Ulysses spacecraft flies over Sun's north pole

The Ulysses spacecraft made a rare flyby on 14 January of the sun's north pole. 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 video clip obtained by SOHO's EIT instrument, was taken in extreme ultraviolet light and covers about 24 hours. It shows two 'EIT waves', a kind of solar storm that blasts out from an active region across a portion of the Sun's surface. The two waves originated in the sunspot whose appearing marked the start of the new solar cycle ('Cycle 24') on 4 January 2008. The first, smaller wave was observed on 6 January 2008, while a stronger wave was observed the next day. Credits: SOHO/EIT (ESA/NASA)

"This important milestone for the joint ESA-NASA mission also coincides with the start of a new cycle of solar activity", explains Richard Marsden, ESA's Ulysses Mission Manager. "It's been calm on the space weather front recently and so we are looking forward to some solar fireworks over the coming months as the number of sunspots increases."

As part of an impressive fleet of interplanetary spacecraft that includes the twin STEREO probes launched by NASA in 2006 and ESA's SOHO observatory, Ulysses is ideally placed to provide a unique perspective as our star gears up for the next peak in its activity cycle. This is expected to occur around 2012.

Although the Sun has been relatively quiet during the past year, this has not meant that the spacecraft operations team has been able to sit back and relax. On the contrary, the rapid transit from south to north through perihelion is one of the most nerve-racking periods in the 6.2-year orbit of Ulysses, now in its 18th year of operations. This is because the spacecraft experiences a dynamic disturbance as it comes closer to the Sun, causing the spinning probe to wobble, a motion called 'nutation'.

"We have to keep the nutation under control", says Marsden, "otherwise we could lose contact with the spacecraft. Fortunately, the operations team has developed a robust strategy to tackle this problem, and there haven't been too many scary moments this time around!" In fact, this strategy is so effective that the scientific measurements have hardly been affected. Nutation is expected to die away over the next few weeks as Ulysses once again heads away from the Sun.

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, Calif.

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 Oct. 1990 from the space shuttle Discovery, Ulysses is a joint mission of NASA and the European Space Agency.

Source: JPL / ESA
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