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Solar news: convection and magnetism.

Earthlings tend to watch the sun very closely. It is, after all, the ultimate source of nearly all our energy, and what we see there may have a profound effect on us. One way by which energy reaches the surface of the sun from the interior is convection. Astrophysicists have suggested that large amounts of matter are circulating up and down in what they call the sun's convection layer. Recent observations that seem to show the existence of convection cells within such a layer were reported at last week's meeting in Tucson, Ariz., of the Solar Physics Division of the American Astronomical Society. A more surprising development reported at the same meeting is that the sun emits bubbles of magnetism that float off into space.

According to the generally accepted theory of solar dynamics, the sun's convection layer is heated from below by energy produced in thermonuclear fusion reactions. The heated gas rises to the surface, where it is cooled by radiation and then descends again. Such a motion should occur in large cells or pieces of the convection zone's volume. Observers have looked for such convection cells, but according to Philip Scherrer, senior research associate at Stanford University's Center for Space Science and Astrophysics (CSSA), they have not been found before now. Scherrer, with Hirokazu Yoshimura of the University of Tokyo and two research associates at the Stanford CSSA, Richard S. Bogart and J. Todd Hoeksema, used Stanford's Wilcox Solar Observatory to find evidence for convection: gas currents moving east and west across the surface of the sun.

These currents move at about 20 meters per second. From earth they are seen added to or subtracted from the sun's surface rotation, which is westward at 2,000 meters per second. The motions of the currents can be compared to plate tectonics on the earth, except that the solar motions take place in an ionized gas and so are much faster than those in the solid earth. Bogart cautions that the observations are not absolute proof of the existence of convection cells, but they are the sort of surface motion that convection cells ought to show.

Motions in the body of the sun generate magnetic fields. The matter in the sun is an ionized gas, and motions in it constitute electric currents, which generate magnetic fields. Yet the interplanetary magnetic field, which originates in the sun, does not reflect this activity. The magnetic field in interplanetary space, according to a number of measurements by spacecraft, remains constant and in fact fairly weak.

The solution to the question of where the magnetism goes, according to Rainer Illing and Arthur Hundhausen of the National Center for Atmospheric Research in Boulder, Colo., is that the sun emits self-contained bubbles of magnetism that float off into space without affecting the overall strength of the interplanetary field. They find the evidence for this in pictures of the solar corona made by the Solar Maximum Mission satellite since it was repaired by a space shuttle crew a year ago.

Occasionally, active regions on the solar surface erupt in large arcs of magnetized matter. Sometimes, says Illing, something pinches these arcs from the sides. Tension of forces in them then causes them to snap like rubber bands and the result is two arcs, the lower one with its ends rooted in the sun's surface, the upper with its ends pointing outward. The upper arc then detaches itself and becomes a magnetic bubble. The earth may occasionally pass through one of these bubbles, Illing says. He is not sure what effect that would have, but suggests that it might cause some disturbance of the magnetosphere.
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Author:Thomsen, D.E.
Publication:Science News
Date:May 25, 1985
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