Apr 2, 1998

Solar Images to be made by unique X-ray telescope

Solar Images to be made by unique X-ray telescopes

April 2, 1998: A unique cluster of telescopes that make X-rays take a U-turn has been selected

Solar image
for a fourth flight to capture "multicolored" images that will help us understand why the sun's outer atmosphere is so hot.

Right: The Sun as seen in the glow of highly ionized iron. Such images are really taken in black and white. Scientists assign them false colors to help in studying different images.

"One of the major objectives is to follow up on something we saw on the first flight 10 years ago," said Dr. Arthur B.C. Walker II of Stanford University, the principal investigator for the Chromospheric/Corona Spectroheliograph telescope. It will actually be a bundle of up to 19 telescopes, each taking pictures of the sun in a slightly different X-ray energy. The array is an upgrade of the Multi-Spectral Solar Telescope Array (MSSTA) which flew on October 23, 1987, May 13, 1991, and November 3, 1994.

The 1987 flight - which also made the September 30, 1988 cover of Science magazine - returned pictures that showed where the sun's atmosphere was as hot as 1 million deg. K (about 1.8 million deg. F) also showed spectral lines that indicated temperatures of about 700,000 deg. K (1.26 million deg F).

"We were mystified by this," Walker said. "We are now convinced that there is material at about 700,000 degrees K in the transition region and which contributes to coronal heating."

NASA recently selected the Chromospheric/Corona Spectroheliograph under the solar physics research program. Richard Hoover of NASA's Marshall Space Flight Center and Troy W. Barbee of Lawrence Livermore National Laboratory are co-investigators with Walker. Their project is entitled Investigation of the Corona/Chromosphere Interface.

This is the same region that will be studied by the Transition Region and Coronal Explorer (TRACE) scheduled for launch Thursday evening from California. The Chromospheric/Corona Spectroheliograph will complement TRACE by providing images of solar gases at temperatures as high as 5 million degrees K (9 million deg. F).

While the sun is more than 99.9 percent hydrogen and helium, it carries significant quantities of carbon, iron, calcium, silicon, and other elements. Heavier elements have more protons (carbon is 6, iron is 26) in their nuclei than do lighter elements (hydrogen is 1, helium is 2). That means that as electrons are stripped from heavier atoms, the charge of the larger number of protons is devoted to the few remaining electrons. It takes ever more energy to strip off another electron.

As a result, light from energetic atoms acts like a tracer that reveals where the sun is hot and at what temperatures. This is important to dissecting activities from the sun's corona - its outer atmosphere - through the transition region and to the chromosphere and photosphere - the visible "surface."


MSSTA composite



Hydrogen Lyman-alpha, H I 121.6 nm, 20,000 K (11,000 deg. F)


Ionized helium, He II 30.4 nm, 80,000 K (44,000 deg. F)


Ionized iron, Fe XIV 21.1 nm, 1,800,000 K (1,000,000 deg F)


Ionized iron, Fe XII 19.3 nm, 1,500,000 K (833,000 deg. F)


Ionized silicon, Si XII 4.4 nm, 2,000,000 K (1,100,000 deg. F).


White light (comparison image by Big Bear Solar Observatory)
800x600-pixel, 200K GIF copies of these individual images are available at the MSSTA web site.

The challenge is that the X-ray emissions are so energetic that they pass through materials rather than being reflected as visible light would be. The usual trick to making X-ray images is called grazing incidence reflection. Just as light will reflect off clear glass (or a rock will skip on a pond) if it strikes at a shallow angle, X-rays will reflect - and be focused - if they, too, strike at an even shallower angle.

Several X-ray telescopes, such as, the Advanced X-ray Astrophysics Facility use this.

The MSSTA works by a different effect. Its multi-layer mirrors comprise an ultrasmooth mirror coated by up to 100 layers of heavy elements like tungsten spaced by layers of lightweight elements like carbon. In effect, the layers work like a Bragg crystal, which will reflect X-rays. Everything is extremely smooth, on the order of 0.1 nm (a 10 billionth of a meter, or 1/250 millionth of an inch). These reflect a little bit of the X-rays at the surface of each layer pair.

The choice of materials and the thickness of the layers determine precisely which wavelength is making the X-rays interfere with each other reflection. In this way, the scientists can fine tune a telescope to observe in a narrow band of wavelengths (a spectral band) or even one wavelength. That makes it possible to measure the temperature of the solar atmosphere. To observe the sun in several wavelengths at once, several telescopes must be flown together.

This unique approach makes it possible to use conventional optical layouts - like the Hubble Space Telescope's Ritchey Chretien design - and get a much larger collecting area and brighter images than are possible grazing incidence optics of the same size. The design was invented by Barbee (and separately by scientists at IBM) and pioneered by Barbee, Walker, and Hoover for use in telescopes.


MSSTA group shot
The MSSTA (right) carries up to 19 telescopes of various sizes, each with a filter designed to admit only radiation of a specific wavelength or wavelength band, each corresponding to a specific temperature in the sun's atmosphere. Even though each image is taken in black-and-white, each represents a different wavelength and a different temperature in the solar atmosphere. To help in studying them, scientists often give them false colors to distinguish one from the other. This is similar to a color print that is really made from four black-and-white negatives, each to print a different color.

On its fourth flight, the array will include a telescope that can see FE XVII; iron stripped of 9 of its 26 electrons. That takes temperatures up to 5 million deg. K.

"It would be a better indicator of the distribution of high-temperature gases in the solar atmosphere," Walker said. This may also reveal small flares that may be one source of energy being pumped into the corona.

For the C/CS flight, expected by early 2000 near the around the time of solar maximum. MSSTA will be upgraded and some new telescopes and detectors installed. As with its first two flights, the telescope will be boosted by a Terrier Black Brant IX launched from the White Sands Missile Range, N.M. The C/CS payload will be boosted to an altitude of 230 km (144 mi) and fall then parachute back to Earth for recovery. During the coast above Earth's atmosphere, the telescope array will be pointed precisely at the sun for about 6 minutes. Each telescope will take 10 to 15 full-disk images. Ground-based observatories will take pictures at the same time in white light and H-alpha, and with telescopes equipped to map magnetic fields.

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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack