Searching for planets and studying their stars, I have had the privilege of using some of the largest telescopes in the world. However, our team recently turned to an even larger system to study the cosmos: Earth’s forests.
We’ve analyzed radioactive signatures left in tree rings around the world to study the mysterious “radiation storms” that have swept across the Earth half a dozen times over the past 10,000 years or so.
Our findings, published today in Proceedings of the Royal Society A, rule out ‘super solar flares’ as the culprit – but the true cause remains unknown.
A story written in the tree rings
When high-energy radiation hits the upper atmosphere, it converts nitrogen atoms into radioactive carbon-14, or radiocarbon. The radiocarbon then seeps through the air and oceans, into sediments and peatlands, into you and me, into animals and plants – including hardwoods with their annual rings.
For archaeologists, radiocarbon is a godsend. After its creation, carbon-14 decays slowly and steadily into nitrogen, which means it can be used as a clock to measure the age of organic samples, in what is called radiocarbon dating.
For astronomers, it’s just as valuable. Tree rings give a year-by-year record of high-energy particles called “cosmic rays” dating back millennia.
The magnetic fields of the Earth and the Sun protect us from cosmic rays crossing the Galaxy. More cosmic rays reach Earth when these magnetic fields are weaker, and fewer when the fields are stronger.
This means that the rise and fall of carbon-14 levels in tree rings encodes the history of the 11-year cycle of the solar dynamo (which creates the Sun’s magnetic field) and reversals in the Earth’s magnetic field.
Miyaké Events
But tree rings also record events that we currently cannot explain. In 2012, Japanese physicist Fusa Miyake discovered a peak in the radiocarbon content of tree rings from 774 AD. It was so large that several ordinary years of cosmic rays must have arrived all at once.
As more teams joined in the search, tree ring evidence was discovered for other “Miyake events”: from 993 AD and 663 BC, and prehistoric events in 5259 BC, 5410 BC and 7176 BC.
These have already led to a revolution in archaeology. Finding one of these short, sharp points in an ancient sample fixes its date to a single year, instead of the decades or centuries of uncertainty of ordinary radiocarbon dating.
Among other things, our colleagues used the 993 AD event to reveal the exact year of the first European settlement in the Americas, the Viking village of L’Anse aux Meadows in Newfoundland: 1021 AD.
Could huge radiation pulses reproduce?
In physics and astronomy, these Miyake events remain a mystery.
How do you get such a radiation pulse? A multitude of articles have implicated supernovae, gamma-ray bursts, explosions of magnetized neutron stars and even comets.
However, the most widely accepted explanation is that the Miyake events are “super solar flares”. These hypothetical flares from the Sun would be possibly 50 to 100 times more energetic than the largest recorded in the modern era, the Carrington event of 1859.
If an event like this happened today, it would devastate power grids, telecommunications and satellites. If these happen randomly, about once every thousand years, that’s a 1% chance per decade – a serious risk.
Noisy data
Our UQ team set out to sift through all available tree ring data and extract the intensity, timing and duration of Miyake events.
To do this, we had to develop software to solve a system of equations that model how radiocarbon filters through the entire global carbon cycle, to determine what fraction ends up in trees in what years, for opposition to oceans, bogs or you and me.
In collaboration with archaeologists, we have just published the first reproducible and systematic study of the 98 trees of published data on the Miyake events. We have also released open source modeling software as a platform for future work.
Solar Flare Storms
Our results confirm that each event delivers between one and four ordinary years of radiation at one time. Previous research has suggested that trees closer to Earth’s poles record a larger spike – which is what we would expect if super solar flares were responsible – but our work, looking at a larger sample of trees, shows that is not the case.
We also found that these events can happen at any point in the Sun’s 11-year activity cycle. Solar flares, on the other hand, tend to occur at the height of the cycle.
More confusingly, a few peaks seem to take longer than can be explained by the slow creep of new radiocarbon through the carbon cycle. This suggests that either events can sometimes take longer than a year, which is not expected for a giant solar flare, or tree growing seasons are not as regular as previously thought.
For my money, the Sun is still the most likely culprit of the Miyake events. However, our results suggest that we are seeing more of a storm of solar flares than a huge superflare.
To determine exactly what happens in these events, we will need more data to give us a better picture of the events we already know. To get this data, we will need more tree rings – and also other sources such as Arctic and Antarctic ice cores.
It really is an interdisciplinary science. Normally I think of beautifully clean and precise telescopes: understanding the complex and interconnected Earth is much harder.
Benjamin Pope, ARC DECRA Fellow, University of Queensland
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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