For a few nights more than 9,000 years ago, at a time when many of our ancestors were wearing animal skins, the northern skies would have been bright with flickering lights.
Chemical isotopes in ancient ice cores indicate that one of the most powerful solar storms ever occurred around 7176 B.C., and it would have been noticed.
“We know that most high-energy events are accompanied by geomagnetic storms,” said Raimund Muscheler, a professor of geology at Sweden’s Lund University. “So it’s likely that there were visible auroras.”
Muscheler is the senior author of a study published in Nature Communications that reports evidence for an ancient event that resulted in a massive flux of high-energy particles or gamma-rays striking Earth.
produced distinct radioactive beryllium and chlorine isotopes in the atmosphere, which then fell to the ground with the yearly seasonal snowfall and were preserved in layers of ancient ice. Although galactic gamma-ray bursts and supernovas would have left a similar chemical signature in the ice, the cause was almost certainly a solar storm of protons, electrons, and ions, known as solar energetic particles, or SEPs.
The scientists have now examined ice cores from several drilling projects in Greenland and Antarctica, which was a difficult and time-consuming task.
They discovered evidence
of three SEP events in ice cores from both regions, which are known to have occurred in A.D. 993 or 994, A.D. 774 or 775, and 660 B.C. and are all associated with solar storms.
They also discovered evidence of a previously unrecorded large SEP event that occurred around 7176 B.C., or about 9,200 years ago.
Because the strength of the solar storm in 7176 B.C. could be estimated by the levels of radioactive isotopes of beryllium and chlorine, they determined that a similar intense storm today could have catastrophic consequences, including knocking out satellites in orbit, disrupting communications networks, and blacking out electricity grids.
“The known events of the last 70 years, where we have instrumental data, were all much smaller,” Muscheler said. These ancient events, he noted, were about 10 times larger.
According to the researchers,
one puzzling aspect of the 7176 B.C. solar storm is that it occurred during a supposedly “quiet” phase of the 11-year solar activity cycle, known as the “solar minimum,” when solar storms are unlikely. They warn that current risk assessments do not properly account for this possibility.
However, solar physicist Dean Pesnell of NASA’s Goddard Space Flight Center, who was not involved in the study, believes the 7176 B.C. storm occurred at the start of a new solar activity cycle, not during the actual solar minimum.
Pesnell, who’s the project scientist for the Solar Dynamics Observatory, said solar storms can also occur at the end of a declining phase of the solar activity cycle. “They’re not typical, but they’re not unexpected either.”
Jan Janssens, a communications specialist at the Solar-Terrestrial Centre of Excellence in Brussels, which coordinates international solar research, agrees with Pesnell that solar storms can occur at either the beginning or end of a solar activity cycle. “It’s possible,” he said in an email. “Clearly, that doesn’t happen too often, and certainly not during a solar cycle minimum, but it apparently does once in a while.”
The solar activity cycle
is caused by the entanglement and disentanglement of the sun’s powerful magnetic fields. Sunspots and solar storms are more common near the cycle’s maximums, and less common near the minimums.
According to Mary Hudson, a Dartmouth College professor of physics and astronomy who studies solar storms, the storm in 7176 B.C. may have been more severe than usual because it occurred near a solar minimum. Storms near a solar maximum, on the other hand, may be less severe than usual as a result, despite being more common. (The solar mechanism behind this apparent peculiarity is not understood, however, and some scientists dispute that it really exists.)
Hudson, who was not involved in the ice core study, pointed out in an email that the famously powerful solar storm observed by astronomers in 1859, known as the “Carrington Event,” occurred near a solar minimum, as did the powerful solar storm of A.D. 774 or 775.
So far, solar storms have had little impact
on the modern world. Janssens stated that they can severely damage satellites, threaten the health of astronauts in space with bursts of intense radiation, and interfere for several hours with radio signals used in communication networks and for ship and aircraft navigation.
They can also cause damage to power grids by generating unexpected electric currents that overwhelm the transformers in the grid. The “Halloween storms” of 2003, which blacked out parts of Europe for several hours and damaged transformers in South Africa, were among the worst solar storms in recent memory, while an intense solar storm in 1989 blacked out the Canadian province of Quebec. However, Pesnell claims that in recent years, power companies have become more aware of the risks and have “hardened” their equipment against solar storm damage.
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