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SOHO craft gets the lowdown on sunspots

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10:21am, November 15, 2001
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Sunspots have fascinated scientists ever since Galileo sketched these dark, Earth-size blemishes in 1611 and shattered the notion that the sun is a divine sphere devoid of flaws. Now known as sites of intense magnetic activity that can hurl flares and belch clouds of ionized gas toward Earth, sunspots have puzzled astrophysicists for decades.

It's easy to see why they've been perplexed. Because the bundles of magnetic fields that cluster in a sunspot all point in the same direction, they should repel each other–just as bar magnets do when their like poles are brought together. Yet somehow, instead of flying apart, the fields of a sunspot stay bundled, enabling sunspots to last for days to weeks.

By using sound waves to obtain the first clear picture of the structure beneath the surface of a sunspot, scientists now have a solution to the puzzle, they say. Beneath the surface lies a planet-size hurricane that pulls in ionized gas. The inflow of gas acts like a collar, keeping the magnetic fields together and the sunspot intact.

Alexander G. Kosovichev of Stanford University in Palo Alto, Calif., reported the findings last week at a press conference in Washington, D.C. He and his colleagues Junwei Zhao of Stanford and Thomas L. Duvall Jr. of NASA's Goddard Space Flight Center in Greenbelt, Md., also had detailed some of their work in the Aug. 10 Astrophysical Journal.

To probe the inside of a sunspot, the team used an instrument aboard the SOHO (Solar and Heliospheric Observatory) spacecraft that detects ripples on the sun's surface. The ripples are caused by low-frequency sound waves generated by roiling gases inside the solar cauldron (SN: 3/18/00, p. 183). The sound waves reveal interior structures because the waves travel faster through regions with higher temperatures and stronger magnetic fields.

Kosovichev and his collaborators used the SOHO detector to study a sunspot in June 1998. Measuring the speed of sound waves in the solar orb, they produced a map of the region extending from the sunspot's surface to some 16,000 kilometers below it.

The surface of a sunspot is cooler and darker than adjacent regions because its bundled magnetic fields act like a plug, capping the flow of heat from the solar interior. The team's analysis reveals that sound waves near the surface of the June sunspot move 10 percent slower than their average speed for the sun's surface. This indicates a lower temperature than the surrounding surface.

Sunspots keep their cool, but it's only skin deep. The study shows that at a shallow depth–less than 1 percent of the distance to the sun's core–the sound waves speed up significantly. This implies that beneath their surface, sunspots are in fact hotter than their surroundings.

The data also indicate that above the shallow magnetic plug, gas cools and grows denser, and it sinks rapidly. This leaves a void, which is filled by hotter gas and associated magnetic fields sucked toward the sunspot from nearby areas. This vortex increases pressure on the sunspot, preventing its magnetic field from dispersing. As a result, the sunspots stay intact, notes Kosovichev.

Moreover, the additional magnetic fields from the inflowing gas strengthen the plug. This prompts further cooling and sinking of gas from the surface and perpetuates the process that maintains sunspots, the team says.

"If confirmed, the reported results represent a major step forward in our understanding of sunspots," says Robert Rosner of the University of Chicago.

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