Lab presents new line of thought on solar corona

Lab physicists and astrophysicists have explained how the two strongest energy emission lines from iron ions in the solar corona differ from laboratory calculations.

The emission lines — a discrete spectral line that corresponds to the emission of energy at a single wavelength — from iron ions in the solar corona have puzzled solar scientists for many years. The observed intensity ratio of the two strongest lines in the soft X-ray region differed by a factor of two or more from calculations. The discrepancy lead to many speculations about the properties of the solar corona that could make such a large change in the line ratio.

This problem seemed like a perfect match for the capabilities of LLNL’s laboratory astrophysics group. Peter Beiersdorfer, a physicist in V Division of the Physics and Advanced Technologies Directorate, heads the group. He teamed with Steve Kahn from Columbia University, a founding member of the group; the NASA Goddard Space Flight Center; the Princeton Plasma Physics Laboratory; and the Harvard-Smithsonian Center for Astrophysics. The group has been using laboratory facilities, particularly Livermore’s electron beam ion trap, to investigate atomic physics problems of observational astronomy for the past 10 years.

In astronomy, a corona is the tenuous outermost layer of the sun’s atmosphere that begins immediately above the chromosphere and contains gas at temperatures of 1 to 2 million degrees kelvin. The corona of other stars may contain much hotter gas.

When the Livermore team first looked at the problem, they found that the theoretical predictions were off. Theory had overestimated the ratio. Recalibrating to the laboratory data made the discrepancy much smaller. But it could not explain all of the discrepancy.

Last year, observations of Capella, a nearby star in the constellation of Auriga, were reported and showed that the corona of Capella had the same problem as our Sun. At that point, the team knew something unusual must have been going on. These observations, made with the Chandra and XMM X-ray Satellite Observatories, allowed astronomers for the first time to see extrasolar objects with high clarity. The hypotheses that might explain the discrepancy in the Sun should not apply to Capella. The fact that data from multi-billion dollar satellites could not be explained satisfactorily made solving the problem even more pressing.

"Using the Livermore electron beam ion trap we tried very hard to look at every aspect of the problem," said Greg Brown, who took the lead on the laboratory measurements at Livermore. "We had lots of support from other Lab scientists, such as astrophysicist Duane Liedahl from V division."

But no matter what process the group looked at, the laboratory data continued to differ from the stellar data.

The first break came when the group collaborated with Manfred Bitter of the Princeton Plasma Laboratory and analyzed iron data from one of the Princeton tokamaks. Tokamaks are used to study the production of energy from nuclear fusion. The tokamak data for the first time reproduced all relevant aspects of the iron emission from the Sun. Therefore, the hypotheses invoked by solar physicists to reconcile the solar data with laboratory data were no longer needed. However, the group still had no explanation for the actual mechanism that produced the discrepancy in the x-ray emission.

Continued research with Livermore’s EBIT-II electron beam ion trap finally paid off. The laboratory astrophysics group noticed that the iron emission could potentially become contaminated with emission from an iron ion of different charge. The emission from the different ions fell right on top of each other, preventing a direct observation of the effect for all these years in the astrophysical plasmas. Moreover, the interfering ion is highly unstable and difficult to produce in the laboratory. The laboratory spectra were simply too "clean" to observe the effect. To prove the interference, the group managed to operate their ion trap far from the equilibrium producing lots of the interfering iron ions. As they maximized the interference, they watched the emission reproduce the solar ratios one by one.

Their findings now show that the interference makes the iron emission a useful tool to measure the temperature of the solar and stellar coronae in the range from 1.5 million to 6 million degrees Celsius.

"There are still more puzzles in the X-ray spectra observed by Chandra and XMM," Beiersdorfer said. "But we are glad that we finally solved one of the major ones."
The results from the group’s ion trap and tokamak experiments are published in the August and September issues of Astrophysical Journal Letters and Physical Review A, respectively.