TELESCOPES from Earth and from space have imaged the sun millions of times but have discerned its secrets very slowly, especially those concerning its ever-changing magnetic fields. Now, extremely detailed images from a NASA satellite called the Transition Region and Corona Explorer (TRACE) are revealing the role played by fluctuating magnetic fields in heating the sun's hot plasma and creating such fantastic features as solar flares, eruptions, and loops. Obtained with a telescope using multilayer-coated mirrors produced by Lawrence Livermore scientists, the images can be assembled into mosaics of the complete solar body or arranged sequentially into astonishing time-lapse movies.
TRACE is part of NASA's Small Explorer program of lightweight spacecraft designed to minimize the cost of scientific space exploration. In Earth orbit since April 2, 1998, the 1,030-kilogram satellite continuously studies the key solar regions: the photosphere (the 6,000-kelvin surface), chromosphere (the part of the sun's atmosphere where the temperature rises to about 10,000 kelvins), transition region (between the chromosphere and corona in which the temperature rises dramatically to 100,000 kelvins), and the corona (the multimillion-kelvin upper atmosphere).
TRACE observes the sun at unprecedented spatial resolution (1 arc second, or 740 kilometers across) and time resolution (from 2 to 30 seconds between images). Images at these resolutions effectively provide time-lapse movies that reveal how extremely fine magnetic loops appear, evolve, and reform.
Key to the high image resolution are the multilayer optical coatings produced by a Lawrence Livermore team led by materials scientist Troy Barbee, Jr. Multilayers are composed of alternating layers of two different materials as thin as a few atoms. (See Atomic Engineering with Multilayers, S&TR, December 1997.)
In recent years, multilayers have proved extremely valuable to astrophysicists for their ability to focus light in previously inaccessible narrow bands of the x-ray, soft x-ray, and extreme ultraviolet wavelengths. Their extraordinarily efficient optical performance reveals astronomical features that cannot be captured by instruments operating at longer wavelengths.

Wanted: Higher Resolution
When planning for the mission began, says Barbee, the TRACE team asked for multilayers that could provide much higher resolution and reflectivity than had ever been achieved. The team also requested more robust materials; previous space-born multilayers have degraded in time as a result of the harsh environment.
In response, Barbee's team developed multilayers made of alternating layers of molybdenum carbide and silicon. The multilayers were applied to a substrate of titanium silicate glass, supplied by the group at the Smithsonian Astrophysical Observatory responsible for TRACE optics.
Barbee reasoned that the carbon in the molybdenum carbide reacts with silicon to form silicon carbide, an extremely stable material, at the interfaces between the layers. To test the multilayers' stability, Barbee annealed some test materials for eight hours at 673 kelvins-equivalent to being in space for about two years at 90 degrees C. No changes were observed in the performance of the annealed structures, demonstrating that they are dramatically more stable than other potential multilayer material combinations. In space, the TRACE multilayers have proved so stable that they are used to calibrate multilayers on other satellites.
The Livermore materials are exceptionally smooth and flat; their 47-percent reflectivity is a factor of 2 better than any multilayer previously used in space. Such high reflectivity results in much shorter exposure times for the satellite's charge-coupled device (CCD) camera.
The 30-centimeter primary TRACE telescope has a different multilayer coating on each quadrant of its surface, making it in effect four different telescopes. The telescope shutter selects one quadrant at a time. Coatings for three quadrants were each designed for a specific band of extreme ultraviolet light corresponding to one of these excited ions of iron: Fe IX, Fe XII, or Fe XV. These three excited states are formed at temperatures of 600 thousand, 1.5 million, and 2 million kelvins, respectively.
The fourth quadrant of the telescope reflects visible and ultraviolet light; a filter wheel near the focal plane selects this light from the regions of 6,000 to 30,000 kelvins. One piece of the surprising information yielded by TRACE's high-resolution images is that temperatures in the outer solar atmosphere vary from less than 30,000 to more than 2,000,000 kelvins over distances of only a few thousand kilometers.






