Image courtesy STEREO/NASA
Published June 14, 2011
Enjoy our stormy sun while it lasts. When our star drops out of its latest sunspot activity cycle, the sun is most likely going into hibernation, scientists announced today.
Three independent studies of the sun's insides, surface, and upper atmosphere all predict that the next solar cycle will be significantly delayed—if it happens at all. Normally, the next cycle would be expected to start roughly around 2020.
The combined data indicate that we may soon be headed into what's known as a grand minimum, a period of unusually low solar activity.
The predicted solar "sleep" is being compared to the last grand minimum on record, which occurred between 1645 and 1715.
Known as the Maunder Minimum, the roughly 70-year period coincided with the coldest spell of the Little Ice Age, when European canals regularly froze solid and Alpine glaciers encroached on mountain villages.
"We have some interesting hints that solar activity is associated with climate, but we don't understand the association," said Dean Pesnell, project scientist for NASA's Solar Dynamics Observatory (SDO).
Also, even if there is a climate link, Pesnell doesn't think another grand minimum is likely to trigger a cold snap.
"With what's happening in current times—we've added considerable amounts of carbon dioxide and methane and other greenhouse gases to the atmosphere," said Pesnell, who wasn't involved in the suite of new sun studies.
"I don't think you'd see the same cooling effects today if the sun went into another Maunder Minimum-type behavior."
Sunspots Losing Strength
Sunspots are cool, dark blemishes visible on the sun's surface that indicate regions of intense magnetic activity.
For centuries scientists have been using sunspots—some of which can be wider than Earth—to track the sun's magnetic highs and lows.
For instance, 17th-century astronomers Galileo Galilei and Giovanni Cassini separately tracked sunspots and noticed a lack of activity during the Maunder Minimum.
In the 1800s scientists recognized that sunspots come and go on a regular cycle that lasts about 11 years. We're now in Solar Cycle 24, heading for a maximum in the sun's activity sometime in 2013.
They noticed a long-term trend of sunspot weakening, and if the trend continues, the sun's magnetic field won't be strong enough to produce sunspots during Solar Cycle 25, Penn and colleagues predict.
"The dark spots are getting brighter," Penn said today during a press briefing. Based on their data, the team predicts that, by the time it's over, the current solar cycle will have been "half as strong as Cycle 23, and the next cycle may have no sunspots at all."
Sun's "Jet Streams," Coronal Rush Also Sluggish
Separately, the National Solar Observatory's Frank Hill and colleagues have been monitoring solar cycles via a technique called helioseismology. This method uses surface vibrations caused by acoustic waves inside the star to map interior structure.
Specifically, Hill and colleagues have been tracking buried "jet streams" encircling the sun called torsional oscillations. These bands of flowing material first appear near the sun's poles and migrate toward the equator. The bands are thought to play a role in generating the sun's magnetic field.
Sunspots tend to occur along the pathways of these subsurface bands, and the sun generally becomes more active as the bands near its equator, so they act as good indicators for the timing of solar cycles.
"The torsional oscillation ... pattern for Solar Cycle 24 first appeared in 1997," Hill said today during the press briefing. "That means the flow for Cycle 25 should have appeared in 2008 or 2009, but it has not shown up yet."
According to Hill, their data suggest that the start of Solar Cycle 25 may be delayed until 2022—about two years late—or the cycle may simply not happen.
Adding to the evidence, Richard Altrock, manager of the U.S. Air Force's coronal research program for the National Solar Observatory (NSO), has observed telltale changes in a magnetic phenomenon in the sun's corona—its faint upper atmosphere.
Known as the rush to the poles, the rapid poleward movement of magnetic features in the corona has been linked to an increase in sunspot activity, with a solar cycle hitting its maximum around the time the features reach about 76 degrees latitude north and south of the sun's equator.
The rush to the poles is also linked to the sun "sweeping away" the magnetic field associated with a given solar cycle, making way for a new magnetic field and a new round of sunspot activity.
This time, however, the rush to the poles is more of a crawl, which means we could be headed toward a very weak solar maximum in 2013—and it may delay or even prevent the start of the next solar cycle.
Quiet Sun Exciting for Science
Taken together, the three lines of evidence strongly hint that Solar Cycle 25 may be a bust, the scientists said today during a meeting of the American Astronomical Society in Las Cruces, New Mexico.
But a solar lull is no cause for alarm, NSO's Hill said: "It's happened before, and life seems to go on. I'm not concerned but excited."
In many ways a lack of magnetic activity is a boon for science. Strong solar storms can emit blasts of charged particles that interfere with radio communications, disrupt power grids, and can even put excess drag on orbiting satellites.
"Drag is important for people like me at NASA," SDO's Pesnell said, "because we like to keep our satellites in space."
What's more, a decrease in sunspots doesn't necessarily mean a drop in other solar features such as prominences, which can produce aurora-triggering coronal mass ejections. In fact, records show that auroras continued to appear on a regular basis even during the Maunder Minimum, Pesnell said.
Instead, he said, the unusual changes to the sun's activity cycles offer an unprecedented opportunity for scientists to test theories about how the sun makes and destroys its magnetic field.
"Right now we have so many sun-watching satellites and advanced ground-based observatories ready to spring into action," Pesnell said. "If the sun is going to do something different, this is a great time for it to happen."
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