Sunspot Cycles: Deciphering the Butterfly Pattern

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.

Continued on Next Page >>




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