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While astronomers who study the sun get more attention during periods of solar maximums, they generally feel somewhat neglected, underfunded and unappreciated, poor cousins to those who observe distant stars and galaxies in the night skies and who consider the sun boring. Then why do solar astronomers persist? "We are driven to an understanding of the sun," says Robert Howard, an astronomer at the National Solar Observatory in Tucson. "It is an enormous lab. It is a Rosetta stone for the study of the stars. With other stars, all you have is a pinpoint of light. By understanding more about the sun, we can learn more about the distant stars."
While all that may be true, says Caltech physicist Robert Leighton, "if the sun didn't have a magnetic field, it would be as dull as most nighttime astronomers think it is." What a difference a field makes. Twisted and stretched by both the sun's rotation and its roiling interior, the magnetic lines of force orchestrate the intriguing solar cycle.
Most explanations of that phenomenon liken the sun to a dynamo. Mighty currents of electricity flowing in the solar interior generate magnetic-field lines that, like the earth's, tend to be oriented in a north-south direction. But because the sun, unlike the earth, is gaseous, it does not rotate uniformly: bands of gases around the equator circle the solar axis once every 27 days, compared with a 34-day rotation rate near the poles.
At the same time, hot gases, being lighter, rise from the interior to the surface, while cooler, heavier gases descend -- a process called convection (similar to what occurs in a hot oven). As a result of these massive convection currents and the differing rates of solar rotation, the magnetic lines of force begin wrapping around the sun like ropes. The wrapping action stretches the ropes and creates magnetic fields so strong that they repel the surrounding solar gases. In effect, this makes the magnetic regions lighter than the gases, and they begin to rise. Some reach the surface and become sunspots, dark because they are cooler than surrounding incandescent gases.
The darker central portions of sunspots, or umbras, have the strongest magnetic fields; the lighter exteriors, or penumbras, the weaker fields. Occasionally, the penumbras of two sunspots of opposite polarity merge as they move past each other, putting the oppositely charged umbras in contact. The results are spectacular. "Because the umbras have opposite polarities, they attract each other," says the Marshall Center's Moore. "The closer they are together, the stronger the pull. Then, as they push past each other, it's like an earthquake fault slipping. In this case the stored energy is released in a flare." In the sunspot group that produced the flares of March, he notes, spots of opposite polarity were close together.