Snapping the sun's sharpest x-ray image.

Snapping the sun's sharpest X-ray image

Soaring to 150 miles above the White Sands Missile Range in New Mexico, a NASA sounding rocket on Sept. 11 carried an instrument that took what its chief scientist calls "the sharpest X-ray image so far of a solar flare and the sun's corona."

The photo, one of 40 taken by an instrument called the Normal Incidence X-Ray Telescope, shows details as small as 1 arc-second -- about twice the resolution of previous images. "I did not expect that the corona would everywhere be structured at this resolution," says Leon Golub of the Smithsonian Astrophysical Observatory in Cambridge, Mass. He had anticipated seeing only "fuzzy loops containing hot plasma, along with sharply defined regions of localized heating." Instead, he says, the images "now look as if somebody adjusted the focus knob. Going from 2 to 1 arc-sec shows us an enormous amount of detail we've never seen before."

The photos reveal bright, active regions with details as small as the telescope's resolution limit. These areas represent temperatures as high as 2 million to 3 million kelvins, their sharpness limited only by the clumping of the grains in the film emulsion. A medium-sized solar flare (top arrow) emerging from such a region is probably as hot as 10 million kelvins. A solar prominence, too cool to emit X-rays, outlines a cavity in the corona (lower right arrow). Small, bright features (lower left arrow) appear as portions of loops, and faint coronal plumes show in the north polar region.

Golub notes that Eberhard A. Spiller of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., produced the telescope's mirror, which had to have the capacity to accurately reflect the short (63.5-angstrom) wavelengths of X-rays emitted by multiply ionized iron.

An important factor was the need to time the rocket's launching so that its 5-minute observing period could take place when the sun was active at the proper wavelength. The National Oceanic and Atmospheric Administration's Space Environment Laboratory in Boulder, Colo., helped by providing measurements directly from the GOES 6 and 7 satellites in "real time." Researchers can seldom time rocket firings to fit rapidly changing conditions such as those on the sun. But in this case, Golub says, "the missile range allowed us to wait until T minus 2 minutes, and hold." This meant the rocket could go through most of its countdown, then pause until the sun looked just right, and take off with only 2 minutes' notice.