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Following the Trail of Heavy Ions in the Solar Corona

by Shadia Habbal, Eclipse Team Leader 

 

eclipse sequence

Eclipse photos taken by Isabelle Scholl with a modified Canon EOS 30D camera. The original infrared-cutoff filter has been replaced to improve the red response of the CCD. The new filter transmits more than 98 percent of the visible spectrum and therefore has nearly full transmission in the H-alpha line, which is at the red end of the optical spectrum.

Total solar eclipses never fail to fascinate, which explains why people travel to remote places to witness one. For solar physicists, they are more than just a fascination. They offer us a unique opportunity to study the solar corona. With the Moon blocking the very bright solar disk, the outer atmosphere of the Sun, the corona, becomes visible for a few fleeting minutes.

The extent of this atmosphere depends on the instrument used to capture it. So far, the human eye is the most sensitive and developed instrument, but advances in detector technology and image processing have significantly improved the ability to image and record the extent of the corona out to a few times the radius of the Sun.

Welcome arch

Arch welcomed observers to "Eclipse City" in the middle of the Gobi Desert in the western part of China near the border with Mongolia. Photo by Isabelle Scholl.

For the total solar eclipse of August 1, 2008, the IfA team chose an observing site in the Gobi desert in the western part of China. The trip was not without difficulties: a change of observing site two weeks before the actual eclipse and no food at the campsite for the first 24 hours. Also, it rained very heavily prior to the eclipse. Fortunately, the skies cleared, enabling the experiments that had been planned for over a year to be carried out successfully.

One of the surprising results of the March 29, 2006, eclipse, which the IfA team observed from the Great Sahara Desert in the southern part of Libya, was the emission from a very tiny part of the solar spectrum for which iron ionized 10 times (Fe XI) is responsible. Although the existence of this spectral line had been known for over a century, few scientists ventured to image the corona in this spectral line because its wavelength, 789.2 nanometers, is in a part of the solar spectrum where the visible and the near infrared meet, and therefore it poses some technical challenges.

Total solar eclipse

Total solar eclipse image of the corona. The image, taken with a PIXIS 1024 BR camera manufactured by Princeton Instruments, a 300-mm focal length lens, and a filter centered at 789.2 nanometers (nm) with a 0.5-nm-wide bandpass, was processed by Miloslav Druckmüller from Brno University of Technology, Czech Republic.

The IfA team took advantage of advances in detector technology and the availability of a perfectly suited camera, manufactured by Princeton Instruments, to image the corona in Fe XI. With a total of 14 seconds of observation time, the IfA team obtained the first image of this spectral line. Two surprises emerged: First, the Fe XI emission extended out to at least two solar radii. Second, the presence of magnetized regions in the solar corona, where the intensity of the emission in Fe XI was locally enhanced, was totally unexpected.

When preparing for the 2008 eclipse, the IfA team planned to repeat the 2006 eclipse experiment with the addition of imaging in three other spectral lines produced by different ionization states of iron. The outcome confirmed the findings of the 2006 eclipse observations. In addition, comparison with images from the other spectral lines showed that the Fe XI 789.2 line is indeed very special, as the extent of its emission was almost double that from all the other spectral lines.

Shadia Habbal with equipment

Shadia Habbal and her large cameras. Photo by Isabelle Scholl.

Analysis of these observations has led us to conclude that the extent of the solar corona as observed in the Fe XI line is an indication that the dominant electron temperature in the corona is close to 1.2 million degrees. The existence of regions of enhanced Fe XI relative to the background white light is an indication that these ions are not receiving enough energy and thus cannot readily escape into interplanetary space with the solar wind flow, which is composed primarily of electrons and protons. These surprising results point to the importance of following the trail of heavy ions. Although they form a minute fraction of the coronal material, they nevertheless hold some of the secrets of the physical processes that enable the ionized coronal gas to reach a temperature often exceeding one million degrees. (The surface of the disk of the Sun is only several thousand degrees.)

Members of the IfA 2008 eclipse team included Martina Arndt (Bridgewater State University), Adrian Daw (Appalachian State University), Judd Johnson (Electricon), Huw Morgan (IfA), Alex Pevtsov (National Solar Observatory), Isabelle Scholl (IfA), and Brian Stuart (IfA). The team also collaborated with Prof. Miloslav Druckmüller from the Brno University of Technology in the Czech Republic, who observed the 2008 eclipse from Mongolia. Funding was provided by grants from the National Science Foundation and NASA.

Eclipse team Back row: Huw Morgan, Shadia Habbal, Judd Johnson, Mindy Lekberg (a high school teacher), Isabelle Scholl, Alex Pevtsov, and Rafif Rifai (Shadia Habbal's sister).Front row: Brian Stuart, Martina Arndt, and Adrian Daw.