New Delhi, Nov. 11: Two Indian scientists who spent months simulating the Sun’s behaviour over multiple millennia have proposed a solution to a long-standing puzzle — its 27 vows of silence over the past 11,600 years.

Solar physicists Arnab Rai Choudhuri and Bidya Binay Karak at the Indian Institute of Science, Bangalore, have proposed a mechanism to explain events called solar grand minima, in which sunspots disappear for decades before reappearing.

Using computer simulations, the duo have reconstructed the disappearance of sunspots at a frequency that closely matches direct and indirect evidence of grand minima since about 9600 BC.

“We were pleasantly surprised at the close match between our theoretical calculations and the observed data,” said Rai Choudhuri, professor of physics at the IISc. The scientists have described their findings in the journal Physical Review Letters.

Scientists have long known that the number of sunspots — zones of intense magnetic activity on the solar surface — increase and decrease over an 11-year cycle. But the grand minima are extended periods, from 30 years to 110 years, of almost no sunspot activity.

“In contrast to the sunspot activity at other times, grand minima are like abrupt periods of silence,” said Rai Choudhuri.

The only grand minimum directly observed after the invention of the telescope had lasted 70 years, from about AD 1645 to AD 1715. But the absence of sunspots increases the amount of cosmic rays striking the Earth, whose chemical signatures on terrestrial elements can serve as indirect evidence for grand minima.

A team of European scientists had five years ago synthesised all such indirect evidence to list 27 grand minima since 9600 BC.

“We had observational data, but the how and why of grand minima remained an open question,” said Ilya Usokin, a cosmic ray physicist at the University of Oulu, Finland, who was a member of the European team that documented the 27 grand minima.

“This new study (by the IISc) offers a possible theoretical explanation,” Usokin told The Telegraph.

Karak and Rai Choudhuri simulated on computer the flow of solar material from the Sun’s equator to its poles, and the changes in the Sun’s magnetic field, and calculated that over 11,000 years, the number of grand minima would be anything between 24 and 30.

“This is remarkably close to the observed 27 grand minima,” said Karak, a research scholar who had studied BSc physics in Bankura before being selected for IISc’s integrated PhD programme.

The IISc physicists have proposed that extreme fluctuations in either the speed of the flow of solar material, called the meridional circulation, or the strength of the Sun’s polar magnetic field are the likely triggers for grand minima.

The simulations also predict the Sun would spend about 10 to 15 per cent of its time in grand minima — another result close to the figure of 17 per cent obtained through the direct and indirect evidence of grand minima over the past 11,600 years.

“The origin of grand minima is one of the most vexing problems in solar physics,” said Kristof Petrovay, head of astronomy at the Eotvos University in Budapest, Hungary. The proposal by the IISc team, he said, “presents the first fully explicit solar dynamo model that displays behaviour similar to grand minima”.

But solar physicists including Karak and Rai Choudhuri say it could take a while before their explanation is accepted.

“Although their model seems to work, it makes certain assumptions about the meridional circulation and properties of turbulence that cannot be confirmed with simulations,” said Alex Brandenburg, a physicist at Alba Nova University Centre, Sweden.

Sunspots are associated with solar flares and eruptions and solar activity called coronal mass ejections that can potentially influence satellite operations, telecommunications and air traffic, particularly over the poles.

“There is some evidence that during the last grand minima in the 17th century, several places in Europe and China went into extreme cold,” said Karak. “The effects, if any, of sunspots on weather is an important research issue.”

A typical computer simulation of solar behaviour over 11,000 years takes about 10 days. The IISc researchers tweaked fluctuations in the meridional circulation and the magnetic field, and observed that extreme fluctuations tend to push the Sun into grand minima.

“We think we now know why the Sun, every once in a while, lapses into this silence,” said Rai Choudhuri. “But we still don’t know what pushes the Sun back into its routine 11-year sunspot activity. That’s an area of future research.”