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March 1998

On to the Caribbean: All eyes on the eclipse

As Staff Notes Monthly was going to press, scientists from NCAR's High Altitude Observatory and several other research institutes were preparing to aim new detectors at the sun's corona during the 26 February solar eclipse, searching for structures they've never observed before. The eclipse was expected to be one of the most heavily studied in recent history, as scientists make observations from a ground station on the Caribbean island of Curaçao, a research aircraft flying out of Panama, and the Solar and Heliospheric Observatory (SOHO) satellite operated by NASA.

FLASH! The eclipse experiment was a success. Check the SN Extra! in this issue for more details on how it went. There's more below on the planning of each experiment.

These folks clearly know how to observe an eclipse. In the top row (left to right) are Greg Card, Steve Tomczyk, Bruce Lites, and Kim Streander. At bottom are David Elmore and Alice Lecinski. The HAO team went to Curaçao with the telescope at right, which sports two new instruments: a very large format camera (the topmost square) for measuring coronal electron density and a low-noise camera (the lower-left instrument) for observing plumes at the sun's poles. (Photo by Carlye Calvin.)

Data from this expedition--which is inspired by new theories and new technology--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.

Porthole in the sky

Last fall, HAO solar physicist Phil Judge predicted that a line of ionized silicon in the infrared region, close to four microns, should be detectable as a faint but distinct line in the spectrum of the sun's corona, or outer atmosphere. Before he set off for the Caribbean, Phil observed that "if this line can be detected, it may prove to be the most sensitive indicator of coronal magnetic field strengths available to researchers." During the eclipse, a team led by former NCAR and HAO director Bob MacQueen (now at Rhodes College) was set to be on board the NSF/NCAR C-130 Hercules aircraft, hunting for the ionized silicon and several other spectral lines that Phil also predicted. He planned to accompany the team and serve as backup for the researchers on the C-130.

Top: the heart of the passenger cabin on the NSF/NCAR C-130 was emptied to make room for an HAO eclipse experiment over the Caribbean on 26 February. Above: a 16-inch hole was cut in the aircraft's roof, allowing the instrument spar to point directly at the sun--or the moon, as shown in this test performed in Boulder. (Photos by Carlye Calvin and Phil Judge.)

"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," Phil explained. Because of the faint signal, exacting observations must be made with minimal interference from the earth's atmosphere. That's why the detection instruments were 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 (0.4-meter) 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. (See photos.) MacQueen, Kuhn, Lin, and Mann planned to ascend to 18,000 feet (5,500 meters) in the unpressurized cabin to track the eclipse.

Alice Lecinski adjusts POISE '98 (polarimeter instrument for solar eclipse '98), the very large format camera used by Tim Brown for planet finding and called into service for this year's eclipse.

Several instruments on the SOHO satellite gathered 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.

Down to the wire

Any eclipse is dramatic, but high corporate drama beyond UCAR made the planning phase of this expedition unusually rocky for HAO. For instance, the observatory had planned to use an infrared camera being built from circuit boards supplied by a subcontracted laboratory. However, that lab's expert on electronics left the company in midstream to go into business for himself, leaving the camera in limbo. According to Kim Streander, HAO hopes to take the optics constructed at NCAR for the ill-fated camera and use them with one of the IR cameras now on the C-130 on the next eclipse expedition (Europe, August 1999). The very large format camera deployed at Curaçao had a similar tale of woe. Large-scale layoffs at Kodak caused delays in the loan authorization of a unique camera originally planned for use within a telescope. In its place, the HAO instrumentation group borrowed a camera used by HAO's Tim Brown in his planet-finding work. They then scrambled to put together the new camera/telescope and its computer interface in a scant month before departing for Curaçao. •BH

A theoretician, Phil has been excited about the possibility of detecting the predicted line. Detection would 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 Phil and his colleagues could then address include how coronal fields evolve during the solar cycle and what launches solar flares and coronal mass ejections (the cause of "space weather"). "This is a first step," Phil explained. "Until we can measure the magnetic field, we won't really know what's going on in the corona."

A new infrared camera, or photometer, 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 1991 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," explained 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," said Kuhn.

On the ground

On the northern tip of Curaçao, an NCAR team led by Steve Tomczyk planned three experiments. The first involved a very large format (2,048 x 2,048 pixels) electron detector based on video technology and called a charge-coupled device, or CCD, camera. Previously deployed by NCAR researchers to hunt for new planets, the camera was brought on this expedition to measure the electron density of the corona. Steve expected the images from the new camera to be of higher quality and easier to calibrate than those obtained with the Newkirk camera on prior eclipse expeditions (see sidebar).

The second ground-based experiment was designed 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 a sign of magnetic waves in the polar plumes," Steve said. Like the coronal magnetic fields themselves, these waves--known as Alfvén waves--have never been observed, although scientists have postulated their existence for many years. The instrument constructed at HAO 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 Alfvén waves was "a long shot," according to HAO's Bruce Lites, but worth the small investment.

In the third experiment (in cooperation with Don Hassler of Southwest Research Institute), the team planned to 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," said Steve. This experiment involved an eight-inch Celestron telescope and a third, midsized CCD camera. •Zhenya Gallon and Carol Rasmussen

Who needs an eclipse, anyway?

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. •CR

Farewell to an NCAR landmark

The Newkirk white light coronal camera was the workhorse of HAO eclipse expeditions from 1966 through 1994. With this eclipse, it's finally been put out to pasture.

The camera was designed and built here by Gordon Newkirk and Lee Lacey of HAO. Its road log would be the envy of many globetrotters; it visited six continents and three Pacific islands to photograph the solar corona in visible (white) light. Since the late 1980s, it has lived in the second-floor lobby of the Mesa Lab between expeditions.

"It's past time for the retirement," said Kim Streander (HAO). The camera "has earned its place as a museum piece," he added. Only a lack of time kept HAO's instrumentation team from building (or finding) a replacement camera for the 1994 Chile eclipse. Technology passed the Newkirk by some years ago, with newer equipment--such as the large-format camera that was taken to Curaçao--offering digital, higher-resolution images that can be calibrated more easily. Further, some of the Newkirk's optical components are degrading because of their age.

Individual parts that are still valuable, such as the camera's lens, may eventually take on a new life by being recycled into future instruments. And like many another retiree, the Newkirk camera won't really be leaving NCAR. It will remain in the lobby, giving visitors a chance to see both a real scientific instrument--with all its well-earned dents and scratches--and a piece of NCAR history. "It truly was a fine instrument," says Kim. •CR

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Edited by Bob Henson, bhenson@ucar.edu

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