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Spring 1998

New technology paves the way for clearer eclipse views

by Carol Rasmussen
and Zhenya Gallon

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.

The corona, or outer atmosphere of the sun, is a million times dimmer than the solar disk. Scientists can observe the corona at any time using a coronagraph--an instrument that blacks out the disk--but sunlight scattered by the earth's atmosphere masks the very faint coronal light. A real eclipse gives much better results because the moon blocks sunlight before it reaches the earth's atmosphere and is scattered.

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.

In the air

Last fall, Judge predicted that some wavelengths in the far infrared region of the spectrum should be detectable as a faint but distinct line (emitted by ionized silicon at about 4 microns) in the corona. "If this line can be detected, it may prove to be the most sensitive indicator of coronal magnetic field strengths available to researchers," said Judge. However, detection would require exacting observations made with minimal interference from the earth's atmosphere. That's why the instruments for this attempt were mounted on the C-130 aircraft, which can fly above most of the absorption introduced by water vapor in the atmosphere.

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.)
Below: NCAR Research Aviation Facility staff prepare the C-130 for the eclipse instrumentation. (Photo by Carlye Calvin.)
Jeff Kuhn (National Solar Observatory) tests equipment on the C-130. (Photo by Philip Judge.)

Several instruments on NASA's Solar and Heliospheric Observatory (SOHO) satellite were used to gather information about the state of the magnetic field in the photosphere--the lower layer of the sun's gaseous surface--before and after the eclipse. Combining the SOHO data with the coronal observations on the ground and from the aircraft helps piece together a better picture of the sun's magnetic structure as a whole. "This is the first time that all of this data will be amassed to address this one question," said 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.

On the ground

On the northern tip of Curaçao, an HAO team consisting of Steven Tomczyk, Lites, Kim Streander, David Elmore, Alice Lecinski, and Gregory Card did three studies during the eclipse. The first used a new instrument that replaced the Newkirk camera, built by HAO in 1966 and taken on every HAO eclipse expedition from then till now. The Newkirk camera used photographic film to take pictures of the eclipses and also measured linear polarization. Although "it truly was a fine instrument," Streander said, its optics are deteriorating, and technological advances have outdistanced it. Its replacement is a large-format charge-coupled device (CCD) camera (2,048 x 2,048 pixels), on loan from HAO's Timothy Brown, who uses it to search for planets around other stars. "Basically we're going to perform the same experiment that we used to do with the Newkirk camera, but using an electronic format," said Tomczyk. "The purpose of this is not just to take a pretty picture but also to measure the electron density of the corona." The images from the new camera will be of higher quality and easier to calibrate.

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.

The bottom line

HAO's three ground-based experiments are underwritten by the division's base funds. The cost, including airfare and shipping, is a modest $50,000. How have the researchers managed to keep costs so low?

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.

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 14:51:09 MDT 2000