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 may not fully awaken for another decade. 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.
Nandy notes that measurements made by Hathaway and his colleagues are limited to the surface, so they may not reflect the true speed of the flow deep in the sun’s interior.
Hathaway says that a fast meridional flow is indeed required to explain both the sun’s long hibernation and the weakness of the current solar cycle, which began in late 2008. But his observations show that the fast flow occurred during the last half of the last solar cycle, not the first. The flow drags the magnetic field along with it, and a fast flow leads to a weaker field at the poles than a slow flow would. Because the polar fields are thought to be the seeds for the next solar cycle, a weak polar field will cause the next cycle to be weak also.
“It is possible that the current sunspot cycle, cycle 24, was seeded with magnetic fields from cycle 23, or even an earlier cycle, cycle 22,” says Matt Penn of the National Solar Observatory in Tucson. “This seed field may have been weaker than normal, somehow, and it may be producing a weaker solar activity cycle now.”
For now, the exact role that the meridional flow plays in the solar cycle remains a matter of debate. But the new research “demonstrates how the inner working of the sun, and variations in the plasma flow deep within our parent star can control its magnetic and energetic output, which in turn, determines the environment in space and affects climate on Earth,” says Nandy.
A weaker solar cycle is accompanied by a slightly dimmer sun, which changes the average temperature on Earth, says Judith Lean of the Naval Research Laboratory. She notes, however, that the sun’s brightness did not hit an all-time low during the past solar minimum, even though the sun was unusually quiet.
Nonetheless, the decline in solar brightness from 2002 to 2008 as solar activity dwindled probably countered the warming on Earth that would otherwise have occurred due to greenhouse gases over that period. “As solar activity now increases,” she says, “we can expect this mitigating effect to stop.”
The most powerful solar flare to erupt from the sun in more than four years can be seen in this video taken by the Solar Dynamics Observatory. The flare appears as a brief flash just below and to the right of center, about two seconds into the video.
Back story | SUNSPOTS, 1750–2010
Credit: D.H. Hathaway/Living Reviews in Solar Physics 2010
The sun’s activity waxes and wanes on a cycle that averages roughly 11 years, though cycles as short as nine years and as long as 14 years have been observed. Chinese astronomers were already tracking the sun’s activity using sunspots more than 2,000 years ago; the modern record of solar output starts in 1755, with cycle 1, and runs through cycle 24, which began in late 2008. Generated by intense magnetic fields, sunspots have proven one reliable indicator of the sun’s overall output and its production of solar storms.