ancient living trees. Instead, the researchers examined ancient timbers used in buildings still standing, like the Abbey Church of St Alban, St Albans, Hertfordshire, UK. Its construction dates back to the eleventh century. The team examined 13 different timbers from 11 different buildings in the UK and Switzerland.
This type of analysis has the potential to teach us even more about the Sun. There are tree ring archives that go back 14,000 years in sub-fossilized wood, which is still rich in carbon. The researchers hope to use their method to measure the C14 concentrations in that wood, which will help them reconstruct solar activity back to the end of the last ice age.
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