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National Geographic : 1974 Oct
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ALL FROMAMERICANSCIENCEANDENGINEERINGANDNASA X-RAYS OUTLINE the sun's magnetic fields in a dramatic series of pictures taken during the second mission. Large bright areas-coronal gases at temperatures above two million degrees F. -move along magnetic loops and arches, or lines of force, suggesting a three-dimensional picture of the magnetic fields. The dark zone, running from the north pole (upper left in these tilted images) to below the equator, repre sents a "hole" in the corona. Such holes, with relatively low temperatures and nearly vertical magnetic fields, may be major sources of high-speed streams of the solar wind that blows outward toward the planets. Rotating with the sun, the hole travels halfway across the disk in this sequence, shown in numerical order on August 19, 1973, August 21, August 23, and finally on August 25. The photographs also reveal the sun's numerous "bright points," whose abundance was unsuspected before Skylab. About the same size as the earth, some 1,500 bright points emerge each day, each lasting ap proximately eight hours. Ground-based in struments show that they carry unexpected amounts of magnetic flux, possibly con tributing to the solar cycle in which the sun's magnetic field reverses every 11 years. 497
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