2010 Solar Eclipse Expedition
by Shadia Habbal
The 2010 solar eclipse expedition observed five iron lines (Fe IX, Fe X, Fe XI, Fe XIII, Fe XIV), one hydrogen line (H-alpha), and a nickel line (Ni XV). It was the first time that the corona was imaged in Fe IX and Ni XV. © 2010 Miloslav Druckmüller, Martin Dietzel, Shadia Habbal, Vojtech Rusin. See Spectroscopy & Science for more information.
Mounting a solar eclipse expedition requires advance planning, patience, and optimism. The best options for the July 11 eclipse were a few atolls in French Polynesia. We needed a usable airstrip and accommodation for 20 people. We also needed 100 liters of liquid nitrogen for Haosheng Lin's polarization experiment.
The largest atoll that seemed to fulfill our requirements was Hao. Bed and breakfast manager François Dantzer assured me that he could find accommodations for us there. A week later, he contacted me to say that Tatakoto, an atoll 745 miles (1,200 km) from Tahiti's capital, Papeete, would offer a longer eclipse, and it had a landing strip. He also said that our group could use the local school as our staging and observing site, and as sleeping quarters. I contacted the school's headmaster, Heifara Lanteires, and got the ball rolling.
Our group consisted of a 12-member IfA team (including several European collaborators) and an eight-member team from Italy. Standing, from left to right: P. Calcidese, L. Casetti, C. Benna, S. Fineschi, G. Capobianco, V. Rusin, H. Lin, J. Saken, A. Daw, M. Druckmüller, A. Ding. Seated: M. Arndt, F. Trevisan, M. Romoli, G. Massone, G. Nitta, J. Johnson, S. Habbal, M. Dietzel, H. Morgan.
Getting to Tatakoto was a challenge. We had to charter a plane from Air Tahiti to transport us and our nearly two tons of equipment from Papeete. The liquid nitrogen barely made it to Tatakoto because of a firefighter's strike in Papeete the week the empty dewars (thermos-like containers) arrived from Honolulu. The dewars were to be filled in time to catch a boat to Tatakoto, but missed the boat by a few hours. Through tactful negotiations, Air Liquide and DHL Danzas (which transported the dewars to Papeete) convinced Air Tahiti to fly the filled dewars to Hao in time to catch up with the boat going to Tatakoto.
Eclipse observations provide unique opportunities for exploring the ionized gas, or corona, that forms the extended atmosphere of the Sun. The temperature of this gas exceeds a million degrees, so it is not possible to replicate coronal processes in laboratories. Curiously, the heavier elements, such as iron, nickel, and sulfur, which only constitute a minute fraction of the bulk protons and electrons forming this gas, offer important clues about its behavior and properties, particularly the escaping component known as the solar wind. Fortunately, these ions emit light at well-known wavelengths. By selecting filters centered on these wavelengths, we can explore the behavior of these ions.
We imaged the corona in seven spectral lines—five iron lines characteristic of plasma temperatures ranging from half a million to over 2 million degrees Kelvin (3.6 million degrees F), a cool 10,000-degree hydrogen line, and a 3-million-degree nickel line. Spectral measurements were also made with three different spectrographs, and Lin set up his very ambitious polarization experiment to measure the magnetic field direction and strength in a region of the corona.
The clouds looked rather ominous at 6 a.m. on the morning of the eclipse. Fortunately, the Sun rose above them before totality began at 8:47 a.m. Despite the rolling clouds during the four minutes of totality, imaging simultaneously in the spectral lines and their neighboring continuum enabled us to remove the effects of the clouds. When the images were subtracted from each other, they exposed the pure emission from hydrogen, iron, and nickel. Preliminary results show that there is a very distinct separation between the solar features seen in the cooler emission lines and those seen in the hotter ones. Some solar features were seen in iron lines but not in white light. The explanation is rather simple and neat: Initially, all the ions are linked together, but if there is not enough energy for all of them to escape the solar gravitational field, the heavier ions lag behind.