Fire in the Sky

Supercomputers and new observational technology are helping astronomers unlock the sun's most stubborn secrets.

By Tim Folger|Saturday, August 01, 1992
Leaping 100,000 miles or more above the surface of the sun, solar prominences look like enormous tongues of flame. Prominences, however, have little in common with earthly fire. The hot, electrically charged hydrogen gas that makes up a prominence can hover over the sun for weeks or months at a time, supported by powerful magnetic fields looping out of the solar surface. But beyond these few facts, astronomers know surprisingly little about how prominences form, why they persist, and why they then suddenly disappear.

Most of the models people have made of solar prominences have been too simplistic, says Michael Raadu of the Royal Institute of Technology in Stockholm. Raadu and his colleague Brigitte Schmieder at the Observatory of Paris have been investigating prominences for several years. They are particularly interested in the velocities and densities of the gas in the prominences. Because such data have been hard to come by, current models of prominences are static, says Raadu.

By analyzing images like the ones on these pages, Raadu and Schmieder are trying to fill in gaps in the data. Key to their research is an instrument at the Pic du Midi observatory in southern France called a multichannel subtractive double-pass spectrograph, one of only three such instruments in the world. It’s an ingenious device, says Raadu. The spectrograph splits incoming light into different beams, forming many different images simultaneously. The spectrum of each image is analyzed separately to extract information about the density, velocity, temperature, or some other property of the gas that emitted the light. So when studying the velocity, say, of gases in a prominence, the astronomers can also look at their temperature at the same time, rather than making two successive measurements that wouldn’t coincide exactly. Typically, people only have one image or perhaps two images simultaneously, but never, as we have here, something like eight images simultaneously, says Raadu. This way you can really piece together a complete picture.

Their most intriguing discovery so far is that the velocities of the gases in a prominence are constantly changing. This suggests that the magnetic field holding up the gases is being twisted like a rubber band by roiling motions just below the surface of the sun. In this scenario, the jostling of the magnetic field shakes up the gases in the prominence.

But little is known about what actually goes on below the sun’s surface. Seen through appropriate filters, the solar surface is mottled and granular. These granules are the tops of convection cells. Convection is what happens to a boiling pot of water--and to the outer layers of stars. In both cases hot fluids rise to the surface, where they cool and sink back down again.

If you took away convection, the sun and other stars would be boring. They would just sit there, says Fausto Cattaneo, an astrophysicist at the University of Chicago. Prominences, sunspots, the solar wind, the sun’s magnetic field, all this complex behavior simply would not be there if the sun were just radiating energy into space. Most of the exciting behavior of the sun occurs because it is boiling.

Cattaneo is simulating convection on a Cray supercomputer at NASA’s Goddard Space Flight Center in Maryland. The simulations allow him and his colleagues to study phenomena beyond the reach of even the best telescopes. The typical size of a granule is about six hundred miles, says Cattaneo. Three hundred to five hundred miles is the limit of observations with present technology. So with telescopes you can see the granules, but you cannot resolve the structure within them.

Cattaneo’s team assigns five numbers to every point in the simulation: three numbers describe the velocity of a particle in three dimensions, one number gives the density, and another the temperature. Each five-number array is updated some 40,000 times during one simulation. The result is a computerized motion picture of convection in the sun. These pages contain a few stills from such movies.

Every time we run this simulation, the gases travel faster than the speed of sound, says Cattaneo. Until computer models recently started spewing out these velocities, no one really knew that solar convection might be supersonic. The speed of the gases is a direct measurement of the energy of convection. It determines how much energy is available to be pumped into prominences and other solar phenomena.

A full understanding of convection and prominences is still a long way off. We’re in very uncharted territory, says Cattaneo. It has turned out to be a much nastier problem than anyone had anticipated.
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