A few years back, the sun went into a lull, its activity tailing off like a rambunctious child settling down for a nap. The lull was no surprise; it is a normal part of the sun's roughly 11-year cycle of activity, over which the number of magnetized regions known as sunspots waxes and wanes. But the sun did not snap out of its slumber as expected. The lull persisted, lingering on to become the deepest solar minimum in about 100 years before sunspots finally started increasing in number around the end of 2008.

The sunspot cycle is no mere scientific curiosity. At the maximum of the solar cycle, when activity is at its peak, eruptions from the sun can cause problems at Earth, damaging satellites and power grids. And the solar cycle governs how much energy reaches Earth, which can affect climate. A better understanding of the drivers of the solar cycle—and better tools to predict it—would be a boon to satellite operators, climate scientists and electric utilities, among others.

Numerous ideas have been put forth to explain the prolonged recent solar minimum, and now a study in the March 3 issue of Nature adds a new explanation. After modeling the circulation of plasma within the sun, the study's authors conclude that variations in the plasma flow speed may be the cause. (Scientific American is part of Nature Publishing Group.) But a prominent solar scientist who studies those flows says that the model does not match observational data.

The sun's meridional flow, a looping stream of plasma (hot, ionized gas) flowing from the equator to the poles and then, at greater depths, back to the equator, speeds up and slows down within the course of a single solar cycle as well as between cycles. By simulating the sun's meridional flows with a computer model, the new study's authors identified a specific pattern of flow variations that would cause a deep sunspot lull: a fast flow at the start of the solar cycle, followed by a slower flow toward the end of the cycle.

"Variations in the meridional flow affect the way magnetic fields form inside the sun," says Dibyendu Nandy, a solar physicist at the Indian Institute of Science Education and Research, Kolkata, who authored a paper on the study results with Andrés Muñoz-Jaramillo of the Harvard–Smithsonian Center for Astrophysics and Petrus Martens of Montana State University in Bozeman. The meridional flows carry magnetic flux along with them and, according to the new research, a fast meridional flow during the first half of the cycle stunts the development of the large-scale magnetic field. With less time to develop, the magnetic field "runs out of steam before the start of the next cycle," Nandy says.

The later slowdown of the meridional flow, in turn, delays the onset of the next cycle, leading to a long gap between sunspot cycles, which means a large number of days with no sunspots at all. That is just what was observed during the most recent solar minimum, which separated solar cycles 23 and 24. (Although Galileo Galilei and his contemporaries began monitoring sunspots in the early 17th century, numbered sunspot cycles date only to the mid-18th century.)

A weak magnetic field measured at the sun's poles during the 2008 minimum can be explained by the same shifting plasma flows, the researchers found. "When you have a fast flow early in the cycle, it sweeps positive- and negative-polarity sunspots to the polar region," Nandy explains. "They cancel each other out, and the next flux imparted is less."

So in one fell swoop the researchers' model explains two solar phenomena observed during the recent solar minimum: a large number of sunspot-free days and a weak polar magnetic field. There's only one catch, and it's a big one, says solar physicist David Hathaway of NASA Marshall Space Flight Center in Huntsville, Ala. "I agree with all three authors that variations in the meridional flow are the key to understanding variations in the sunspot cycle," Hathaway says. "The problem is that the variation in the meridional flows that they want is the opposite of what we observed."

Hathaway and Lisa Rightmire of the University of Memphis published a paper a year ago in Science showing meridional flow variations speeding up, not slowing down, as the sunspot cycle headed toward its prolonged minimum. "We definitely have a controversy here," Hathaway says, "and it's only going to get more interesting as time goes on." Nandy notes that those data concern shallow features on the sun and that the vast majority of the plasma resides farther down, where flows could behave differently.

Hathaway says that the measurements on which his Science study and a subsequent follow-up were based are the most accurate available and should bear on the model Nandy and his colleagues developed. "They have tried to marginalize the observations by saying it doesn't matter, it only concerns the surface layer," he says. "What we measured were the motions of magnetic elements, precisely the flux transport that they have in their model."

NASA's Solar Dynamics Observatory, which launched in 2010, should be able to peer deeper into the sun's churning plasma to help settle the issue. "This is a point that is still being debated," Nandy concedes. "I would say that this is something that needs to be resolved with more observations."