Putting Observations Together
By combining observations taken at different wavelengths only a few seconds apart, scientists can follow the evolution of the sun's magnetic fields from the photosphere into the highest reaches of the corona. The quality of the images is further refined by internal stabilization of the telescope optics against spacecraft jitter.
To date, more than two million TRACE images have been captured and relayed to Earth for analysis. Viewed individually, combined into giant mosaics, or rapidly sequenced as time-lapse movies, the images have sparked a revolution in understanding solar atmosphere dynamics, especially those events occurring in the transition region and corona. "We're seeing for the first time how magnetic fields determine solar phenomena," says TRACE principal investigator Alan Title, solar astrophysicist at the Lockheed-Martin Solar and Astrophysics Laboratory and physics professor at Stanford University.
Title says that aside from observations from a couple of five-minute rocket flights into the Earth's atmosphere, the sun's corona had not been seen with such resolution. "Suddenly, TRACE extended our observing window from five minutes to several years."
TRACE observations, he says, reveal a richly detailed portrait of the corona, where magnetic fields play a dominant role. Especially obvious are bright loops of different lengths of plasma that connect regions of opposite magnetic polarity.
"The images show that virtually all of the sun's loops have a structure at scales near or below the CCD camera's resolution of 1 arc second," says Title. "No one had appreciated that the solar atmosphere was as finely detailed and as rapidly evolving as we're seeing with the TRACE movies. We really hadn't expected so much information."
Title cites the light-gathering capabilities of the multilayer mirrors, which permit taking rapid sequences of images. "If you're only taking a picture every 15 minutes, magnetic waves with periods of a few minutes don't get recorded."





Important Data with Multiple Uses
The data are important to NASA and other agencies because magnetically induced solar events, such as flares and coronal mass ejections, can have huge consequences millions of miles away. Although they last only a couple of minutes, large flares emit enormous amounts of high-energy radiation and fast particles that can endanger astronauts, disrupt satellites in orbit, and even cause power outages in electrical grids on Earth.
The problem of flare emission and coronal mass ejection is sufficiently important that a National Sun Weather Program has been established. A better understanding of the physical processes in the outer solar atmosphere is also important to astronomers for understanding the processes of other stars and to magnetic fusion scientists for designing methods to confine hot plasmas and produce magnetic fusion energy.
The wealth of information contained in images from TRACE and other solar satellites was the focus of an international conference held at Monterey, California, in August 1999 that was attended by scientists from the U.S., Japan, China, Europe, Russia, and Canada. Barbee, who attended the conference, says the proceedings showed that TRACE observations are forcing a reconsideration of traditional theories underlying the physics of the sun and its atmosphere.
In particular, he says, TRACE movies are providing new insights into how the corona becomes heated to extremely high temperatures by magnetic fields. Astrophysicists have long been perplexed by the fact that the sun's outer atmosphere is so much hotter than its surface; computer models have not accounted for this heating satisfactorily.
Images and movies are not reserved for scientific investigators. TRACE is the first U.S. research mission with a completely open policy; all data are available to other scientists, students, and the general public soon after they become available to investigators. Sample movies of TRACE images can even be seen on the Internet at http://vestige.lmsal.com/TRACE/.
Barbee points out a final and seemingly unrelated payoff to the successful application of multilayers in solar astrophysics: computer chip manufacturing. Multilayer mirror coatings are a key technology for extreme ultraviolet lithography (EUVL), now under development by Lawrence Livermore and its industrial research partners. The technology promises manyfold increases in computer performance by shrinking the size of lines and features within chips. "The first images of the sun with multilayers in 1987 really put EUVL in motion," says Barbee. "We demonstrated we could get these resolutions, so the researchers pushed ahead in the lithography area."
From the vast atmosphere of the sun to the tiniest computer chips, multilayers are helping scientists push ahead on many fronts.
-Arnie Heller

Key Words: chromosphere, corona, extreme ultraviolet lithography (EUVL), multilayers, photosphere, sun, Transition Region and Corona Explorer (TRACE).

For further information contact Troy Barbee, Jr. (925) 422-7796 (barbee2@llnl.gov).


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