First clear images of solar flux tubes - relation of magnetically active areas and sun spots - Brief Article

Science News, Sept 26, 1992 by Kathryn Hoppe

Using a new technique to examine the sun, a Swiss astronomer has obtained high-resolution images that reveal a solar surface covered with small-scale, magnetically active regions related to the more extensive sunspots.

Researchers have long theorized about the form and evolution of these features, called magnetic flux tubes, but have been unable to directly observe them in detail until now, says Christopher U. Keller of the Institute for Astronomy in Zurich. In the Sept. 24 NATURE, Keller announces the first simultaneous observations of the visible-light and magnetic-fields signatures of flux tubes.

He obtained the images using a modification of an observing method known as speckle interferometry. The new technique should reveal details of the energy dynamics of the sun, which affect such things as radio communications and electrical power transmission on Earth, explains Carol Jo Crannell, a solar physicist at NASA's Goddard Space Flight Center in Greenbelt, Md.

Flux tubes represent concentrated regions of intense magnetic activity similar to that observed in sunspots (SN: 6/20/92, p.404). However, unlike the massive sunspots, whose diameters often span several thousands of kilometers, flux tubes reach less than a few hundred kilometers across. While these areas may account for more than 90 percent of the magnetic flux outside of sunspots, their small size has prevented researchers from directly observing them in detail, Keller says.

Turbulence in Earth's atmosphere blurs images of the sun, limiting the resolution of observations from ground-based telescopes. Traditionally, astronomers have eliminated some of this blurring in visible-light images by using speckle interferometry, which creates an image composed of many short exposures. By modifying this technique to include magnetic flux data, Keller observed individual flux tubes with diameters as small as 200 kilometers. He found that such small tubes correspond to previously observed bright spots on the solar surface, whereas many tubes larger than 300 kilometers in diameter appear darker than their surroundings. Keller notes that flux tubes seem to evolve rapidly, changing noticeably within 15 minutes.

He made his observations using the 50-centimeter Swedish Vacuum Solar Telescope at La Palma, in the Canary Islands.

Such findings may help astronomers determine the exact relationship between flux tubes and sunspots, says Douglas M. Rabin, an astronomer with the National Solar Observatory in Tucson, Ariz. Rabin says the new study represents "a promising line of investigation," which may finally verify theories that have been debated for decades.