Outta sight! A crafty peek at the sun's back

Lightest region indicates storm activity on the farside, recorded July 20, 1996. The sun's radiation carves a bubble in the hydrogen gas in which the solar system is embedded. Ultraviolet light from a solar eruption strikes the inside of the bubble's surface, generating a hot spot. Inset: Celestial hemisphere illuminated by ultraviolet radiation from the sun's farside. (European Space Agency, NASA)

 When it comes to studying the far side of the sun, astronomers are no longer in the dark. Researchers reported last week that employing a detector on an orbiting spacecraft, they have had their first glimpse of the sun's hidden half.

 Scientists may be able to use this new capability to provide earlier warnings of solar storms that will strike Earth, damaging satellites and disrupting power grids. Solar storms are expected to be on the rise as the sun reaches the maximum of its 11-year activity cycle next year.

 The new discovery relies on detection of ultraviolet radiation emitted by the hydrogen gas that bathes our solar system. Radiation from the sun clears a bubble in this gas about the size of Earth's orbit, and the inside of the bubble can act like a giant theater screen. When energy emitted by a solar outburst strikes the screen, it produces ultraviolet hot spots.

 An instrument aboard the SOHO (Solar and Heliospheric Observatory) spacecraft can detect these hot spots even from outbursts on the sun's hidden face. "We can monitor the back side of the sun without looking at it directly," says Jean-Loup Bertaux of the CNRS Service d'Aronomie in Verrieres le Buisson, France. He presented the findings at a SOHO workshop in Paris.

"Bertaux's work represents an entirely new way, and to my knowledge the only successful way, to identify the patterns of activity on the far side of the sun," says Craig DeForest of Stanford University.

 Viewing hydrogen gas over the entire sky, a SOHO detector known as Solar Wind Anisotropies (SWAN) records a wavelength of ultraviolet radiation called Lyman alpha. Such light cannot be seen through Earth's atmosphere, but from SOHO's vantage point 1.5 million kilometers from our planet, it readily detects the radiation. When a solar storm erupts, the bubble of hydrogen gas radiates 5 to 15 percent more ultraviolet light than it normally does, says Bertaux.

 Every solar disturbance gets carried along with the sun's 28-day rotation. As a result, each outburst is like a lighthouse beam, sweeping across the screen of hydrogen gas. Bertaux reports, "We have verified that the large areas of enhanced emission that we see are indeed moving on the sky with the 28-day period."

 Bertaux told Science News that his team has shown that storm activity on the sun's near side, imaged directly by another SOHO detector, also produces the expected ultraviolet glow recorded by SWAN. Moreover, when an active region on the near side rotates out of view, SWAN continues to detect an enhanced ultraviolet glow from the hydrogen gas.

 The technique has also uncovered disturbances that originate on the farside and then rotate into Earth's view. Monitoring the sky at the Lyman-alpha wavelength "therefore offers a unique opportunity to detect in advance some new solar activity," Bertaux notes.

 Although the technique is for now only a research tool, it "has the potential to give us a much longer lead time," as much as 14 days, for predicting solar storms, agrees Ron D. Zwickl of the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder, Colo.

 Richard C. Canfield of Montana State University in Bozeman points out that the method cannot distinguish outbursts likely to pose a threat to Earth from those that may turn out to be weaklings. Estimating the power of outbursts requires other diagnostic tools now available only for the sun's near side. These include direct images of twisted magnetic fields in the sun's atmosphere, Canfield says.