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Article 1 of the citation list.

July 4, 2000, Tuesday    Late Edition - Final
Section: F    Page: 3    Column: 1     Desk: Science Desk    Length: 1149 words

Satellite Observes Solar Flares' Snap, Crackle and Pop


Like the painter Winslow Homer, who dismissed the North Atlantic as ''a duck pond'' when it was not blowing a good storm, solar physicists find the sun most fascinating when its atmosphere is erupting, flaring and flinging energetic particles into space.

The physicists believe that the energy for those outbursts somehow comes from the strong magnetic fields that are generated deep inside the sun and emerge from its surface like wild hair.

Much less clear is exactly how the energy flows from the tangle of field lines to the solar equivalent of a northeaster.

By capturing images of the sun that are as beautiful as they are revealing, the Yohkoh satellite, a collaboration between the Institute of Space and Astronautical Science in Japan and the National Aeronautics and Space Administration in the United States, has moved scientists a step closer to pinpointing the precise mechanism for that release.

Yohkoh, which observes hot, X-ray-emitting gas that is trapped by the magnetic field lines, has seen the first evidence that the lines snap back toward the solar surface like rubber bands after being stretched out during great eruptions of solar material into space.

The observation supports the theory that solar flares accompanying those eruptions are powered by sudden breakages in the field lines, called reconnection events, that generate tremendous electrical currents in the gas and heat it to high temperatures.

Scientists had often seen the stretching, but because the much more rapid recoil had eluded them, they had begun to question their model of how flares -- the most violent events in the solar system -- are set loose.

Evidence for the recoil turned up when a solar physicist, Dr. David McKenzie of Montana State University, analyzed Yohkoh images of 17 cases in which solar eruptions were followed by flares.

Dr. McKenzie presented the results in late June at a meeting of the solar physics division of the American Astronomical Society in Lake Tahoe, Nev.

''It's something we all thought was happening, but it's something we had never seen before,'' said Dr. Eric Priest, a solar physicist and applied mathematician at the University of St. Andrews in Scotland. ''In fact, the absence of evidence was really casting some doubt on it.''

Referring to the theory that magnetic reconnections drive flares, Dr. Priest said the new analysis was ''a beautiful piece of supporting evidence for that basic process.''

The sun's luminous output is powered by nuclear fusion reactions in its central core. Because the sun's interior, and its atmosphere, are heated to such high temperatures by the same energy source, the electrons that normally surround gas atoms have been knocked loose and roam freely, creating an electrified gas called a plasma.

Unlike ordinary gas, plasmas and magnetic field lines interact in such a way that they tend to remain stuck together. As the sun spins on its axis, this interaction within the roiling solar interior often generates twisted bundles of strong magnetic fields that bob to the surface like corks in a lake. The fields can keep rising until they protrude from the surface and form a variety of structures resembling cowlicks, arches or tethers, with one end anchored on the sun and the other drifting into space.

One sprawling type of structure, called filaments because of their dark, stringy appearance against the face of the sun, can support billions of tons of relatively cool plasma above the surface of the sun, like giant hammocks. It is the sudden eruption of the filaments, heaving the plasma into space and stretching the field lines beneath, that begins the train of events studied by Dr. McKenzie.

''The analogy that I can think of is a hot air balloon taking off and stretching its tethers up like an elastic band,'' Dr. McKenzie said.

Exactly what sets off the eruptions, called coronal mass ejections, is not known, but physicists suspect that instabilities in the filament's twisted magnetic field lines may be the cause. Whatever the reason, the plasma rises up, stretching its magnetic tethers to the breaking point.

Magnetic field lines, unlike normal tethers, cannot have exposed ends. So once one breaks free, Dr. McKenzie said, ''some of the magnetic field will reconnect or rearrange its connection underneath the erupting coronal mass ejection.'' In effect, the ends of broken tethers splice together, or reconnect.

The powerful electrical currents driven by that reconnection are thought to heat the plasma violently, leading to the spectacular displays that come next: solar flares, which consist of explosions of energetic particles and high-energy radiation like X-rays.

But the reconnection should also let the tethers snap back toward the surface of the sun, an effect that had never been seen. So Dr. McKenzie examined a series of pictures of each event as taken by Yohkoh using the same X-rays the flares generated. Since magnetic structures would be revealed by some of the hot plasma still sticking to them, the recoil should be evident in the pictures, taken 15 to 20 seconds apart.

And they were. Dr. McKenzie turned up both light and dark striations moving back toward the sun, though somewhat more slowly than theory had predicted.

''This is immeasurably helpful in understanding flares and coronal mass ejections,'' said Dr. Hugh Hudson, of the Solar Physics Research Corporation in Tucson, who collaborated with Dr. McKenzie in an early phase of the work last year. ''It is the clearest confirmation yet that the long-suspected process of large-scale magnetic reconnection really does work.''

Other physicists, including Dr. McKenzie, said that because the reconnection had not been seen directly, the case was far from settled.

''There are so many compelling pieces of evidence that reconnection may be underlying flares and coronal mass ejections that it's a plausible connection,'' said Dr. Amitava Bhattacharjee of the University of Iowa. ''But not a watertight case yet, in my mind.''

Images: Photo: (Photograph by Institute of Space and Astronautical Science in Japan/NASA; The New York Times)

Chart: ''Seeing The Sun's Magnetic Fields''
An ACTIVE REGION contains strong magnetic fields spread out over large areas in the outer atmosphere, or corona.

A cylindrical ARCADE OF LOOPS occurs when solar flares or coronal mass ejections release matter and energy from the sun's surface.

A CORONAL HOLE, a dark region of the corona, appears empty, though magnetic fields stream from it, into space.

Active regions can be joined by INTERCONNECTING LOOPS, which become visible as they fill with high- temperature, ionized gas.

A large-scale ISOLATED LOOP traps hot gas at high pressures. Such loops may be the building blocks of the sun's corona.

Copyright 2000 The New York Times Company

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