One
pole of the
sun is cooler than the other. That's the surprising conclusion announced today by scientists who have been analyzing data from the ESA-NASA Ulysses spacecraft. Ulysses is the only ship in the NASA or European fleet capable of flying over the sun's poles, a result of the spacecraft's uniquely-tilted orbit. Its ability to study the sun's unexplored polar regions is prized by
solar physicists. Ulysses' first polar flybys in 1994 and 1995 revealed the asymmetry—a 7 to 8 percent difference in temperature,
says Ulysses science team member George Gloeckler of the University of Maryland. The measurement was both mysterious and a little hard to believe. What would make the sun lopsided in this way? There's still no definitive answer to that question, but now at least researchers know the effect is real. Ulysses has returned to the sun's South Pole in 2007 and "recent observations show that the average
temperature ... is virtually identical to what we saw 12 years ago, says Gloeckler. Taking the sun's temperature is tricky business. The
spacecraft can't descend to the surface and insert a thermometer. Instead, Ulysses samples the solar
wind at a safe remove of 300 million km. "We measure the abundance of two oxygen ions found in the solar wind. The ratio O6+/O7+ tells us the temperature of the gas," explains Gloeckler. He is the principal investigator of the instrument onboard Ulysses that does this, the Solar Wind Ion Composition Spectrometer or SWICS.
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