Jasna Hodžić racconta per Big Think cosa accadde durante alcuni giorni nella tarda estate del 1859:

“The whole sky appeared to undulate something like a field of grain in a high wind; the waters of the Bay reflected the brilliant hues of the Aurora,” wrote one journalist in the San Francisco Herald on September 5, 1859. “Nothing could exceed the grandeur and beauty of the sight; the effect was almost bewildering and was witnessed with mingled feelings of awe and delight by thousands.”

Almost immediately, the world’s 100,000 miles of telegraph lines fell silent, victim to a wave of space-borne electric current strong enough to fry the systems. The communications system of the time fell “so completely under the influence of the Aurora Borealis that it was found utterly impossible to communicate between the telegraph stations.”

Questo evento prese il nome di evento Carrington dall’astronomo che ne propose una spiegazione: una forte tempesta solare.

L’evento Carrington, però, non è nè il primo nè il più potente evento cosmico di questo tipo mai individuato: nel 2012, infatti, la dottoranda Fusa Miyake scoprì un aumento del 12% della concentrazione del carbonio-14 (che è influenzata dall’alto flusso di particelle energetiche nell’atmosfera terreste) in alcuni anelli di un antico cedro di Yakushima.

Thanks to previous research, she knew there had been a pronounced carbon-14 spike sometime in the late eighth century. Eventually, she found an unmistakable signal: Between 774-775 AD, she noted a 12% jump in carbon-14 that suggested an event 20 times larger than ordinary cosmic phenomena. Other researchers confirmed Miyake’s findings with European and North American trees. Scientists found a similar signal in beryllium isotopes present in Antarctic ice cores. The collective findings provided abundant evidence that the event in question was a global, rather than a local, phenomenon.

A questa prima scoperta, ne sono seguite molte altre, come riporta Science:

Each of its rings held a trace of ¹⁴C. The radioactive isotope forms continuously in the upper atmosphere as cosmic rays—high-energy particles from space—collide with gas molecules, spawning neutrons. When one of these neutrons knocks out a proton in a nitrogen atom, that nitrogen is transformed into ¹⁴C. As it inhaled CO₂, the cedar had incorporated the ¹⁴C into its wood.

In 2013, they found a second, slightly smaller ¹⁴C spike in the same Yaku cedar at 993–94 C.E. Other scientists started calling the phenomena Miyake events.

La presenza di questi improvvisi aumenti nella concentrazione del carbonio-14 è estremamente utile per ottenere una datazione più precisa di vari oggetti storici:

When Wacker read about Miyake’s 774–75 C.E. and 993–94 C.E. spikes, he recognized the radiocarbon beacon’s untapped potential. He trained it on a chapel in Müstair, Switzerland, supposedly built by the first Holy Roman Emperor, Charlemagne, on a site where he and his party had survived a horrendous blizzard. Another team of scientists had dated the chapel to 785 C.E. by meticulously analyzing the widths of tree rings in a wooden beam—wider rings often indicate wetter years—then matching the patterns with more recent timber from the area until they had an unbroken record stretching from the present back to the beam.

In 2020, Kuitems and Dee used the technique to clear up a longstanding mystery surrounding rectangular ruins on a lake island in southern Siberia. Despite decades of archaeological work, nobody knew for sure how old the Por-Bazhyn site is, who built it, or what its purpose was.

Miyake events also promise to date natural disasters that have altered the course of human history. “This technique can help answer questions about the rise and fall of civilizations,” Pearson says.

L’esatta origine di questi eventi, però, è tuttora argomento di dibattito: in un articolo del 2022 pubblicato su Proceedings of the Royal Society gli autori sembrano escludere le eruzioni solari come causa. Ne parla Jacinta Bowler sulla rivista Cosmos:

“The leading theory is that these are huge solar flares, and so that’s what we set out to prove. We sort of haven’t proved it,” says Dr Benjamin Pope, an astrophysicist at the University of Queensland.
This lack of relationship to the solar cycle means that Miyake events probably aren’t due to a solar flare, as flares occur more during the solar maximum.

Pope suggests that this could mean that the Miyake events could have something to do with ‘grand solar minimums’ or ‘Dalton minimums’ – which are decades or centuries where solar activity is significantly lower than usual.

The other problem with the solar flare theory is that these events are lasting much longer than a solar flare would normally.

Katherine Kornei su Scientific American parla a sua volta di questo recente studio, in particolare del problema della lunga durata degli eventi Miyake.

One in particular, which occurred in 663 B.C.E., lasted for roughly three years, the researchers concluded. That’s perplexing, Pope says, because solar flares, coronal mass ejections and other eruptions from the sun typically last only a few days or weeks. Such a relatively short burst of high-energy particles would presumably be captured in a single tree ring, which is assembled over the course of a year, he says. Finding evidence of a multiyear signal, then, is rather befuddling.

Lo studio degli eventi Miyake non è solo un argomento di interesse per pochi scienziati, infatti il ripetersi di un evento Miyake potrebbe avere serie conseguenze per la nostra società:

After all, a cosmic barrage intense enough to show up in tree rings would likely be disastrous to the thousands of satellites that encircle the planet. Their sensitive electronics would be essentially fried, Pope says, and that could have ripple effects in the spheres of navigation and communication, technologies we take for granted in modern society. If—or when—the next Miyake event occurs, he says, “good luck to telecommunications.”