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Science At The 1998 Eclipse: Heating Of The Solar Corona

Williams College

WILLIAMSTOWN, Mass.--A group of scientists and students from Williams College will study the heating of the solar corona during the total solar eclipse of Feb. 26.

Preparing to observe his 26th solar eclipse, Professor Jay M. Pasachoff, its leader, described their experiments, which they will be carrying out from Aruba.

Pasachoff is Field Memorial Professor of Astronomy and Director of the Hopkins Observatory at Williams College, Williamstown, Massachusetts, USA. He is also Chair of the Working Group on Eclipses of the International Astronomical Union and Chair of the Astronomy Division of the American Association for the Advancement of Science.

Two of Pasachoff's experiments will deal with the still open question of how the corona, the outermost layer of the sun's atmosphere, can reach a temperature of two million degrees Celsius (about four million degrees Fahrenheit), even though the everyday surface of the sun below it is only 6,000 degrees Celsius (about 11,000 degrees Fahrenheit). A third experiment is in liaison with scientists in charge of an experiment on the Solar and Heliospheric Observatory (SOHO) spacecraft. The experiments are in collaboration with Dr. Bryce Babcock, staff physicist at Williams College.

The observations are possible only during the brief minutes of a total solar eclipse, when the everyday sun is hidden by the moon, allowing the faint corona to be observable from earth. On ordinary days, the corona is hidden by the blue sky, since it is about a million times fainter than the layer of the sun we see shining every day, the photosphere.

Pasachoff, together with Dr. Leon Golub of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, recently completed the first textbook about the solar corona to be written in decades; it was published last summer.

The first experiment is a search for rapid oscillations in the corona, with periods of about one second. Pasachoff has developed techniques over the last two decades to observe in the so-called "coronal green line," a color in which the corona emits light especially strongly, with time resolution so fast that such short periods can be detected. Oscillations with periods in that short range are predicted by some theories that hold that the extreme coronal heating is caused by vibrations of magnetic loops. The loops of gas, held in place by the sun's magnetic field, have been observed, and the question is whether their vibrations bring enough energy into the corona to heat it sufficiently. The experiment is supported by a grant from the Atmospheric Sciences Division of the National Science Foundation.

The second experiment is a map of the temperature of the corona, using a technique of comparing electronic images of the corona taken at special ultraviolet wavelengths. Following theoretical work, these wavelengths are chosen to include two such at which the difference between the shape of the everyday sun's spectrum and the corona's spectrum is especially striking. The experiment is supported by a grant from the Committee for Research and Exploration of the National Geographic Society.

The third experiment is to image the solar corona during the eclipse at the same scale and with the same green filter as a filter in the coronagraph experiment on board the Solar and Heliospheric Observatory (SOHO). This observation is in collaboration with Dr. Guenter Brueckner of the Naval Research Laboratory, principal investigator of that experiment, LASCO (Large Angle Spectrographic Coronagraph). The comparison of the eclipse image with an image taken with one of LASCO's coronagraphs will provide a calibration of how much light is scattered in the process of making an artificial eclipse on board the spacecraft. Such artificial eclipses cannot quite match the quality of a natural eclipse, in which the moon hides the sun's light before it reaches a telescope. The comparison of the eclipse result with the SOHO image taken at the same time should allow improvements in data reduction of future SOHO images.

An unusual aspect of Pasachoff's experimental team is that it includes so many undergraduate students, eight in total, all astrophysics majors at Williams College: Timothy McConnochie, Johan Kongsli, Mac Stocco, James Bates, Laura Brenneman, and Craig Westerland. They are supported by the NSF and National Geographic grants; by the Keck Northeast Astronomy Consortium, a group of eight colleges whose astronomical student research is supported by the W. M. Keck Foundation; and by the Safford Fund, set up by his descendants in honor of the second director of the Hopkins Observatory, Truman Henry Safford. Also through the Keck grant, undergraduates Carolina Artacho Guerra of Bryn Mawr College and Lisa Reinker of Wellesley College will participate in the expedition.

Scientific staff also include Lee Hawkins of Wellesley College; Stephan Martin of Williams College; and Jonathan Kern, an optics designer from New Orleans. Also participating in the scientific work are Prof. Joseph Hollweg of the University of New Hampshire; Prof. Edw. S. Ginsberg of the University of Massachusetts Boston; and Dr. Donald Lubowich of Hofstra University and the American Institute of Physics.

The main team will spend two weeks on site in Aruba setting up, aligning, and testing the ton of equipment they will bring. They will arrive in Aruba on February 15.

References: Donald H. Menzel and Jay M. Pasachoff, Peterson Field Guide to the Stars and Planets (Houghton Mifflin, updated 1997); Jay M. Pasachoff and Michael Covington, Cambridge Guide to Eclipse Photography (Cambridge University Press, 1993); Fred Espenak and Jay Anderson, NASA Reference Publications 1383 for the 1998 eclipse and 1398 for the 1999 eclipse; Leon Golub and Jay M. Pasachoff, The Solar Corona (Cambridge Univ. Press, 1997). Jay M. Pasachoff, 1973, "The Solar Corona," Scientific American 229, #4 (October), 68-79. Donald H. Menzel and Jay M. Pasachoff, 1970, "Solar Eclipse," National Geographic 138, #2 (August), 222-233. Jay M. Pasachoff, 1992, RThe Darkness That Enlightens,S National Geographic 181, #5 (May), 36-37.

For more information, call: Prof. Jay M. Pasachoff, Williams College--Hopkins Observatory, Williamstown, MA 01267, phone: 413 597 2105; fax 413 597 3200; home 413 458 8346. e-mail: or

During the eclipse expedition: February 16-28: The Mill Condominium, J.E. Irausquin Blvd. 330, Palm Beach, Aruba, 011 297 8 67 700; fax 011 297 8 67271. also February 23-28, Hyatt Regency Aruba, L.G. Smith Blvd #85, Palm Beach, Aruba, 011 297 8 61234; fax 011 297 8 61682.

Williams College is consistently ranked one of the nation's top liberal arts colleges. Founded in 1793, it is the second oldest institution of higher learning in Massachusetts. The college of 2,000 students is located in Williamstown, which has been called the best college town in America. You can visit the college in cyberspace at


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