Ulysses: a magnetic odyssey, by Jove.

The Ulysses spacecraft swung close to Jupiter during the past two weeks, using that planet's gravity as a slighshot to lift the craft out of the plane in which the planets orbit the sun. The Jovian encounter--a prerequisite for an unprecedented exploration of the sun's polar regions in 1994 and 1995--marks a turning point not only for the spacecraft but also for scientific understanding of Jupiter's magnetic field.

Comparing measurements made by Ulysses--a joint European-U.S. mission--with those from previous missions, researchers have discovered that Jupiter's magnetic field expands and contracts over a period of years. The Ulysses data reveal that the field on the sunward side stretches some 7 million kilometers from Jupiter's core, or about 100 times the planet's radius -- double the distance indicated by the Voyager mission in 1979, but similar to that recorded by Pioneer 10 in 1973.

Scientists presented the new findings last week during a press briefing at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

Several researchers, including Ulysses project scientist Edward J. Smith of JPL, speculate that the density of the solar wind (charged particles streaming from the sun toward the planets) determines the extent of the Jovian field. A drop in solar wind density may allow the magnetic field to expand, Smith says.

Ulysses detected such a drop on Feb. 2, just six hours before it encountered Jupiter's bow shock, the region where the solar wind meets the outer edge of the Jovian magnetic field, reports David J. McComas of Los Alamos (N.M.) National Laboratory, He suggests that Jupiter's magnetic field contracts or expands depending on the solar wind's strength.

The solar wind may exert another influence on the magnetic field. While probing the outer edge of the field, Ulysses identified layers of electrons that appear to come from the sun. Researchers had thought that solar-wind particles could not penetrate the Jovian field; the electron finding suggests that solar particles provide a previously overlooked component of the field, McComas says.

As it approached Jupiter, Ulysses pierced a doughnut-shaped region of ions that girdles the planet. These ions come from Io, one of Jupiter's moons, where volcanoes spew out sulfur, oxygen and sodium atoms. Ionized by intense radiation, the particles are hurled into orbit around Jupiter, following the looping paths of the planet's magnetic field lines.

The Ulysses data indicate that the doughnut's ion density is only half that detected by the Voyager mission. Instead of finding an unbroken ring of charged particles, as the Voyager craft had, Ulysses' radio observations reveal that the doughnut now consists of six "hot spots," where ions cluster, separated by gaps where few ions reside. This suggests that Io's volcanoes have temporarily died down, Smith says.

Ulysses' observations also show that the strength of the magnetic field varies as the field rotates in sync with Jupiter's 10-hour day. In addition, the craft has confirmed a puzzling finding from Pioneer 10: Energetic electrons far from the planet, where the magnetic field is weak, nonetheless rotate along with the field. Such motion, which can create beams of X-rays and radio waves, may help explain light emissions from rotating stars that have large magnetic fields, says K.-Peter Wenzel, Ulysses project scientist for the European Space Agency in Noordwijk, the Netherlands. He adds that researchers now look forward to analyzing the data gathered as Ulysses exited the field on the dusky side of the planet. Ulysses is the first spacecraft ever to probe that region.