Visit Comes After New Solar Cycle Detected
NASA's Ulysses spacecraft made a
rare flyby of the sun's north pole Monday. Unlike any other
spacecraft, Ulysses is able to sample winds at the sun's poles,
which are difficult to study from Earth.
Ulysses has flown over the sun's poles three times before, in
1994-95, 2000-01 and 2007. Last week, solar physicists announced
the first indications of a new solar cycle. Visiting the pole at
this time may lead to new insights about solar activity.
"This is a wonderful opportunity to examine the sun's north pole
within a transition of cycles," said Arik Posner, Ulysses program
scientist at NASA Headquarters in Washington. "We've never done
this before."
Many researchers believe the sun's poles are central to the
11-year ebb and flow of solar activity. When sunspots break up,
their decaying magnetic fields are carried poleward by vast
currents of plasma. This makes the poles a sort of graveyard for
sunspots. Old magnetic fields sink beneath the polar surface
200,000 kilometers deep (about 124,000 miles), all the way down to
the sun's inner magnetic dynamo, which generates the solar magnetic
field. There, dynamo action amplifies the fields for use in future
solar cycles.
"Just as Earth's poles are crucial to studies of terrestrial
climate change, the sun's poles may be crucial to studies of the
solar cycle," said Ed Smith, Ulysses project scientist at NASA's
Jet Propulsion Laboratory, Pasadena, CA.
Each previous flyby revealed something interesting and
mysterious. One puzzle has been the temperature of the sun's poles.
In the previous solar cycle, the magnetic north pole was about
80,000 degrees Fahrenheit (more than 44,000 degrees Celsius), or 8
percent cooler than the south. The current flyby may help solve
this puzzle because it comes less than a year after a similar south
pole flyby in Feb. 2007. Mission scientists will be able to compare
temperature measurements, north versus south, with hardly any gap
between them.
Ulysses also discovered the sun's high-speed polar wind. At the
sun's poles, the magnetic field opens up and allows solar
atmosphere to stream out at a million miles per hour. By flying
around the sun, covering all latitudes in a way that no other
spacecraft can, Ulysses has been able to monitor this polar wind
throughout the solar cycle and has found that it is acting a bit
odd.
"Twelve years ago, just before the previous 'sea change' between
solar cycles, the polar wind spilled down almost all the way to the
sun's equator. But this time it is not. The polar wind is bottled
up, confined to latitudes above 45 degrees, " said Posner.
Launched in October 1990 from the space shuttle Discovery,
Ulysses is a joint mission of NASA and the European Space
Agency.