JUPITER'S SUPERSONIC WINDS
Violent winds race around the poles of Jupiter at thousands of miles an hour like cars round a racetrack, sometimes reaching supersonic speeds. And these winds - known as "auroral electrojets" - may help to explain why temperatures at the top of the jovian atmosphere are much higher than would be expected for a planet five times farther away from the Sun than Earth.
Writing in this week's 'Nature', an international team of astronomers report that the first detection of the auroral electrojet on Jupiter, in which electrically charged molecules - ions - are accelerated by electro-magnetic forces to an average 2.8 kilometres per second. Co-ordinated by Dr. Steve Miller, of the Department of Physics and Astronomy at University College London, the team used Nasa's Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, to measure the speed of rapidly moving molecular hydrogen ions, H3+.
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.
The auroral region of the atmosphere links though the magnetic field to a giant "plasmasheet", consisting of electrically charged gas and dust, which swirls around Jupiter like a spinning ballerina's skirt. 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. Miller. "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.
"What is happening is that the plasmasheet is siphoning off some of the reservoir of rotational energy that is stored up in Jupiter. Our discovery of the auroral electrojet shows how the plasmasheet couples to the planet by a sort of electromagnetic friction, which involves electric currents flowing through the plasmasheet, along Jupiter's magnetic field and then closing the switch across the aurorae. We've had a model that predicted this for some while, but now we really know it's true."
The Group's technique for detecting the jovian electrojet consisted of carefully measuring the wavelength of lines of H3+ emission using the spectrometer on the IRTF telescope. The rapid motion of these ions in the electrojet caused their lines to be "Doppler shifted".
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 and helps explain why the temperature near the top is around 1000K, 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.
'Supersonic Winds in Jupiter's Aurorae', published in the 13 May 1999 issue of 'Nature', is by:
Daniel Rego (1,2), Nicholas Achilleos (1), Tom Stallard (1), Steve Miller (1), Renee Prange (2), Michele Dougherty (3) and Robert D. