Jovian auroras in the infrared ...

It was a dark and stormy night.

On Aug. 8, 1997, astronomers at NASA's Infrared Telescope Facility on Mauna Kea in Hawaii were taking routine observations of Jupiter when a magnetic storm struck the distant planet's polar regions. The auroras that grace Jupiter's poles--souped-up versions of the northern and southern lights that shimmer in Earth's upper atmosphere--intensified. For the first time, researchers detected winds of charged particles whipping around the poles like cars around a racetrack.

Known as electrojets, these high-speed winds may explain how energy from the auroral regions spreads around the planet. This energy maintains temperatures throughout the upper atmosphere that are hundreds of degrees higher than what the meager sunlight shining on Jupiter could ever produce.

Jovian auroras are fueled by charged particles, mostly electrons, belched by the planet's volcanically active moon, Io. Captured by the planet's powerful magnetic field, these particles crash into the atmosphere a few hundred kilometers above Jupiter's magnetic poles, where they collide with hydrogen molecules. The battered molecules collect into oval patches centered on the poles and emit ultraviolet light.

Slightly higher in the atmosphere, hydrogen molecules ionized by the incoming electrons combine with hydrogen atoms to become ionized triatomic hydrogen ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]). Swept around by the planet's magnetic field, these triatomic ions form the bulk of the circulating electrojet stream. By analyzing the infrared glow from these ions, Steve Miller of University College London and his colleagues measured the velocity of the electrojets. During the 1997 storm, they whirled faster than the speed of sound, the team reports in the May 13 Nature.

The electrojets rotate in the direction opposite that of the planet, causing friction between these winds and the rest of the atmosphere. This generates heat that can be transported around the planet by small whirlpools and eddies, Miller suggests. He notes that although [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is only a tiny constituent of the Jovian atmosphere, it easily imparts its energy to the much larger population of neutral hydrogen molecules.

Electrojets may also explain another phenomenon. Like a twirling ballerina's skirt, a giant sheet of charged particles spins around Jupiter, keeping pace with the planet's rotation. That requires a lot of energy. Miller suggests that the electrojets siphon rotational energy from Jupiter, electrically transferring it to the giant sheet.