The Sun goes through a regular 11-year cycle, and it is now emerging from the quietest part of the cycle, or solar minimum. But even for this phase it has been unusually quiet, with little roiling of the magnetic fields that induce sunspots.
“It’s starting with a murmur,” said David H. Hathaway, a solar physicist at NASA’s Marshall Space Flight Center in Huntsville, Ala.
As of Thursday, the 276th day of the year, the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center in Boulder, Colo., had counted 205 days without a sunspot.
In another sign of solar quiescence, scientists reported last month that the solar wind, a rush of charged particles continually spewed from the Sun at a million miles an hour, had diminished to its lowest level in 50 years.
Scientists are not sure why this minimum has been especially minimal, and the episode is even playing into the global warming debate. Some wonder if this could be the start of an extended period of solar indolence that would more than offset the warming effect of human-made carbon dioxide emissions. From the middle of the 17th century to the early 18th, a period known as the Maunder Minimum, sunspots were extremely rare, and the reduced activity coincided with lower temperatures in what is known as the Little Ice Age.
Compared to the Maunder Minimum, the current pace of sunspots “makes it look like we’re having a feast, not a famine,” Dr. Hathaway said.
Scientists expect that sunspot activity will pick up in the coming months, but exactly what will happen next is open to debate. Dr. Hathaway had predicted two years ago, based on the Sun’s behavior near the end of the last cycle, that the maximum this time would be ferocious.
“I’m getting worried about that prediction now,” he said. “Normally, big cycles start early, and by doing that, they cut short the previous cycle. This one hasn’t done that.”
But many of the other competing predictions — more than 50 over all — pointed to a quieter-than-average cycle. “They do kind of go all over the map,” said Douglas Biesecker, a physicist at the Space Weather Prediction Center who led an international panel that reviewed predictions.
The solar wind is another piece of the puzzle. David J. McComas of the Southwest Research Institute in San Antonio and one of the researchers who analyzed data from the Ulysses Sun-watching spacecraft, said that the strength of the solar wind seemed to be in a long-term decline. The pressure exerted by the solar wind particles during the current minimum is about a quarter weaker than during the last solar minimum, Dr. McComas said.
Dr. McComas said scientists were still trying to figure out how all the data fits together.
“There are a number of researchers who predict the next solar cycle,” he said. “There are also a number of investment counselors who predict the future of the stock market.”