Violent winds race around the poles of Jupiter at thousands of kilometres per hour. Known as "auroral electrojets", they may reach supersonic speeds and explain why temperatures at the top of the Jovian atmosphere are much higher than expected.

An international team of astronomers used Nasa's Infrared Telescope Facility (IRTF) in Hawaii, to measure the speed of rapidly moving molecular hydrogen ions. In Jupiter's upper atmosphere, electrically charged particles - ions - are accelerated to several kilometres per second.

The poles of Jupiter are ringed by aurorae, like the Earth's Northern and Southern Lights, only a thousand times more powerful. These aurorae trace out a bright oval track around which the fast ion winds flow.

They are produced when energetic particles - mainly electrons - are fired along Jupiter's magnetic field and crash into the upper atmosphere.

Ballerina's skirt

The auroral region is connected to Jupiter's magnetic field though a giant "plasmasheet." This consists of electrically charged gas and dust, which swirl around Jupiter like a spinning ballerina's skirt.

Io pumps up Jupiter's winds

This plasmasheet extends from the orbit of Jupiter's moon Io, some 350,000 kilometres above the planet's surface, outwards for nearly 3 million kilometres, swirling round, as Jupiter rotates, roughly once every 9 hours 55 minutes.

Most of the plasma in the sheet comes from the highly active volcanoes on Io and then drifts outwards into space.

"You need a lot of energy to keep that plasmasheet rotating along with Jupiter," explains Dr. Stephen Miller of University College London. "At the rate that Io is pumping out gas and dust - about 1 tonne per second - we estimate that up to ten million megawatts of power is required."

A thousand degrees

The group's technique for detecting the Jovian electrojet consisted of carefully measuring the wavelength of lines of certain ions using the spectrometer on the IRTF telescope.

Jupiter seen by the IRTF

The friction between the electrojet and the rest of Jupiter's atmosphere also produces a great deal of energy, which can go into heating the rest of the planet.

This may help explain why the temperature near the top is around a thousand degrees, several hundred degrees hotter than can be maintained by sunlight alone.

"Although Jupiter is one of the best studied of the planets - the Galileo orbiter will have been circling the planet for four years by the time its mission finishes at the end of the year - it still has many secrets and many puzzles to solve. Understanding the dynamics of Jupiter is the key to unravelling many of these," Dr. Miller comments.

The research is published in the journal Nature.

Main image from the BBC Science TV series The Planets