The Stellar Dynamo - Scientific American Digital

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SUNSPOTS ARE 2,000 DEGREES C COOLER than the surrounding surface; they would GLOW ORANGE-RED AGAINST A NIGHT SKY. are 2,000 degrees Celsius cooler than the surrounding surface of the sun; they would glow orange-red if seen against the night sky. The spots form when strong magnetic fields suppress the flow of the surrounding gases, preventing them from carrying internal heat to the surface. Next to the sunspots are often seen bright areas called plages (after the French word for “beach”). The magnetic field lines tend to emerge from the surface at one spot to reenter the sun at another, linking the spots into pairs that resemble the two poles of a bar magnet that is oriented roughly east-west. At the start of each 11-year cycle, sunspots first appear at around 40 degrees latitude in both hemispheres; they form closer to the equator as the cycle progresses. At sunspot minimum, patches of intense magnetism, called active regions, are seen near the equator. Aside from the sunspots, astronomers have observed that the geographic poles of the sun have weak overall magnetic fields of a few gauss. This large-scale field has a “dipole” configuration, resembling the field of a bar magnet. The leading sunspot in a pair—the one that first comes into view as the sun rotates from west to east—has the same polarity as the pole of its hemisphere; the trailing sunspot has 1920 1940 1960 1980 2000 MAXIMUM 1988 1990 PARIS OBSERVATORY Sunspot Number NOAA; PETER SAMEK Slim Films the opposite polarity. Moreover, as Hale and Seth B. Nicholson had discovered by 1925, the polarity patterns reverse every 11 years, so that the total magnetic cycle takes 22 years to complete. But the sun’s behavior has not always been so regular. In 1667, when the Paris Observatory was founded, astronomers there began systematic observations of the sun, logging more than 8,000 days of observation over the next 70 years. These records showed very little sunspot activity. This important finding did not raise much interest until the sunspot cycle was discovered, prompting Rudolf Wolf of Zürich Observatory to scrutinize the records. Although he rediscovered the sunspot lull, Wolf’s finding was criticized on the grounds that he did not use all the available documents. During the late 1880s, first Gustav F. W. Spörer and then E. Walter Maunder reported that the 17th-century solar anomaly coincided with a cold spell in Europe. That astonishing observation lay neglected for almost a century, with many astronomers assuming that their predecessors had not been competent enough to count sunspots. It was only in 1976 that John A. Eddy of the High Altitude Observatory in Boulder, Colo., reopened the debate by examining the Paris archives and establishing the validity of what came to be known as the Maunder minimum. Minze Stuiver, while at Yale University, Hans Suess of the University of California at San Diego and others had discovered that the amount of carbon 14 in tree rings increased during the dearth of sunspots. This radioactive element is created when galactic cosmic rays transmute nitrogen in the upper atmosphere. Their findings suggested that when the magnetic fields in the solar wind—the blast of particles and energy that flows from the sun—are strong, they shield Earth from cosmic rays, so that less carbon 14 forms; the presence of excess carbon 14 indicated a low level of magnetic activity on the sun during the Maunder phase. Eddy thus reinforced the connection between the paucity of sunspots and a lull in solar activity. Aside from the rarity of sunspots during the Maunder minimum, the Paris archives brought to light another oddity: from 1661 to 1705, the few sunspots that astronomers sighted were usually in the southern hemisphere. They were also traveling much more slowly across the sun’s face than present-day sunspots do. Only at the beginning of the 18th century did the sun assume its modern appearance, having an abundance of sunspots rather evenly distributed between the two hemispheres. The Solar Dynamo THE MAGNETIC ACTIVITY of the sun is believed to reside in its convective zone, the outer 200,000 kilometers where churning hot gases bring up energy from the interior. The fluid forms furious whorls of widely different sizes: the best known is an array of convective cells or granules, each 1,000 kilometers across at the surface but lasting only a few minutes. There are also “supergranules” that are 30,000 to 50,000 kilometers across and even larger flows. Rotation gives rise to Coriolis forces that make the whorls flow counterclockwise in the northern hemisphere (if one is looking down at the surface) and clockwise in the southern hemisphere; these directions are called cyclonic. Whether similar cyclones exist underneath the surface is not known. Deep within, the convective zone gives way to the radiative zone, where the energy is transported by radiation. The core of the sun, where hydrogen fuses into helium to fuel all the sun’s activity, seems to rotate rigidly and slowly compared with the surface. www.sciam.com SCIENTIFIC AMERICAN 37 COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.
a b c North Pole North Pole North Pole SOLAR DYNAMO generates the sun’s magnetic field and also causes it to change orientation every 11 years. Suppose that the initial magnetic field (a) resembles that of a bar magnet with its north pole (+) near the sun’s geographic north pole. The magnetic field lines are carried along with the electrically charged gases. The faster flow at the equator therefore distorts the field lines (b) until they wrap tightly (c) around the sun. But the field lines then resist the stretching and unwind, QUADRUPOLE DIPOLE The first description of how the sun’s gases conspire to create a magnetic field was proposed in 1955 by Eugene N. Parker of the University of Chicago. Because of the high temperature, the atoms of hydrogen and helium lose their electrons, thereby giving rise to an electrically charged substance, or plasma. As the charged particles move, they generate magnetic fields. Recall that the lines describing magnetic fields form continuous loops, having no beginning or end—their density (how closely together the lines are packed) indicates the intensity of the magnetic field, whereas their orientation reveals the direction. Because plasma conducts electricity very efficiently, it tends to trap the field lines: if the lines were to move through the plasma, they would generate a large, and energetically expensive, electric current. Thus, the magnetic fields are carried along with the plasma and end up getting twisted. The entwined ropes wrap together fields of opposite polarity, which tend to cancel each other. But the sun’s rotation generates organizational forces that periodically sort out the tangles and create an overall magnetic field. This automatic engine, which generates magnetism from the flow of electricity, is the solar dynamo. The dynamo has two essential ingredients: the convective cyclones and the sun’s nonuniform rotation. During the mid-1800s, Richard C. Carrington, an English amateur astronomer, found that the sunspots near the equator rotate faster, by 2 percent, than those at midlatitudes. Because the spots are floating with the plasma, the finding indicates that the sun’s surface rotates at varying speeds. The rotation period is roughly 25 days at the equator, 28 days at a latitude of 45 degrees and still longer at higher latitudes. This differential rotation should extend all the way through the convective zone. Now suppose that the initial shape of the sun’s field is that of a dipole oriented roughly north-south. The field lines get pulled forward at the equator by the faster rotation and are deformed in the east-west direction. Ultimately, they lie parallel to the equator and float to the surface, erupting as pairs of sunspots. But Coriolis forces tend to align the cyclones and thereby the sunspots, which are constrained to follow the plasma’s gyrations. The cyclones arrange the sunspots so that, for example, a trailing sunspot in the northern hemisphere lies at a slightly higher latitude than a leading one. As the equatorial field lines are stretched, they eventually unwind and drift outward. The trailing sunspot reaches the pole first, effectively reversing the magnetic field there. (Recall that the trailing spot has a polarity opposite that of the nearest pole.) Those field lines that initially extended far beyond the sun reconnect into loops and are blown away by the solar wind. In this manner, the overall magnetic field flips, and the cycle begins again. There is, however, a caveat. This simple picture seems to be at odds with results from helioseismology, the science of sunquakes. The model requires the sun to rotate faster at the interior; in contrast, results from the Global Oscillation Network Group, an international collaboration of observatories, show that the rotation velocity near the equator decreases downward. Such experiments are providing accurate information on internal motions of the sun and thereby help- PETER SAMEK Slim Films 38 SCIENTIFIC AMERICAN THE SECRET LIVES OF STARS COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.
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Page 7 and 8: Intensity of Calcium Emission Lines

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