© NASAScientists say fewer sunspots than expected have been appearing on the solar surface recently.
Centuries of observations have shown that sunspot activity goes through a 22-year cycle, with an 11-year period of sunspot activity from greatest to least, then an ascent to another peak.
The sun currently is in an 11-year valley, or solar minimum. However, it seems to be idling there, rather than ticking back upward. The least active stage for sunspots was in August 2008, which led scientists to expect an uptick by March of this year - but it hasn't yet occurred.
Sunspots are evidence of solar regions with increased magnetic activity. They can have a strength thousands of times stronger than the Earth's magnetic field. They usually come in groups, normally with two sets of spots, and are the sources of solar flares, coronal mass ejections, and intense ultraviolet radiation.
When sunspots do appear on the surface, they typically last for days, although very large ones may last for weeks. Decades-long periods of low sunspot activity may be associated with spells of lower-than-usual temperatures, though scientists say the link is still being debated.
Tony Phillips of NASA's heliophysics news team recently reported that there were no sunspots observed on 266 of the 366 days in 2008. The last time the sun was that quiet for such a long time was 1913, when about 311 spotless days were recorded, according to Phillips' report. As of March 31, there were no sunspots on 78 of 2009's 90 days.
Since Galileo Galilei made the first European observations of sunspots in 1610, there have been many prolonged solar minimums. Among the most notable was the Maunder Minimum, which also has been deemed the "Little Ice Age" of Europe and North America.
According to data from the solar physics team at NASA's Marshall Space Center in Huntsville, Ala., early sunspot records indicate the sun went through a period of low activity in the late 17th century, meaning very few sunspots were seen on the sun from about 1645 to 1715.
Normally ice-free rivers froze, and snow fields remained year-round at lower altitudes during that 70-year period. In both North America and Europe, winter temperatures cooled as much as 2 to 4 degrees, enough to freeze rivers and change agriculture and the economy.
Sunspots were not the only factor at work, however. According to NASA's Goddard Space Flight Center, some scientists say a major volcanic eruption in 1258 and another in 1600 also helped cool northern hemisphere temperatures during those periods.
Astrophysicist Terry Forbes, a University of New Hampshire research professor, said he believes the planet is in its deepest solar minimum since 1956.
Scientists who monitor solar activity on a consistent basis may know within six months or so if we can expect a prolonged minimum, he said.
He added that he would've expected solar activity to flare up nearly a month ago and is surprised at the lack of spots on the sun's surface.
"I kind of expected April to show sign," he said. "The sun was very quiet last week."
Sunspots happen when the outer layers of the sun are in a state of conductive turbulence, he said.
"It's kind of like if you took a cup of mercury and you stirred it with a metal spoon, and as you're stirring, it would shock you," Forbes said.
The effect is so strong it's noticeable from the Earth's surface.
He said because the state of the sun is so chaotic, predictions are speculative, and things could change in a month or so.
"My own feeling is that it's far too early to call the police quite yet," he said of the lull in sunspots.
Eberhard Moebius, physics department chairman at the UNH Space Science Center, characterizes the sunspots and solar cycles as space weather.
He cautions that the lack of sunspots is nothing to be overly worried about, and it's not rare for a prolonged solar minimum to last as long as 12 to 13 years.
He said the impact a prolonged minimum could have on the Earth's weather and climate has been the subject of a scholarly debate for quite some time.
"As long as we have solar cycles that may be changing ... there's not much impact on our weather and climate," he said.
But should the current solar minimum extend out 50 to 80 years, the story could change, he said.
"It could be that we are getting into such a minimum again, but nobody knows. It has happened in the past going all the way back to the Middle Ages," he said.
One period in which the sun experienced a prolonged solar minimum, called the Sporer Minimum, occurred much earlier than the "Little Ice Age," he said.
It lasted from about 1420 to 1570 and had such a profound effect on certain parts of the Earth that it could be credited with ending the Viking settlements on Greenland, Newfoundland and Iceland, he said.
At the time, land in northern Canada normally used to grow fruit for wine got so cold it was useless to settlers, he said.
Hundreds of years later, the Maunder Minimum made summer a rarity in New England and brought a lot of summer rain, according to Moebius.
The Dalton Minimum occurred during the 1800s. That period of low solar activity lasted from about 1790 to 1830 and was named after English meteorologist John Dalton.
Much like the Maunder and Sporer minimums, the Dalton Minimum coincided with a period of lower-than-average global temperatures. Also, an event called "The Year Without a Summer" occurred in 1816.
Moebius said things could get so prolonged that in theory, temperature increases from global warming could be offset, although he doesn't see anything like the "Little Ice Age" happening now.
"In a way, it could give humankind a short reprieve from man-made global warming," he said.
But in the end, the solar cycle could flip and things could begin to heat up again. Overall, throughout time, there have been more warmer periods than colder, with stronger solar cycles creating more sunspots, he said.
"At the moment we don't have the predictive tools, so there's no way we can predict what the solar cycles will be. Scientists are working on that," he said.
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