Feb. 12, 1998 BOULDER--Scientists from the National Center for Atmospheric Research (NCAR) and several other research institutes will aim new detectors at the sun's corona during the February 26 solar eclipse, searching for structures they've never observed before. The total eclipse over the Caribbean promises to be one of the most heavily studied in recent history, as scientists make observations from a ground station in Curacao, a research aircraft flying out of Panama, and the Solar and Heliospheric Observatory (SOHO) satellite operated by the National Aeronautics and Space Administration (NASA). The data they gather could eventually lead to better prediction of the coronal mass ejections that launch solar storms--the magnetic disturbances that play havoc with communications and electric power grids here on earth. The expedition is inspired by new theories and new technology. NCAR's primary sponsor is the National Science Foundation (NSF).
Porthole in the sky
Last fall, NCAR solar physicist Philip Judge predicted that a line of ionized silicon in the infrared region, close to 4 microns, should be detectable as a faint but distinct line in the spectrum of the sun's corona, or outer atmosphere. "If this line can be detected, it may prove to be the most sensitive indicator of coronal magnetic field strengths available to researchers," according to Judge. During the eclipse, a team led by Robert MacQueen (Rhodes College) will be on board NSF's C-130 Hercules aircraft, hunting for that and several other spectral lines also predicted by Judge. Judge will accompany the team and serve as backup for the researchers on the C-130. The NSF C-130 is operated by NCAR.
"Nobody has actually measured the strength of the magnetic field under average coronal conditions, because it's so weak. That's why we have to go into the far infrared to get any signature at all of the magnetic field there," Judge explains. Because of the faint signal, exacting observations must be made with minimal interference from the earth's atmosphere. That's why the detection instruments are mounted on the C-130, which can fly above most of the absorption introduced by water vapor in the earth's 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 aircraft's roof allows the instrument spar designed by Ingrid Mann (Max Planck Institute for Aeronomy, Germany) to point directly at the sun. MacQueen, Kuhn, Lin, and Mann will ascend to 18,000 feet in the unpressurized cabin to track the eclipse.
Several instruments on the SOHO satellite will be used to gather information about the state of the corona and the magnetic fields in the photosphere-the lower layer of the sun's gaseous surface- before and after the eclipse. Combining the SOHO data with the aircraft observations will help piece together a better picture of the sun's magnetic structure as a whole.
A theoretician, Judge is excited about the possibility of detecting the predicted line. Detection could build the case for constructing a measuring device called a coronal magnetograph for future deployment in space or on the ground. 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 cause disrupting "space weather"). "This is a first step. Until we can measure the magnetic field, we won't really know what's going on in the corona," he explains.
A new infrared camera, or photometer, will make 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 have 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," explains Kuhn. Invisible to sensors so far, the dust particles--if they're there--will be heated to a few thousand degrees, which is hot enough to emit infrared light. The glare of the sun obscures such infrared emissions, so an eclipse is a rare opportunity to look for the dust with this new technology. "Whether we find dust rings or not, the photometer will tell us more about the sun's magnetic fields," says Kuhn.
On the ground
On the northern tip of the island of Curacao, an NCAR team led by Steven Tomczyk will conduct three experiments. The first uses a very-large-format (2,048 x 2,048 pixels) electron detector called a charge-coupled device, or CCD camera, that uses video technology. Previously deployed by NCAR researchers to hunt for new planets, on this expedition the camera will measure the electron density of the corona. Tomczyk expects the images from the new camera to be of higher quality and easier to calibrate than those obtained with photographic film on prior eclipse expeditions.
The second ground-based experiment will 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 a sign of magnetic waves in the polar plumes," Tomczyk says. Like the coronal magnetic fields themselves, these waves--known as AlfvŽn waves--have never been observed, although scientists have postulated their existence for many years. The instrument constructed at NCAR for this experiment is a very-high-speed, low- noise CCD camera. It takes pictures in the red end of the visible- light spectrum. The possibility of recording the AlfvŽn waves is "a long shot," according to NCAR scientist Bruce Lites, but worth the small investment.
The third experiment, in cooperation with Don Hassler of Southwest Research Institute in Boulder, Colorado, will measure the white light of the corona above a magnetically active region, at fairly high resolution. "We'll be looking for very fine structures that outline the magnetic fields in the active region," says Tomczyk. This experiment will use an eight-inch Celestron telescope and a third, midsized CCD camera.
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. It also allows scientists to look at the lower layers of the corona, much closer to the sun's photosphere than is possible using coronagraphs.
Solar physicists can't send probes too close to their subject because its heat would melt their instruments. But the structure of the magnetic fields has been theorized since the late 1800s. These fields, while weak, are sufficiently strong that they underlie and organize everything that happens in the corona. Because ions and electrons in the highly ionized coronal plasma cannot cross the lines of magnetic force, the shape of the plasma indicates where the lines are. But since the corona itself is a very thin soup of plasma, its magnetic fields are equally meager.
NCAR is managed by the University Corporation for Atmospheric Research (UCAR). UCAR is a consortium of more than 60 universities offering Ph.D.s in the atmospheric or related sciences.
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