Study Blames Plasma Flow for Spotless Sun

A new computer model suggests the shifting speeds of plasma inside the sun could have shut off sunspots at the end of the most recent solar cycle. The model, described in the March 3 Nature, attempts to explain why the most recent lull in solar activity was so long and so quiet. The sun’s magnetic […]
Image may contain Light and Lightbulb

A new computer model suggests the shifting speeds of plasma inside the sun could have shut off sunspots at the end of the most recent solar cycle.

The model, described in the March 3 Nature, attempts to explain why the most recent lull in solar activity was so long and so quiet. The sun's magnetic activity ramps up and calms down on a fairly regular 11-year cycle. The highs are full of sunspots, dark splotches that mark where knots of magnetic field have risen from the solar interior to pop up at the surface like a cork. During the lows, some days have no sunspots at all.

Sunspots can give rise to solar flares and other magnetic storms that can wreak minor havoc on Earth, knocking out power grids and communications satellites.

The last solar cycle peaked in 2001 and was supposed to end in 2008. But the sun stayed asleep, displaying a weak magnetic field and an unusually high number of sunspotless days, for an extra 15 months beyond what astronomers expected.

Now, Dibyendu Nandy of the Indian Institute of Science Education and Research and colleagues offer an explanation: A "conveyor belt" of plasma inside the sun ran quickly at first and then slowed down.

Nandy and colleagues at Montana State University and the Harvard-Smithsonian Center for Astrophysics ran a computer simulation of magnetic flow inside the sun for 210 sunspot cycles. They randomly varied the speed of plasma flow around a loop called the meridional circulation, which carries magnetic fields from the sun's interior to its surface and from the equator to the poles.

Observations suggest that the fastest flow runs around 22 meters per second (49 miles per hour). Nandy's model looked at speeds between 15 and 30 meters per second (33 to 67 miles per hour).

The model found that a fast flow followed by a slow flow reproduced both the weak magnetic field and the dearth of sunspots observed in the last solar minimum.

"This is the first paper that is able to provide a rationale and reproduce two of the main characteristics of the extended solar minimum," said NASA solar physicist Madhulika Guhathakurta, who was not involved in the new work. "For something as complicated as the solar dynamo and solar cycle, this relatively simple model has produced remarkable results."

The model makes physical sense, Nandy says. The seeds of sunspots form when the magnetic field is strong in a region Nandy calls the "creation zone," about a third of the way down into the sun. A faster meridional flow means magnetic plasma spends less time in the creation zone, making a weaker magnetic field and fewer sunspots.

"What you’re doing by having a very fast flow early on in the cycle is you’re producing a sunspot cycle which is not very strong," he said. "It runs out of steam before the next cycle can start."

A slower flow in the second half delays the onset of the next solar maximum, leaving a sunspot-free gap between the two cycles.

Unfortunately, observations of the sun's surface seem to directly contradict the new model.

"We're in this quandary, this clash between theory and observations," said NASA astronomer David Hathaway, who analyzed 13 years of data from the Solar and Heliospheric Observatory (SOHO) that tracked the movement of charged material near the surface of the sun.

Hathaway agrees that a fast flow can cause weak magnetic fields and fewer sunspots. But his observations, published March 12, 2010 in Science, suggest that the meridional flow was slow in the first half of the last solar cycle, from about 1996 to 2000. Only after the solar maximum did the flow speed up.

"That's where there's a problem," Hathaway said. "We see one thing, they want the opposite to explain the observations."

Nandy and colleagues point out that the SOHO observations only see plasma moving at the surface of the sun, not in the deep interior where sunspots are born. The surface flows might not reflect what's going on underneath, he says.

"In an analogy that you might be able to relate to, one could ask, do ripples on the surface of the sea indicate how ocean currents determine the migration of aquatic animals deeper inside?" Nandy said.

Hathaway argues that changes in the surface should be transmitted to the interior at the speed of sound, and should reach the creation zone in half an hour or less. The disagreement between theory and data means there must be a problem with the models, he says.

"Since 1999, I was a huge champion of these models. They so nicely explained why the sunspot zones drift toward the equator at the speeds they do," he said. "But I'm worried now. I'm really worried."

More observations, especially with NASA's fairly new Solar Dynamics Observatory, should clear things up.

"The sun will ultimately tell us how to resolve this conflict because only it knows what the next cycle will bring," Guhathakurta said.

Image: 1) A spotless sun in September 2008. Credit: SOHO/ESA/NASA. 2) William T. Bridgman (NASA/GSFC), Dibyendu Nandy (IISER Kolkata), Andrés Muñoz-Jaramillo (Harvard Smithsonian Center for Astrophysics) and Petrus C.H. Martens (Montana State University).

Citations:
"The unusual minimum of sunspot cycle 23 caused by meridional plasma flow variations." Dibyendu Nandy, Andrés Muñoz-Jaramillo and Petrus C. H. Martens. Nature, Vol 471, 3 March 2011. DOI: 10.1038/nature09786.

"Variations in the Sun’s Meridional Flow over a Solar Cycle." David H. Hathaway and Lisa Rightmire. Science, Vol. 327 no. 5971, 12 March 2010. DOI: 10.1126/science.1181990

See Also: