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Professor predicts 'average' peak for sun cycle
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The sun's magnetism, manifest by sunspots, can seriously disrupt satellite communication, telephones, power grids and space flights. Yet despite wild predictions about the sun on the Internet, ranging from misconceptions about coronal mass ejections to suggestions of a solar megacycle, the sunspot cycle we're currently in&emdash;called Cycle 23&emdash;will be average in strength, states Herschel Snodgrass, professor of physics at Lewis & Clark College. "Cycle 23 is reaching its maximum earlier than expected, but it is not a strong maximum," he says. Moreover, torsional oscillations may be the key to making reliable predictions of upcoming solar cycles, Snodgrass announced at a press conference, hosted by the American Geophysical Union, in San Francisco, Dec. 16. Snodgrass, who was recently interviewed by National Geographic, is an expert on the solar magnetic cycle and torsional oscillations. Torsional oscillations are jet stream-like patterns that appear to link the polar fields of one cycle to the sunspot fields of the next. Some observers, Snodgrass notes, find evidence that Cycle 23 has already peaked, ahead of schedule. But most indicators suggest it will peak in mid-2000. Cycle 19, the strongest cycle on record, peaked around 1960, he notes. Cycle 20 was average and Cycles 21 and 22 were both stronger than average. "We expected to see the torsional oscillations begin in 1992, but instead, the pattern didn't begin until late 1995," Snodgrass comments. "Its late and weak start suggests that Cycle 23 will be definitely weaker than Cycles 21 and 22. "That does not mean that there will not be significant flares and so forth," he said. "There always are, especially during the declining phase of the cycle when the active regions become large. But at present, it looks as though the next 1,000 years will dawn under the sun's rather gently smiling face." Torsional Oscillations Detailed mapping of the sun's surface motions and magnetic fields shows connections between patterns in the magnetic field and patterns of large-scale motion of ionized solar gases, called plasma, according to Snodgrass. The sun rotates on its axis as does the Earth, but unlike the Earth, the sun does not rotate rigidly, he explains. The poles take more time to make a full revolution than does the equator. That differential rotation is possible because the sun is not solid but gaseous throughout. Scientists believe that differential rotation functions as a dynamo that twists and intensifies the magnetic fields. In 1980, scientists at Mount Wilson Observatory discovered a weak but large-scale fluctuation in that differential rotation and dubbed it torsional oscillations. "The fluctuation looks like a wave in which the rotation is slower than normal in one zone of latitudes and faster than normal in the adjacent zone closer to the equator," Snodgrass says. "It is as if the sun had, in each hemisphere, two jet stream-like winds blowing in opposite directions. The higher latitude band flows in the direction opposite the sun's rotation, and the lower latitude band flows in the same direction as the rotation," he says. "The remarkable thing about this pattern of motion," Snodgrass points out, "is that it migrates in latitude in a way that is linked with solar activity. It begins in high latitudes, following the reversal of the polar fields, and moves gradually toward the equator and continues to be present through solar minimum. When the sunspots appear, they do so along the path of this migration, and the bands of flow persist to the end of the cycle. "That suggests that the torsional flow forms a link between the polar field reversal of one cycle and the lower latitude activity of the following cycle," he continues. "Based on evidence found in 1998 by an orbiting solar observatory, we now know that the flow extends deeply beneath the sun's surface and, therefore, that it may channel the fields into the equatorward-migrating zone where they intensify." Sunspots and the Solar Activity Cycle The sun's magnetism undergoes a fascinating sequence of changes that takes 20 to 24 years to go full cycle. The activity of the cycle is maximum when the number of sunspots is the greatest, and it is minimum when the number is the least, usually near zero. For every full 20- to 24-year cycle, there are two 10- to 12-year sunspot cycles, Snodgrass explains. At minimum, the sun's magnetic field is similar to the Earth's, with magnetism concentrated toward the poles. As the cycle progresses, the number of sunspots increases. Individual spots last from a few days to about a year. At first, sunspots appear in each hemisphere at mid-latitudes. But as time progresses, they appear closer to the equator, and the sun's magnetic field becomes more concentrated in these spots than at the poles. After about four years, the activity cycle reaches its maximum, and a year or so later, the fields at the poles reverse sign: what was a north magnetic pole becomes a south magnetic pole, and vice versa. That begins the declining phase: the spots become larger but fewer in number, and in the course of five to seven more years, the cycle of activity reaches its next minimum. That minimum is like the previous one, except the fields at the poles now have the opposite signs.
'At present, it looks as though the next 1,000 years will dawn under the sun's rather gently smiling face.' The second half of full cycle is just like the first half except the magnetic fields have reversed polarity. The cycle ends with another minimum in which the polar fields have gone back to the same polarity they started with. Thus, for every full 20- to 24-year cycle, in which everything returns to its initial state, there are two 10- to 12-year sunspot cycles. Sunspots have been coming and going in these 10- to 12-year cyclic patterns since the early 18th century. Prior to that, there was a 75-year period, during which the cycle was dormant. During this period, known as the Maunder, Earth underwent a little ice age. "We now know that when spots are not present on the sun, its surface is slightly cooler overall," Snodgrass says. "As we unravel the mechanism of the solar cycle, we will also come to understand such extended periods of minimum." |
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