Earth-Based Solar Eclipse Research

The July 22 total solar eclipse, visible from China and India (but not the United States), will be the longest in the 21st century. Teams of scientists from around the world will gather in China to study the corona, the sun’s outermost atmosphere, for almost six minutes, unusually long for totality.

Most will be stationed at a 3,000-foot mountain site selected by Prof. Jay Pasachoff, a Caltech and Williams College astronomer and planetary scientist, in Tianhuangping, China, not far from Hangzhou or Shanghai.

The July event will be the 49th solar eclipse that Pasachoff has viewed. A champion of using eclipse observations to study the solar atmosphere, he describes the science of eclipses in the cover story of the international journal Nature (June 11 issue). Pasachoff, who is chair of the International Astronomical Union’s Working Group on Solar Eclipses, was invited to write the article as part of Nature’s coverage of the International Year of Astronomy.

The article describes the history of eclipse discoveries, such as the element helium and the verification of Einstein’s general theory of relativity, as well as current themes in eclipse research.

One recent development in eclipse studies is the new computer capability of bringing out low-contrast features. One such spectacular image, involving processing by Miloslav Druckmüller of the Brno Institute of Technology in the Czech Republic, was selected by Nature for its cover.

The detailed structure of the corona is caused by the sun’s magnetic field. Pasachoff’s work with Druckmüller and with Vojtech Rusin and Metod Saniga of the solar observatory in Slovakia has led to several joint papers in the Astrophysical Journal on views of the changing corona. The corona changes not only from year to year with the sunspot cycle but also even within minutes, as the scientists saw by comparing their observations from Siberia and Mongolia at the last solar eclipse on Aug. 1, 2008. They plan to extend that work this summer with observations from India, China, and islands in the Pacific.

Pasachoff’s team in China includes Bryce Babcock, staff physicist at Williams and several undergraduate students from Williams, where Pasachoff is Field Memorial Professor of Astronomy. He chose the site on a visit over two years ago to southern China together with Naomi Pasachoff, a research associate at Williams, and Beijing scientists Yihua Yan and Jin Zhu.

Pasachoff and his colleagues have been studying, in particular, why the solar corona has a temperature of millions of degrees, much hotter than the sun’s surface. They do so by using a special rapid-readout electronic camera and single-color filters chosen to show only coronal gas. They look for oscillations with periods in the range of one second, which would signify certain classes of magnetic waves. The detailed structure of the corona, revealed by imaging in the visible and x-ray regions of the spectrum, and the correspondence of bright coronal regions with sunspot groups, shows that magnetism is the cause of coronal heating and the coronal structure. A competing set of ideas of how the corona is heated to millions of degrees involves ubiquitous nanoflares, that is, relatively tiny solar flares going off all the time.

Studies of eclipses, transits of Mercury and Venus across the face of the sun, and occultations of Pluto and other objects in the outer solar system proceed in tandem. For his eclipse studies, Pasachoff uses a set of electronic cameras provided by NASA’s Planetary Sciences Division, primarily for use in studying Pluto and other objects in the outer solar system. His studies of Pluto’s atmosphere started with similar cameras that had been provided for eclipse work.

Pasachoff’s research this summer, as much of his work in the past, is supported mainly by a grant from the Committee for Research and Exploration of the National Geographic Society.


International Astronomical Union’s Working Group on Solar Eclipses: http://www.eclipses.info

Williams College eclipse expeditions: www.williams.edu/astronomy/eclipse

Pasachoff’s books and other publications: http:// www.solarcorona.com

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