Sun's burning question answered

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IT'S A question with which you could, for more than 50 years, shut up any physicist who was bothering you at a party. Why, exactly, is the sun's atmosphere hotter than its surface?

After all, the source of the heat from the fusion reactions at its core should be carried outwards. Yet the surface temperature is about 6,000C, while that in the atmosphere is several million degrees. Observers could find no explanation.

But now the puzzle seems to have been solved. New data from an orbiting satellite called Yohkoh - Japanese for "sunbeam" - has shown that a clash of magnetic fields is probably the cause of the super-hot conditions above the sun's surface. Or, it works like an electric fire.

The outer part of the sun's atmosphere consists of huge, intensely hot loops of material that arch above the surface.

A team, led by Professor Eric Priest, of St Andrew's University, Scotland, has for the first time been able to measure how the temperature varies along these giant loops, using an X-ray telescope on the satellite, jointly funded by Britain, Japan and the United States. The observations were compared with predictions from theoretical models.

"Some felt that the heat should be dumped at the feet of the loop and then conducted, like the flow of heat along a red-hot poker, to the rest of the loop. Others felt that the heat should be deposited at the summit of the loop, while a third camp predicted a uniform release of heat along the loop," said Professor Priest, whose results were published in the journal Nature today.

The observations showed clearly that the heat was deposited uniformly, just as the element of an electric fire is uniformly hot. The likeliest explanation was a clash of magnetic field lines, which would tangle in the solar atmosphere and then break. As they snapped, they caused dozens of explosions in tiny regions of intense electric current. This released energy across the loop, heating the atmosphere and producing the difference in temperature.