The Sun is still blank (mostly).
Ever since Samuel Heinrich Schwabe, a German astronomer, first noted in 1843 that sunspots burgeon and wane over a roughly 11-year cycle, scientists have carefully watched the Sun's activity. In the latest lull, the Sun should have reached its calmest, least pockmarked state last fall.
Indeed, last year marked the blankest year of the Sun in the last half-century — 266 days with not a single sunspot visible from Earth. Then, in the first four months of 2009, the Sun became even more blank, the pace of sunspots slowing more.
"It's been as dead as a doornail," David Hathaway, a solar physicist at NASA's Marshall Space Flight Center in Huntsville, Ala., said a couple of months ago.
The Sun perked up in June and July, with a sizeable clump of 20 sunspots earlier this month.
Now it is blank again, consistent with expectations that this solar cycle will be smaller and calmer, and the maximum of activity, expected to arrive in May 2013 will not be all that maximum.
For operators of satellites and power grids, that is good news. The same roiling magnetic fields that generate sunspot blotches also accelerate a devastating rain of particles that can overload and wreck electronic equipment in orbit or on Earth.
A panel of 12 scientists assembled by the National Oceanic and Atmospheric Administration now predicts that the May 2013 peak will average 90 sunspots during that month. That would make it the weakest solar maximum since 1928, which peaked at 78 sunspots. During an average solar maximum, the Sun is covered with an average of 120 sunspots.
But the panel's consensus "was not a unanimous decision," said Douglas A. Biesecker, chairman of the panel. One member still believed the cycle would roar to life while others thought the maximum would peter out at only 70.
Among some global warming skeptics, there is speculation that the Sun may be on the verge of falling into an extended slumber similar to the so-called Maunder Minimum, several sunspot-scarce decades during the 17th and 18th centuries that coincided with an extended chilly period.
Most solar physicists do not think anything that odd is going on with the Sun. With the recent burst of sunspots, "I don't see we're going into that," Dr. Hathaway said last week.
Still, something like the Dalton Minimum — two solar cycles in the early 1800s that peaked at about an average of 50 sunspots — lies in the realm of the possible, Dr. Hathaway said. (The minimums are named after scientists who helped identify them: Edward W. Maunder and John Dalton.)
With better telescopes on the ground and a fleet of Sun-watching spacecraft, solar scientists know a lot more about the Sun than ever before. But they do not understand everything. Solar dynamo models, which seek to capture the dynamics of the magnetic field, cannot yet explain many basic questions, not even why the solar cycles average 11 years in length.
Predicting the solar cycle is, in many ways, much like predicting the stock market. A full understanding of the forces driving solar dynamics is far out of reach, so scientists look to key indicators that correlate with future events and create models based on those.
For example, in 2006, Dr. Hathaway looked at disturbances in the Earth's magnetic field that are caused by the Sun, and they were strong. During past cycles, strong disturbances at minimum indicated strong fields all over the Sun at maximum and a bounty of sunspots. Because the previous cycles had been shorter than average, Dr. Hathaway thought the next one would be shorter and thus solar minimum was imminent. He predicted the new solar cycle would be a ferocious one, consistent with a short cycle.
Instead, the new cycle did not arrive as quickly as Dr. Hathaway anticipated, and the disturbances weakened. His revised prediction is for a smaller-than-average maximum. Last November, it looked like the new cycle was finally getting started, with the new cycle sunspots in the middle latitudes outnumbering the old sunspots of the dying cycle that are closer to the equator.
After a minimum, solar activity usually takes off quickly, but instead the Sun returned to slumber. "There was a long lull of several months of virtually no activity, which had me worried," Dr. Hathaway said.
The idea that solar cycles are related to climate is hard to fit with the actual change in energy output from the sun. From solar maximum to solar minimum, the Sun's energy output drops a minuscule 0.1 percent.
But the overlap of the Maunder Minimum with the Little Ice Age, when Europe experienced unusually cold weather, suggests that the solar cycle could have more subtle influences on climate.
