An analysis of radioactive chemicals in ice cores indicates one of the most powerful solar storms ever hit Earth around 7176 B.C.
Discarded ice cores at a drilling site in Greenland.
(Inside Science) -- 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.
Telltale chemical isotopes in ancient ice cores suggest one of the most massive solar storms ever took place 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 January in the journal Nature Communications reporting evidence for the ancient event that resulted in a massive flux of high-energy particles or gamma-rays striking Earth.
The event created distinctive radioactive varieties of beryllium and chlorine in the atmosphere; those isotopes then fell to the ground with the yearly seasonal snowfall and were preserved in layers of ancient ice. The cause was almost certainly a solar storm of protons, electrons and ions -- called solar energetic particles, or SEPs -- although galactic gamma-ray bursts and supernovas would have left a similar chemical signature in the ice.
The researchers have now examined ice cores from several drilling projects in Greenland and Antarctica -- a difficult and time-consuming task.
In ice cores from both regions, they saw evidence of three SEP events that are known to have occurred in A.D. 993 or 994, in A.D. 774 or 775, and in 660 B.C. and are all associated with solar storms.
But they also found evidence of another large SEP event -- unrecorded before now -- that occurred in about 7176 B.C., or about 9,200 years ago.
Because its strength could be estimated by the levels of the radioactive isotopes of beryllium and chlorine, they determined the solar storm in 7176 B.C. was so severe that if a similarly intense storm happened today, it 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.
The researchers say a puzzling feature of the 7176 B.C. solar storm is that it occurred during a supposedly "quiet" phase -- known as the "solar minimum" -- of the 11-year solar activity cycle, when solar storms are unlikely. They warn current risk assessments don’t properly take this possibility into account.
But solar physicist Dean Pesnell of NASA's Goddard Space Flight Center, who wasn't involved in the study, calculates the 7176 B.C. storm occurred not during the actual solar minimum, but at the start of a new solar activity cycle.
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 studies of the sun, agrees with Pesnell that solar storms can occur at the very start or very 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."
And if the solar storm did not happen during the solar minimum, but instead occurred at the start of a new solar cycle, that would undermine the researchers' warning that such storms can happen at that time and are not being properly accounted for.
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.
Mary Hudson, a professor of physics and astronomy at Dartmouth College who studies solar storms, said that if the 7176 B.C. storm happened near a solar minimum, it might have been more severe than usual as a result. Storms near a solar maximum, however, might be less severe than usual as a result, although they are more common. (The solar mechanism behind this apparent peculiarity is not understood, however, and some scientists dispute that it really exists.)
Hudson, who also wasn't involved in the ice core study, noted in an email that the famously powerful solar storm observed by astronomers in 1859, called the "Carrington Event," also occurred near a solar minimum, as did the powerful solar storm of A.D. 774 or 775.
So far, the modern world hasn't been very badly affected by solar storms. Janssens noted they can badly damage satellites, threaten the health of astronauts in space with bursts of intense radiation and interfere for several hours with the radio signals used in communication networks and for navigation by ships and aircraft.
They can also damage power grids by creating unexpected electric currents that overcome a grid’s transformers. Some of the worst solar storms in recent memory, the "Halloween storms" of 2003, blacked out parts of Europe for several hours and damaged transformers in South Africa, while an intense solar storm in 1989 blacked out the Canadian province of Quebec. But Pesnell said power companies have become more aware of the risks in recent years and have "hardened" their equipment against solar storm damage.
