Sun may be entering a long period of reduced activity, several studies suggest

By Ron Cowen

Web edition: June 15, 2011
Print edition: July 16, 2011; Vol.180 #2 (p. 12)

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SPOTTED

A close-up of a group of sunspots shows their dark, highly magnetized cores surrounded by a lighter, less magnetized region known as the umbra. These familiar solar blemishes may disappear, perhaps for decades, if the sun goes into hibernation around 2020 as some researchers now predict.

Credit: NSO

Already sluggish, the sun may be slipping into several decades of hibernation that could exert a cooling effect on Earth’s climate, several new studies suggest.

During the last extended period of solar dormancy, from 1645 to 1715, Europe plunged into some of the coldest winters on record. But Earth’s atmosphere, which now contains an abundance of greenhouse gases, differs in composition compared with three centuries ago, and solar physicists say they’re unsure how a long solar hiatus would affect the planet’s 21st century climate.

It’s also possible that the beginning of the next 11-year solar cycle — which is marked by the emergence of dark blemishes called sunspots at high solar latitudes — may simply be delayed by a few years, rather than shut down for decades.

The scientists base their findings on multiple observations of the sun’s outer atmosphere, visible surface and the movement of magnetic fields inside the sun. The sun’s 11-year cycle is governed by flows of hot gas, or plasma, in the sun that transport parcels of the solar magnetic field. 

Three teams presented their results June 14 during a press briefing at a meeting in Las Cruces, N.M., of the American Astronomical Society’s Solar Physics division.

The evidence “all indicates that the next solar cycle will be delayed by two to five years,” but that a longer break is possible, says Dean Pesnell of NASA’s Goddard Space Flight Center in Greenbelt, Md., who did not participate in the studies.

In one of the new studies, Frank Hill of the National Solar Observatory in Tucson and his colleagues used ground-based detectors to monitor changes in the solar surface due to the reverberation of sound waves inside the sun. These solar sonograms previously revealed an east-west flow of material deep inside the sun that presaged, years in advance, the onset of the last two solar cycles. But the flow marking the beginning of the next solar cycle, which the team expected to see in 2008 or 2009, still isn’t there. The flow’s absence “is leading us to believe that the next cycle may be very much more delayed, or may not happen at all,” Hill says.

In a second study, Richard Altrock, manager of the Air Force’s coronal research program at the National Solar Observatory in Sunspot, N.M., and his colleagues mapped the distribution of highly ionized iron atoms in the sun’s outer atmosphere. The distribution of these ions acts as a tracer for the movement of magnetic fields rooted in the sun.

Around the peak of a solar cycle, the magnetic field at the sun’s poles reverses direction, with the old field erased by the new, oppositely directed field. But the current delayed cycle may not be strong enough to fully erase the old field. The findings suggest that this cycle’s solar maximum, predicted to occur in 2013, may be weak or not occur at all, Altrock says.

A third study finds that the strength of the magnetic fields that produce sunspots has steadily declined over the past 13 years. If that decline continues, the fields will fall below the threshold required to support the cold gas in sunspots, and the familiar blemishes will all but disappear around 2022, notes study coauthor Matt Penn of the National Solar Observatory in Tucson.

Douglas Biesecker of the National Oceanic and Atmospheric Administration in Boulder, Colo., says he’s skeptical that the studies indicate the sun will go on a full hiatus, especially because most of the observations only encompass the last few solar cycles, which have either been atypically weak or late in getting started.

Nonetheless, the findings have caught Biesecker and other researchers by surprise at an auspicious time, with an armada of sun-staring telescopes on the ground and in space poised to provide answers.

 “It’s a great time to be a solar physicist,” says Pesnell.