swirling around Jupiter at thousands of miles an hour is a huge sheet of electically charged gas and dust that extends outward about two million miles. This plasma sheet rotates like a filmy skirt on a spinning ice-skater. The gas and dust are from volcanoes on the moon Io.

Something must be coupling the sheet's rotation to that of the planet. One theory holds that a kind of electromagnetic friction is at work, as electric current flows through the plasma, along Jupiter's magnetic field and across the polar auroras. In this model, where the auroras meet Jupiter's upper atmosphere, ion winds known as electrojets should be present. A team of researchers using a nasa in Hawaii has now reported the existence of these electrojets, by measuring the shifting wave-lengths of molecular hydrogen. The researchers, writing in Nature, reported that the average speed of the ions was 2.8 km per second.

The researchers note that the friction between these winds and the rest of Jupiter's atmosphere may help explain why the temperature of the upper atmosphere is several hundred degrees higher than would be explained by the effects of sunlight alone ( New York Times ).