Spring 1998
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Spring 1998 |
For three-and-a-half minutes on 26 February, that puzzling, ethereal phenomenon, the solar corona, was the biggest tourist attraction in the Caribbean. During those brief moments of total eclipse, scientists from NCAR's High Altitude Observatory (HAO) gained data that will occupy analysts for months.
The local weather cooperated with the eclipse teams, offering cloud-free skies above Curaçao and Panama, where the researchers were based. HAO scientist Bruce Lites reported, "All of our equipment seemed to work very well. We've looked at the images, and they seem to be really good." (For a preview of the results, visit HAO's eclipse image on the Web.)
| Alice Lecinski (HAO) works on one of the ground-based instruments. (Photo by Carlye Calvin.) Inset: The 26 Feburary eclipse as observed from Curaçao. |
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Since an HAO team last went to an eclipse (in Putre, Chile, in 1994), technological advances in cameras and other detectors have created the possibility of observing coronal features that have long been theorized but never measured. Using the new equipment, scientists have a chance to uncover a gold mine of new information. Thus HAO has fielded a record-sized team and diverse experiments.
Solar physicists can't send probes too close to their subject because its heat would melt their instruments. Even with this limitation, however, theorists have pieced together over the years a picture of a roiling, vibrating star dominated by intense magnetic fields from its core far out into space. These fields underlie and organize everything that happens in the corona.
The structure of the magnetic fields has been theorized since the late 1800s, as soon as the corona could be seen in sufficient detail. The early scientists realized that the corona's shape gave evidence of magnetic fields, because the electrons in the highly ionized coronal plasma cannot cross lines of magnetic force. Theories of coronal magnetism have grown apace over the last century, but there's still one problem. Since the corona itself is a very thin soup of plasma, its magnetic fields are equally meager. "Nobody has ever measured the strength of magnetic fields in the typical one-million-degree corona," said Philip Judge (HAO).
One of HAO's experiments was designed to pave the way toward solving that problem by finding detectable features in the electromagnetic spectrum. This experiment took place on board NSF/NCAR's C-130 Hercules aircraft, which flew out of Panama City, Panama. Other airborne and ground-based experiments (performed on the island of Curaçao) observed the magnetic fields indirectly, by looking at their effects.
A theoretician, Judge was clearly excited about the possibility of detecting the predicted lines. The experiment could build the case for constructing a coronal magnetograph (whether in space or on the ground depends on which detected line is strongest). The kinds of problems Judge and his colleagues could then address include the nature of the evolution of coronal fields during the solar cycle and what launches solar flares and coronal mass ejections, which are important factors in space weather.
To hunt for the spectral line, Jeff Kuhn (National Solar Observatory and Michigan State University) and Haosheng Lin (NSO) designed an instrument package for the C-130. A 16-inch hole in the C-130's roof allowed the instrument spar, designed by Ingrid Mann (Max Planck Institute for Aeronomy, Lindau, Germany), to point directly at the sun. Robert MacQueen (Rhodes College), Kuhn, Lin, and Mann ascended to 18,000 feet in the unpressurized cabin to track the eclipse. Although skies over Howard Air Force Base, where the plane was based, were cloudy on the day of the eclipse, the team caught 4 minutes and 40 seconds of totality flying over the Darien province of Panama, just inside the Colombia border.
| Right: Robert MacQueen (Rhodes College) and Dan Edmonds (National Solar Observatory) on a test flight in the C-130. (Photo by Philip Judge.) |
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| Below: NCAR Research Aviation Facility staff prepare the C-130 for the eclipse instrumentation. (Photo by Carlye Calvin.) | |
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| Jeff Kuhn (National Solar Observatory) tests equipment on the C-130. (Photo by Philip Judge.) |
Kuhn modified the spectrograph he put together for the 1994 eclipse in Chile, improving the grating to home in on the coronal infrared emission lines. The spectrograph also was trained on a helium emission line in the outer corona first observed during the 1994 eclipse. A third role for the instrument was measuring the shift of dark lines (Fraunhofer lines) in the solar spectrum, which indicate absorption by gases in the outer parts of the sun, in order to determine the motion of interplanetary dust near the sun.
A new infrared camera made its debut on the C-130. The camera's infrared array detector, made by Rockwell International and employed in missile guidance systems during the Persian Gulf war, was recently declassified for peacetime use. MacQueen and Kuhn enlisted the instrument in their search for interplanetary dust structures. "The dust from the whole solar system should be accumulating around the sun and forming dust rings, like Saturn," explained Kuhn. Invisible to sensors so far, dust particles around the sun--if they're there--would be heated to a few thousand degrees, at which point they would become emitters of infrared light. The glare of the sun obscures infrared emissions, so an eclipse is a rare opportunity to look for them with this new technology. "Whether we find dust rings or not, the photometer will tell us more about the sun's magnetic fields," said Kuhn.
A second ground-based experiment, under the direction of Lites, attempted to observe the sun's polar plumes--fingerlike structures that radiate upward from the poles. "We're going to see if they wiggle around, which could be indicative of magnetic waves in the polar plumes," Tomczyk said. Like the coronal magnetic fields themselves, these waves--known as Alfvén waves--have never been observed, although people have postulated their existence for many years. The CCD camera for this experiment was made at HAO and taken to the 1994 Chilean eclipse, but cirrus clouds blocked its use. "It's a very high speed, low-noise camera," Lites explained. "The idea is to build up a body of images with good signal-to-noise ratio." The camera takes pictures in the red end of the visible-light spectrum. Before the eclipse, Lites thought that the possibility of recording the Alfvén waves was a long shot. The day after the eclipse, he reported, "We've made a short movie of the data from the [camera], and it looks extremely good--no problems."
The third experiment, in cooperation with Don Hassler of Southwest Research Institute in Boulder, Colorado, measured the white light of the corona above a magnetically active region, at fairly high resolution. "We're looking for very fine structures that outline the magnetic fields in the active region," said Tomczyk. This experiment used an eight-inch Celestron telescope and a midsized CCD camera.
For one thing, scientists used to arrive at an eclipse site as much as a month early to prepare the site and test their equipment. Now they set up in about a week. "That cuts down on your logistics cost," said Streander. Technological advances explain most of this change. For example, with the Newkirk camera and spar, the observers could only practice pointing and tracking for an hour and a half each day; with the new telescope, they can practice all day long, because the telescope can scan the entire horizon.
But the logistical savings alone don't explain the low cost. "We're cooperating; we're borrowing equipment; we're renting rather than owning," said Tomczyk. For this expedition, the HAO team borrowed equipment from the National Solar Observatory, a private laboratory, and the Air Force.
With all the new technology--owned and borrowed, airborne and on the ground, in infrared and visible frequencies--and with the atmosphere's cooperation in providing clear skies, the data gathered from this eclipse are likely to yield answers to some of the many questions that remain about the corona's magnetic fields.