An artist's concept of Ulysses. Photo: ESA

Space physicists predict gusty winds for the next few months at the Sun's north pole, an area that will be observed when the Ulysses spacecraft passes over it starting on Aug. 31.

This pass over the pole occurs at a time of solar maximum, when the Sun is more active. "The solar wind is patchier -- gustier -- during solar maximum, because the sources of the wind are more irregular," said Dr. Edward Smith, the project scientist at NASA's Jet Propulsion Laboratory on Ulysses, which is managed jointly by JPL and the European Space Agency.

This will be Ulysses' second pass over the Sun's north pole. It completed a circuit of the Sun in 1996 when the Sun was less active, then passed by the Sun's south pole last fall when the Sun was much more active. Now scientists will use what they've learned from previous passes to analyze data in the coming months.

Earth's magnetic field reverses itself only once every 200,000 years or so, but the Sun's internal magnet flips every 11 years, associated with a rise in solar activity. As it flips, the magnetic field becomes disordered, and the surface of the Sun becomes more active, shooting out bubbles of gas and energy called coronal mass ejections in every direction. The reversal of the magnetic field has a particularly dramatic effect on the poles. The Sun's magnetic field will be weak and X-ray coronal holes at the north pole should have shrunk during this solar maximum, because the magnetic field is unevenly spread around the surface of the Sun.

In 1995, Ulysses saw strong and simple magnetic fields at both poles of the Sun. Large, dark X-ray coronal holes formed in the solar atmosphere over the polar caps. The solar wind is fast and hot when coronal holes in the Sun remain open over long periods of time, as they do at solar minimum.

As Ulysses passed by the south pole of the Sun a few months ago, scientists expected to find that magnetic lines were pointing outward, because observations from Earth show that the magnetic field has already reversed at the Sun's surface. Instead, they found that the magnetic lines were still pointing inward, just as they had been throughout solar minimum. They theorize that this could be caused by the magnetic lines forming small, closed loops next to the pole instead of escaping into space. For now, the magnetic field and solar wind seen over the pole appear to originate near the Sun's equator rather than from the polar cap.

The Ulysses science team will also study energetic charged particles, which speed away from the Sun's surface and escape into space. These are a regular feature of solar maximum. "These energetic solar particles can be as hot as 100 billion Kelvin (180 billion Fahrenheit) and can cause damage to spacecraft," said Dr. Bruce Goldstein, deputy project scientist on Ulysses at JPL. "However, they still follow the magnetic field lines."

At solar minimum, Ulysses saw very few particles coming from the poles. This was because there were fewer particle events occurring on the Sun and also because the particles follow magnetic lines of force that did not reach into the Sun's polar regions.

As it studied the south pole during solar maximum, Ulysses saw energetic particles from many of the past year's solar flares, though they originated at the Sun's equator. Those observations are further evidence that the magnetic field and solar wind seen at the polar caps really originate close to the equator.

Ulysses, launched in 1990, is a joint venture of NASA and the European Space Agency. JPL manages Ulysses for NASA's Office of Space Science, Washington, D.C.