Sun's doldrums may last up to a decade

Despite a recent flare, solar physicists project low activity for up to a decade 

Science News 3/2/11: A powerful explosion that erupted on the solar surface on February 14 was the most powerful flare in more than four years, and heralds an approaching peak in the sun’s 11-year activity cycle. But as the sun pulls out of an exceptionally quiet period of low activity, researchers predict the coming solar maximum won’t be very exciting either.

“This cycle continues to fall below expectations. And those expectations were pretty low two years ago,” says David Hathaway of NASA’s Marshall Space Flight Center in Huntsville, Ala.

The number of sunspots — dark, highly magnetized regions on the solar surface — is one indicator of solar activity, and scientists now predict this will be the weakest sunspot cycle in 200 years. “We are off to a good start for a below-average cycle peaking in late 2013 or early 2014,” says Dean Pesnell of NASA’s Goddard Space Flight Center in Greenbelt, Md.

Understanding how present activity affects future cycles is important to gauging both the sun’s influence on climate and its likelihood of producing powerful and destructive solar storms.

Solar physicists say they are homing in on the complex internal interactions that could explain why the sun has been hibernating for more than four years now and  

 Hathaway and other researchers say they’re now convinced that a flow of ionized gas, or plasma, known as the meridional flow controls the strength of the solar cycle (SN: 4/10/11, p. 8).On either side of the equator, the flow moves like a conveyor belt that stretches just beneath the solar surface from the equator to the two poles and then dives into the sun’s interior, flowing from the poles back to the equator to complete the loop.

The speed of the flow appears to be a critical parameter. But Hathaway and other researchers disagree on exactly how the meridional flow affects solar activity.

In the March 3 Nature, Dibyendu Nandy of the Indian Institute of Science Education and Research in Kolkata and his colleagues report new computer simulations suggesting one way that the flow determines future solar activity. Their simulations show that a fast flow during the first half of a solar cycle, followed by a slower flow during the second half, creates a weak polar magnetic field. This generates an unusually weak and prolonged solar minimum, Nandy says, like the most recent one. For a 780-day period ending in March 2010, not a single sunspot was observed. During a more typical solar minimum, the sun is spot-free for about 300 days.

“The Nandy article is an excellent study of the effect of the meridional flow on what we see at the surface over a solar cycle,” says Pesnell.

But the problem, says Hathaway, is that his observations indicate that the speed of the meridional flow was exactly opposite to that required by Nandy and his colleagues. Another solar physicist, Yi-Ming Wang of the Naval Research Laboratory in Washington, D.C., says he is baffled by the apparent contradiction between the model and Hathaway’s observations.

Remainder at SOURCE