Hubble images Jupiter's aurora.

Last February, while the Ulysses spacecraft probed the magnetic field surrounding Jupiter (SN: 2/22/92, p.118), the Hubble Space Telescope recroded the first ultraviolet image of an aurora above Jupiter. The image represents the sharpest picture of this Jovian phenomenon ever taken at any wavelength.

Auroras occur when charged particles strike the magnetic field surrounding a planet and then sprial inward toward the planet's magnetic north and south poles. Above Earth, charged particles from the solar wind trigger auroras. In the case of Jupiter, astronomers believe that its volcanically active moon, Io, provides the charged particles. Io spews out sulfur and oxygen atoms that become ionized and fall under the influence of Jupiter's intense magnetic field.

The oval aurora imaged by Hubble corresponds roughly to the region where Jupiter's magnetic field lines would pass through Io's orbit and enter the Jovian atmosphere. Thus, the picture offers supporting evidence that Io indeed supplies the ions that creat Jupiter's aurora, says John Caldwell of York University in North York, Ontario.

Researchers can't fully explain why the western edge of the aurora appears brighter, he adds. But the effect may stem partly from the time of day -- late afternoon on the planet -- when Hubble's faint-object camera took the picture, Caldwell notes. At that time, the orientation of the solar wind pushing on Jupiter's magnetic field may allow charged particles to penetrate one section of the Jovian atmosphere more easily, causing part of the aurora to glow more brightly.