Thanks to the study of the amounts of radiocarbon stored in the rings of ancient trees, a team of researchers from the University of Queensland, in Australia, has managed to collect worrying data on a mysterious and powerful type of cosmic event, unpredictable and potentially devastating.
Until now, it was believed that the greatest space radiation ‘attack’ suffered by man was the one that came from the so-called ‘Carrington event’, the most powerful solar storm of all those recorded in historical times, which in 1859 caused the collapse of the then incipient telecommunications system. But according to the new study, published in the Proceedings of the Royal Society A, the Earth has been periodically hit, too, by levels of radiation up to a hundred times more powerful.
“These huge bursts of cosmic radiation, known as ‘Miyake events’ (for the Japanese scientist who first discovered them), they have happened about once every thousand years, but we don’t know what causes them, explains Benjamin Pope, who has led the research. The main theory is that it is huge solar flaresbut we need to know more, because if one of these events were to happen today, it would destroy technology, including satellites, internet cables, long-distance power lines, and transformers. The effect on global infrastructure would be unimaginable«.
So far, the scientist continues, six of these events have been identified “over a period of 10,000 years.” The most recent took place in the years 774 and 993, in the Middle Ages.
Tracks in the tree rings
Many tree species show growth rings on their trunks. These rings, at a rate of one per year, make it possible to ascertain their ages, but they also help scientists to study, and accurately date, numerous characteristics of the environment and to know the climate in which the tree has been living since it was born. And in this case what was interesting was to know how much radioactive carbon 14, or radiocarbon, had been stored in the different rings of the trees studied, all of them millennia old, throughout their long existence. Radiocarbon, in effect, is produced due to the interaction of high-energy particles, such as those from cosmic rays and solar storms, with the atmosphere.
To extract this data from the rings, Qingyan Zhang, also from the University of Queensland and the first author of the study, developed specific software. “Because you can count the rings of a tree to identify its age, you can also observe historical cosmic events that go back thousands of years,” Zhang explains. When radiation hits the atmosphere, it produces radioactive carbon-14, which filters through the air, oceans, plants and animals, and produces an annual record of radiation in tree rings. We modeled the global carbon cycle to reconstruct the process over a 10,000-year period, to gain insight into the scale and nature of the Miyake events.”
Until now, the causes of such events were thought to be gigantic solar storms. “But our results challenge that explanation,” says Zhang. We have shown that there is no correlation with sunspot activity, and some of them have lasted for years. So instead of a single instantaneous explosion or flash, what we may be seeing is a kind of astrophysical ‘storm’ or outburst.”
Six times in 10,000 years
The study data shows that, during the 10,000 years analyzed, these mysterious cosmic radiation storms occurred approximately once every millennium, and not only in the periods of maximum activity of the Sun (the solar maximum that occurs once every 11 years). but at any time of the solar cycle. In addition, many of the detected radiation ‘spikes’ lasted much longer than a typical solar storm. For example, the event that took place in 663 BC lasted three years, and much earlier, the one that took place in 5840 BC lasted for a decade.
“At least two, maybe three of these events,” Pope says, “lasted longer than a year, which is surprising because that doesn’t happen if it’s a solar flare. At first, we thought we were going to be able to prove that the Miyake events were caused by the Sun, but the big result is a huge shrug. We don’t know what’s going on.”
All in all, it could still be that the longest spikes were due not to one, but to a rapid succession of large solar storms. “Not a single solar flare,” says Pope, “but recurring solar flares that go off over and over again.” Something, however, that has never been seen by scientists.
Whatever its origin, researchers are concerned that they have no idea how to predict this kind of event, or when the next one will occur. They only know that if one of them were to take place, even if it were of the shortest duration, it would be powerful enough to plunge the world into chaos for years.
“It’s very important,” Pope concludes, “that we find out what it is, because the Carrington Event was 100 times smaller in terms of radiation output than these Miyake events.”