On February 20,
2007, NASA put out information--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?
Right: Ulysses over the sun's
South Pole, an artist's concept. Credit: ESA.
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."
According to SWICS, the average temperature of the
sun's polar wind is about one million degrees C.
But over one pole the wind is about 80,000 degrees
cooler than over the other pole.
Researchers believe the solar wind at Ulysses is
telling them something about polar conditions
close to the surface of the sun. "The solar wind
comes from the poles," explains Arik Posner,
Ulysses Program Scientist at NASA headquarters.
"The sun's magnetic field opens up over the poles
and allows some of the sun's atmosphere to
escape." These openings are called "coronal
holes," and the hot atmosphere that rushes out is
the solar wind.
Back to the original question: What does the
temperature difference mean? "Perhaps the
structure of the sun's atmosphere over the two
poles is different," he speculates.
We have an analogy here on Earth. The stratosphere
over the South Pole is colder, on average, than
the stratosphere over the North Pole. The reason
has to do with the uneven distribution of land on
Earth (most land is in the north) plus complex
atmospheric circulation patterns.
In the case of the sun, the difference is not land
but magnetism. Apparently, something about the
sun's north magnetic pole keeps the solar
atmosphere above it a trifle cooler. Proof: The
"cool spot" follows the north magnetic pole when
the sun's poles flip.
Above: Solar wind temperatures in
millions of degrees Kelvin measured by Ulysses.
Credit: R. von Steiger and G. Gloeckler. [Larger
image]
"The sun's magnetic poles have reversed polarity
since the 1994 flyby—an effect of the 11-year
sunspot cycle," notes Posner. Lo and behold, "the
temperature asymmetry has also reversed. So it
appears to be a magnetic phenomenon."
When Ulysses finishes its current South Pole
flyby, it will proceed to the other end of the sun
for a North Pole flyby in early 2008. This will
provide more clues to what's shaping up to be a
very cool solar mystery.
Credit: Science@NASA