One possibility proposed a decade ago by Henrik Svensmark and other scientists at the Danish National Space Center in Copenhagen looks to high-energy interstellar particles known as cosmic rays. When cosmic rays slam into the atmosphere, they break apart air molecules into ions and electrons, which causes water and sulfuric acid in the air to stick together in tiny droplets. These droplets are seeds that can grow into clouds, and clouds reflect sunlight, potentially lowering temperatures.
The Sun, the Danish scientists say, influences how many cosmic rays impinge on the atmosphere and thus the number of clouds. When the Sun is frenetic, the solar wind of charged particles it spews out increases. That expands the cocoon of magnetic fields around the solar system, deflecting some of the cosmic rays.
But, according to the hypothesis, when the sunspots and solar winds die down, the magnetic cocoon contracts, more cosmic rays reach Earth, more clouds form, less sunlight reaches the ground, and temperatures cool.
"I think it's an important effect," Dr. Svensmark said, although he agrees that carbon dioxide is a greenhouse gas that has certainly contributed to recent warming.
Dr. Svensmark and his colleagues found a correlation between the rate of incoming cosmic rays and the coverage of low-level clouds between 1984 and 2002. They have also found that cosmic ray levels, reflected in concentrations of various isotopes, correlate well with climate extending back thousands of years.
But other scientists found no such pattern with higher clouds, and some other observations seem inconsistent with the hypothesis.
Terry Sloan, a cosmic ray expert at the University of Lancaster in England, said if the idea were true, one would expect the cloud-generation effect to be greatest in the polar regions where the Earth's magnetic field tends to funnel cosmic rays.
"You'd expect clouds to be modulated in the same way," Dr. Sloan said. "We can't find any such behavior."
Still, "I would think there could well be some effect," he said, but he thought the effect was probably small. Dr. Sloan's findings indicate that the cosmic rays could at most account for 20 percent of the warming of recent years.
Even without cosmic rays, however, a 0.1 percent change in the Sun's energy output is enough to set off El Niño- and La Niña-like events that can influence weather around the world, according to new research led by the National Center for Atmospheric Research in Boulder, Colo.
Climate modeling showed that over the largely cloud-free areas of the Pacific Ocean, the extra heating over several years warms the water, increasing evaporation. That intensifies the tropical storms and trade winds in the eastern Pacific, and the result is cooler-than-normal waters, as in a La Niña event, the scientists reported this month in the Journal of Climate.
In a year or two, the cool water pattern evolves into a pool of El Niño-like warm water, the scientists said.
New instruments should provide more information for scientists to work with. A 1.7-meter telescope at the Big Bear Solar Observatory in Southern California is up and running, and one of its first photographs shows "a string of pearls," each about 50 miles across.
"At that scale, they can only be the fundamental fibril structure of the Sun's magnetic field," said Philip R. Goode, director of the solar observatory. Other telescopes may have caught hints of these tiny structures, he said, but "never so many in a row and not so clearly resolved."
Sun-watching spacecraft cannot match the acuity of ground-based telescopes, but they can see wavelengths that are blocked by the atmosphere — and there are never any clouds in the way. The National Aeronautics and Space Administration's newest sun-watching spacecraft, the Solar Dynamics Observatory, which is scheduled for launching this fall, will carry an instrument that will essentially be able to take sonograms that deduce the convection flows generating the magnetic fields.
That could help explain why strong magnetic fields sometimes coalesce into sunspots and why sometimes the strong fields remain disorganized without forming spots. The mechanics of how solar storms erupt out of a sunspot are also not fully understood.
A quiet cycle is no guarantee no cataclysmic solar storms will occur. The largest storm ever observed occurred in 1859, during a solar cycle similar to what is predicted.
Back then, it scrambled telegraph wires. Today, it could knock out an expanse of the power grid from Maine south to Georgia and west to Illinois. Ten percent of the orbiting satellites would be disabled. A study by the National Academy of Sciences calculated the damage would exceed a trillion dollars.
But no one can quite explain the current behavior or reliably predict the future.
"We still don't quite understand this beast," Dr. Hathaway said. "The theories we had for how the sunspot cycle works have major problems."
This article has been revised to reflect the following correction:
Correction: July 22, 2009
An article on Tuesday about the recent decline in the frequency of sunspots incorrectly described the research of David Hathaway, a scientist at
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