Jupiter as mirror for the sun's X rays.

Scientists have tracked down the source for the X rays that stream from Jupiter's midriff. Rather than deriving from the planet itself, those X rays arise in the sun and are reflected by the giant planet's atmosphere, the researchers found.

"It's a very small proportion of the solar X rays which are reflected, but [they] can be seen by sophisticated instruments," says study leader Anil Bhardwaj of NASA's Marshall Space Flight Center in Huntsville, Ala.

Using data from two satellites, XMM-Newton and TIMED, the researchers compared X-ray measurements of Jupiter and the sun over a 3-day period in November 2003. They found that fluctuations in solar X rays were followed by fluctuations in the X rays emanating from Jupiter, with a delay of 2 hours and 20 minutes. That's just the time it took sunlight to travel to Jupiter, bounce off, and reach Earth on those days. X rays and light travel through space at the same speed. The team reports its results in the Feb. 1 Geophysical Research Letters.

A gradual increase in the sun's X-ray output over the 3 days was mirrored in the X-ray intensity of Jupiter's equatorial regions. A spike in the sun's X-ray output from a solar flare on the third day was also matched by Jupiter's X rays.

Astronomers could take advantage of this effect to observe solar flares that would otherwise not be visible from Earth. When flares erupt on the side of the sun not visible from Earth, their X rays could still be detectable if they reflected off Jupiter toward Earth.--D.S.