Sunspot Cycles: Deciphering the Butterfly Pattern

John Roach
National Geographic News
February 4, 2005
A little more than 150 years ago, scientists learned that the number of sunspots (temporarily cool, dark areas) on our sun waxes and wanes over a period of about 11 years. About 90 years ago, scientists learned that there's a butterfly-shaped pattern to this cycle. Now they are trying to learn what drives that pattern.

Understanding what generates the sunspot pattern may allow scientists to provide better forecasts of solar storms, which can cause power outages and disrupt satellite communications on Earth.

But first, what are sunspots? What's the sunspot cycle? And what's this pattern?

Sunspots are thought to result from a shifting magnetic field inside the sun, explains Aimee Norton, a solar astronomer with the High Altitude Observatory at the National Center for Atmospheric Research in Boulder, Colorado.

The number of sunspots fluctuates over time, reaching a peak every 11 years. This 11-year pattern is known as the sunspot cycle and was discovered in 1843 by German astronomer Samuel Heinrich.

Not only does the number of sunspots fluctuate over the 11-year period, but so too do their locations, Norton said. Over the period, the sunspots migrate from about 35 degrees north and south latitude toward the sun's equator.

In 1904 English astronomer Edward Maunder noticed an artful pattern to the cycle.

When the latitude and time of sunspots from an entire cycle are plotted on a map, the migration of sunspots toward the equator looks like two wings of a butterfly. Several cycles plotted together look like a trail of butterflies.

Solar Dynamo

Scientists are now trying to understand why the sunspot belt moves toward the equator over the course of the 11-year cycle. To understand this, Norton said, requires understanding the so-called solar dynamo.

"This is one of the major mysteries in solar physics," she said. "The dynamo is a process by which the mechanical motions on and in the sun are converted into magnetic energy."

Since sunspots are believed to be regions of intense magnetic field and since they increase and decrease over an 11-year cycle, scientists believe that the sun's magnetic field must also increase and decrease in time.

"The cyclical nature of the sunspot cycle is strong evidence that the magnetic field within the sun is being regenerated during this cycle," Norton said.

Generated by the flow of hot gases, the sun's electric currents in turn generate magnetic fields.

Norton and her colleagues are building computer models of the various flows on and in the sun to help them understand the solar dynamo. This should, in turn, explain the reason for the sunspot migration pattern.

"Some details of the migration pattern as observed in spot behavior is beyond the current capability of dynamo models to produce, but it may be possible with more elaborate models now under development," said Peter Gilman, a colleague of Norton's at the High Altitude Observatory.

Best Theory

Gilman said there is no scientific consensus on why sunspot-migration diagrams take the shapes of butterflies. A leading theory is based on computer modeling by colleague Gilman's colleague Mausumi Dikpati.

Dikpati's models link the migration to a current of plasma called the meridional flow, which circulates between the sun's equator and its poles. It's all part of a process called the Hale cycle.

The flow is like a system of two conveyor belts, one in the northern hemisphere and one in the southern hemisphere. Each belt travels along the surface of the sun, from the equator to the pole (north or south, depending on the hemisphere). At its pole, each belt turns the corner, diving into the sun's interior.

The flow makes its return trip to the equator through the convection zone, the outermost layer of the sun's interior. As the belt approaches the equator, it turns and follows a path toward the sun's surface, and the cycle begins again.

A single Hale cycle takes about 22 years, or two sunspot cycles. The thinking is that the two halves of the "conveyor belt" have similar sunspot patterns on them, which is why sunspot activity follows an 11-year cycle—half a Hale cycle.

According to Dikpati's theory, sunspots leave an imprint on the surface flow. This imprint is carried into the interior, where scientists believe the sunspot-producing magnetic fields are generated. New sunspots form based on the imprints created during the most recent cycle.

By understanding the variation of the meridional flow's speed and the sun's past sunspot cycles, Dikpati and colleagues believe they may be able to forecast the timing and intensity of sunspot activity—and therefore of solar storms.

"In fact, in a very recent work, we are predicting the onset of the next cycle—cycle 24—will be late, because the meridional flow slowed down in the current cycle," Dikpati said.

According to the forecast, the next solar cycle will begin in 2007 to 2008. That means that cycle 24 would begin about a half year late, or about 11 years and six months after the beginning of cycle 23.

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