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Дата изменения: Unknown Дата индексирования: Sat Dec 22 09:26:54 2007 Кодировка: Поисковые слова: earth's atmosphere |
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11. The Sun's Magnetic Cycle |
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What were sunspots? Galileo had guessed they were clouds floating in the Sun's atmosphere, obscuring some of its light. Their true nature only emerged in 1908 when George Elery Hale, leader among US astronomers, showed that they were intensely magnetic. Their magnetic field was as strong as that of a small iron magnet, some 3000 times stronger than the field near the surface of the Earth--yet those fields often extended over areas larger than the entire surface of the Earth. Apparently the magnetic field somehow slowed down the flow of heat from the Sun's interior, causing the sunspots to be slightly darker than the rest of the Sun.
The method was later improved by Babcock and others, allowing astronomers to observe not only the magnetic field of sunspots but also the weak fields near the Sun's poles. It turned out that the Sun has a polar field somewhat like the Earth's, but that it reverses its polarity during each 11-year cycle. Sunspots have also led us to a better understanding of the Earth's own magnetic field. The face of the Sun consists of ionized hot gas ("plasma"), hot enough to conduct electricity. Sunspot fields were evidently produced by electric currents, and it was well known that such currents could be generated by a "dynamo process," by the motion of an electric conductor (e.g. the flow of solar plasma) through a magnetic field. In 1919 Sir Joseph Larmor proposed that the fields of sunspots were due to such dynamo currents. He suggested that a closed chain of cause-and-effect existed, in which the field created by these currents was also the field which made them possible, the field in which the plasma's motion generated the required currents. Many features of sunspots remain a mystery, but Larmor's idea opened an era of new understanding of magnetic processes in the Earth's core.
Click here for Schwabe's original article in which he first reported his observations of a sunspot cycle. Solar Activity and Geomagnetic Storms: The First 40 Years E.W. Cliver, Eos, 6 December 1994
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