Sunspot Cycles: Deciphering the Butterfly Pattern

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"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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