Today (27 November), Ulysses becomes the first space probe to fly over the south pole of
the Sun twice. The European spacecraft has reached a maximum latitude of 80.1 degrees south.
The international teams of scientists working with the mission are raising their glasses to toast
the intrepid solar explorer only weeks after celebrating the 10th anniversary of its launch.
The first visit took place on 13 September 1994 when the Sun was settling down to an extended period of calm known as solar minimum. Things are quite different this time around. Powered by turbulent magnetic fields below the surface, the Sun’s 11-year activity cycle is now peaking. And this is exactly what the scientists wanted. “The Sun is the only star we can study at close quarters,” says Richard Marsden, the European Space Agency’s project scientist for Ulysses. “We need to get to know it in all its moods.”
By comparing measurements made over the solar pole six years ago with the new data, scientists are hoping that more pieces of the complex puzzle that is the Sun will fall into place. The prospects look good. “This time around, Ulysses has not seen the very fast solar wind streams that were so typical of the polar regions at solar minimum,” says Marsden.
Instead of reaching the gale-force speeds of 750 km/s seen earlier, the fastest gusts of solar wind measured by Ulysses now are barely topping 600 km/s. “By tracing this intermediate solar wind back to its source on the Sun, we will gain a much better picture of the conditions in the solar atmosphere that give rise to these different flows,” says Marsden.
As if it knows that it has a special visitor, the Sun put on a spectacular firework show on 8 November. The solar storm, which sprayed ESA’s SOHO spacecraft and the Earth with one of the most intense bursts of energetic particles since 1976, was recorded by Ulysses’ instruments, even though the spacecraft was already high above the Sun’s south pole.
Ulysses’ unique perspective is helping to unravel the processes that give rise to such violent outbursts of ‘space weather’. Such understanding is important, since bombardment by high energy particles can pose a threat to satellites and astronauts in space.
Ulysses’ journey of exploration is far from over. Following the completion of the second polar pass in January, the space probe will swing back down to the ecliptic plane on its return to high northern latitudes in October 2001. By then, solar activity will be declining and the Sun’s magnetic field should have reversed its polarity. Under these conditions, no one can predict what Ulysses will find. “Whatever it is, we’re looking forward to finding out,” says Marsden. In the meantime, there are still plenty of questions to be answered about the Sun’s south polar regions.
