The size of Saturn's magnetosphere is determined by external pressure of the solar wind. When Voyager 2 entered the magnetosphere, the solar-wind pressure was high and the magneto- sphere extended only 19 Saturn radii (1.1 million kilometers or 712,000 miles) in the Sun's direction. Several hours later, however, the solar-wind pressure dropped and Saturn's magneto- sphere ballooned outward over a six-hour period. It apparently remained inflated for at least three days, since it was 70 percent larger when Voyager 2 crossed the magnetic boundary on the outbound leg.

Unlike all the other planets whose magnetic fields have been measured, Saturn's field is tipped less than one degree relative to the rotation poles. That rare alignment was firstmeasured by Pioneer 11 in 1979 and was later confirmed by Voyagers 1 and 2.

Saturn's ribbonlike cloud structure (Click on the image for a larger view)

Several distinct regions have been identified within Saturn's magnetosphere. Inside about 400,000 kilometers (250,000 miles) there is a torus of H+ and O+ ions, probably originating from water ice sputtered from the surfaces of Dione and Tethys. (The ions are positively charged atoms of hydrogen and oxygen that have lost one electron.) Strong plasma-wave emissions appear to be associated with the inner torus.

At the outer regions of the inner torus some ions have been accelerated to high velocities. In terms of temperatures, such velocities correspond to 400 million to 500 million Kelvins (700 to 900 million degrees Fahrenheit).

Outside the inner torus is a thick sheet of plasma that extends out to about 1 million kilometers (600,000 miles). The source for material in the outer plasma sheet is probably Saturn's ionosphere, Titan's atmosphere, and the neutral hydrogen torus that surrounds Titan between 500,000 kilometers (300,000 miles) and 1.5 million kilometers (1 million miles).

Radio emissions from Saturn had changed between the encounters of Voyager 1 and 2. Voyager 2 detected Jupiter's magnetotail as the spacecraft approached Saturn in the winter and early spring of 1981. Son afterward, when Saturn was believed to be bathed in the Jovian magnetotail, the ringed planet's kilometric radio emissions were undetectable.

During portions of Voyager 2's Saturn encounter, kilometric radio emissions again were not detected. The observations are consistent with Saturn's being immersed in Jupiter's magnetotail, as was also the apparent reduction in solar-wind pressure mentioned earlier, although Voyager scientists say they have no direct evidence that those effects were caused by Jupiter's magnetotail.