Report From The 1998 Solar Eclipse: The Williams College Expedition

March 01, 1998

WILLIAMSTOWN, Mass.--A group of scientists and students from Williams College studied the heating of the solar corona during the total solar eclipse of February 26. The expedition observed the eclipse from Aruba.

"The eclipse was a fantastic experience, with the clouds that had plagued the partial phases parting about 15 minutes before totality," said Professor Jay M. Pasachoff, leader of the Williams College expedition. "We were then able to observe the three minutes of totality in a very clear part of the sky. The fact that the diamond ring effect that opened totality persisted for so long indicates that the sky was especially clear. Otherwise the bright sunlight in the diamond ring would have obscured the corona."

Two of the experiments dealt 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). The third experiment is in liaison with scientists in charge of an experiment on the Solar and Heliospheric Observatory (SOHO) spacecraft. The experiments were in collaboration with 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.

The first experiment was a search for rapid oscillations in the corona, with periods of about 1 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. Babcock and Williams College undergraduate Timothy McConnochie '98 concentrated on this experiment. A first look at the eclipse observations showed very strong signals recorded by the equipment. The experiment is supported by a grant from the Atmospheric Sciences Division of the National Science Foundation.

The second experiment was to 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. Unfortunately, a power failure shortly before the eclipse led to a loss of tracking of the sun, and the experiment will have to be repeated at the next eclipse to obtain data. Lee Hawkins of Wellesley College and the Keck Northeast Astronomy Consortium and Bryn Mawr College undergraduate Caroline Artacho concentrated on this experiment. The experiment is supported by a grant from the Committee for Research and Exploration of the National Geographic Society.

A third experiment was 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. Stephan Martin of Williams College supervised that experiment, which obtained 7 excellent images of the corona during totality. This experiment was supported in part by the National Geographic Society.

Jonathan Kern of New Orleans made a coronal image through a filter that was specially graded in density to adjust for the wide range of coronal intensities. This type of image shows coronal structures especially well; the film has not yet been developed. Kern also participated in the other experiments, and designed the optical path for the oscillation experiment.

The expedition recorded a wide variety of coronal and other solar images in the visible and infrared parts of the spectrum. These films are now being developed in Boston. Also collaborating were Robert Eather of Boston and John Weiley of Australia, who made an IMAX large-scale time-lapse movie of the eclipse. Indications so far are that they were successful, though the film has to go to Los Angeles to be developed.

An unusual aspect of Pasachoff's experimental team is that it included so many undergraduate students, 8 in total. All the astrophysics majors at Williams College were included: Timothy McConnochie '98, Johan Kongsli '98, Mac Stocco '98, James Bates '98, Laura Brenneman '99, and Craig Westerland '99. They were supported by the NSF and National Geographic grants; by the Keck Northeast Astronomy Consortium, a group of 8 colleges whose astronomical student research is supported by the W. M. Keck Foundation; by the Safford Fund, set up by his descendents in honor of the second director of the Hopkins Observatory, Truman Henry Safford; and by the Brandi Fund. Also through the Keck grant, undergraduates Carolina Artacho Guerra of Bryn Mawr College and Lisa Reinker, currently an exchange student at Williams College from Wellesley College, participated in the expedition.

Scientific staff included Bryce Babcock, Physicist and Coordinator of the Bronfman Science Center of Williams College; Lee Hawkins, Instructor at Wellesley College; Stephan Martin, Instructor in Astronomy at Williams College; and Jonathan Kern, an optics designer from New Orleans.

The main team spent two weeks on site in Aruba setting up, aligning, and testing the two tons of equipment they brought. They arrived in Aruba on February 15. Winds at the site often gusted to more than 30 knots, and were measured with anemometers provided by Williams College professors Henry Art and David Dethier. The winds did not diminish during the eclipse, remaining in the 13-20 mph range. The temperature, though, dropped by about 10 degrees during totality. These measurements were carried out by Eric Plesko, one of several alumni and friends of Williams College who traveled to the eclipse in association with the expedition.

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. He is the author of the Peterson Field Guide to the Stars and Planets.

Williams College

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