The Ulysses spacecraft passes over the sun's north pole

Ulysses has passed the Sun's poles three times before, in 1994-1995, 2000-2001 and 2007. Last week, solar physicists announced initial signs of a new solar cycle. A visit to the Pole during this period may lead to new discoveries about solar activity

The Ulysses spacecraft made a rare flyby over the sun's north pole in recent days. Unlike any other spacecraft, Ulysses can sample winds at the poles of the Sun, which is difficult for an explorer from Earth.

Ulysses has passed the Sun's poles three times before, in 1994-1995, 2000-2001 and 2007. Last week, solar physicists announced initial signs of a new solar cycle. A visit to the Pole during this period may lead to new discoveries about solar activity.

"This is a wonderful opportunity to examine the North Pole of the Sun during transition between cycles," says Eric Posner, Ulysses Project Scientist at NASA Headquarters in Washington. "We never did."

Many researchers believe that the solar poles are among the key factors in understanding the ebb and flow cycle of solar activity that is renewed every 11 years. When sunspots break up, their waning magnetic fields are pulled poleward by huge streams of plasma. This turns the poles into sort of sunspot cemeteries. Old magnetic fields sink about 200,000 km below the polar surface, all the way to the Sun's internal magnetic dynamo, which creates the Sun's own magnetic field. There dynamo activity amplifies the fields for use in future solar cycles.

"Just as the NASA polars are essential to global climate change studies, the solar polars may be essential to understanding solar cycles," says Ed Smith, Ulysses project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.

Each previous passage over the poles revealed something interesting and mysterious. One puzzle was the temperature of the sun's poles. In the previous solar cycle, the north magnetic pole was about 44,000 degrees Celsius, or about 8 percent colder than the south pole. This flight over the pole may solve the puzzle, because it comes less than a year after a similar flight over the South Pole that took place in February 2007. Mission scientists will be able to compare temperature measurements, north versus south, with almost no time gap between them.

Ulysses also discovered the Sun's fast polar wind. At the sun's pole, the magnetic field opens up and allows the solar atmosphere to flow out at a speed of over a million kilometers per hour. By flying around the Sun, covering all latitudes in a way that no other spacecraft can, Ulysses was able to track this polar wind throughout the solar cycle and found it behaving a little strangely.

"Twelve years ago, before the previous 'sea changes' in the solar cycle, the polar wind glided almost all the way to the Sun's equator. But this time she didn't. The solar wind is confined to latitudes above 12 degrees, Posner says.

Ulysses was launched in October 1990, from the space shuttle Discovery, as a joint project of NASA and the European Space Agency.

On the same topic on the science site

The Ulysses spacecraft flies over the sun's south pole (February 2007)