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The Stellar Dynamo Sunspot cycles—on other stars—are helping astronomers study the sun’s variations and the ways they might affect Earth By Elizabeth Nesme-Ribes, Sallie L. Baliunas and Dmitry Sokoloff MAGNETIC FIELDS on the sun are rendered visible in this x-ray photograph by the curving contours of solar flares. The lines of magnetic fields erupt from the sun’s surface and heat the gases of the surrounding corona to up to 25 million degrees C, causing them to glow. Flares are more frequent during sunspot maxima. In 1801, musing on the vagaries of English weather, astronomer William Herschel observed that the price of wheat correlated with the disappearance of sunspots. But the pattern soon vanished, joining what scientists at large took to be the mythology connecting earthly events with solar ones. That the sun’s brightness might possibly vary, and thereby affect Earth’s weather, remained speculative. Thus, in the mid-1980s, when three solar satellites—Solar Maximum Mission, Nimbus 7 and Earth Radiation Budget—reported that the sun’s radiance was declining, astronomers assumed that all three instruments were failing. But the readings then perked up in unison, an occurrence that could not be attributed to chance. The sun was cooling off and heating up; furthermore, the variation was connected with the number of spots on its face. In recent years one of us (Baliunas) has observed that other stars undergo rhythmic changes much like those of our sun. Such studies are helping refine our understanding of the “dynamo” that drives the sun and other stars. Moreover, they have revealed a strong link between “star spots” and luminosity, confirming the patterns discovered in our sun. And yet astrophysicists, including the three of us, are still debating the significance of the sun’s cycles and the extent to which they might influence Earth’s climate. Updated from the August 1996 issue 35 COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.
NATIONAL SOLAR OBSERVATORY/SACRAMENTO PEAK; NATIONAL SOLAR OBSERVATORIES/KITT PEAK (inset) SUNSPOTS are relatively cool regions formed where magnetic fields emerge from the sun, thereby suppressing the upwelling of hot gases from the interior. Elsewhere on the surface, tightly coiled cells of cyclonically flowing gases show up as granules. Near a sunspot the Sunspots THE EARLIEST KNOWN sunspot records are Chinese documents that go back 2,000 years, preserving observations made by the naked eye. From 1609 to 1611 Johannes Fabricius, Thomas Harriot, Christoph Scheiner and Galileo Galilei, among others, began telescopic studies of sunspots. These records, as German astronomer Samuel Heinrich Schwabe announced in 1843, displayed a prominent periodicity of roughly 10 years in the number of observed sunspot groups. By the 20th century George magnetic fields organize the gaseous flow into lines resembling iron filings near a bar magnet. The magnetogram (inset) shows field lines emerging at one sunspot (yellow) and reentering at another (blue); such sunspot pairs are common. Ellery Hale of the Mount Wilson Observatory in California found those dark surface irregularities to be the seat of intense magnetic fields, with strengths of several thousand gauss. (Earth’s magnetic field is, on the average, half a gauss.) Sunspots appear dark because they Sunspot Number Maunder minimum 1620 1640 1660 1680 1700 1720 1740 1760 1780 1800 1820 1840 1860 1880 MAXIMUM MINIMUM ELEVEN-YEAR CYCLES of sunspot activity were interrupted between 1645 and 1715 by a period of quiescence. This dearth of sunspots, called the Maunder minimum, coincided with unusually cool temperatures across northern Europe, indicating that solar fluctuations influence Earth’s climate. The regular pulsing of the sun’s activity (right) was observed over one cycle at the Paris Observatory. These photographs were taken in violet light emitted by ionized calcium; the technique that produced them is now used to study the magnetic activity of other stars. 1979 1982 1986 36 SCIENTIFIC AMERICAN THE SECRET LIVES OF STARS COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.
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Page 5 and 6: a b c North Pole North Pole North P
Page 7 and 8: Intensity of Calcium Emission Lines

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