Researchers have discovered that one of the mysterious forces that sweep the sun's surface shows an unexpectedly strong connection with the number of sunspots, magnetic disturbances that can affect Earth's weather and telecommunications. The findings should improve predictions of the sun's dynamics and might even help scientists develop better climate models.
Along with heat and light, the sun emits x-rays and magnetically charged particles that can endanger astronauts, fry circuits aboard satellites orbiting Earth, and overload electric power lines on the ground. Because of those potential threats, for several decades scientists have been examining the sun's magnetic behavior, particularly what drives its 11-year sunspot cycle, which at its peak generates dozens of the magnetic disturbances a day and can give rise to gigantic storms, some of which sweep past Earth.
One curious aspect of the solar cycle is meridional flow, which acts like a conveyor belt carrying magnetism to the sun's poles. Scientists haven't been able to model it accurately or determine how it might be connected to the sunspot cycle.
Solar scientists David Hathaway of NASA's Marshall Space Flight Center in Huntsville, Alabama, and Lisa Rightmire of the University of Memphis in Tennessee set out to determine how meridional flow correlates with variations in the sunspot cycle. Using data collected by the Solar and Heliospheric Observatory (SOHO), a spacecraft launched jointly by NASA and the European Space Agency, the two scientists tracked the motion of small zones of magnetism on the sun's surface carried poleward by meridional flow. They used SOHO data collected every 8 hours from nearly all of the most recent sunspot cycle, which ended in December 2009. (The data here went until June 2009.)
Those observations revealed that as the number of sunspots declined, the meridional flow did the opposite. Its average speed increased from about 30 kilometers per hour at the peak of the solar cycle, in 2000 and 2001, to over 47 km per hour in 2008 and 2009, after the solar cycle should have ended but was still lingering. Hathaway and Rightmire report their findings in today's issue of Science.
But knowing that the meridional flow correlates with the number of sunspots—and may even be driving them—doesn't solve every mystery here. For example, this solar cycle lasted longer than normal, and the meridional flow sped up unexpectedly toward the end—why this happened remains a "big unknown," says Hathaway. Studying the flow in earlier sunspot cycles might help us understand that, although those data are much less detailed than those from SOHO.
"We're getting information that can test our ideas about how the sunspot cycle works," says solar physicist Neil Sheeley of the U.S. Naval Research Laboratory in Washington, D.C.
The findings should also help climate scientists refine their long-term models, says solar physicist Philip Judge of the NationalCenter for Atmospheric Research in Boulder, Colorado. Although our understanding of meridional flow remains "crude," he says, the study improves the ability to measure it, and this could help refine the sun's influence on long-term climate models. That's important, Judge says, because the flow is connected to the solar cycle, and the cycle is helping to drive shifts in climate.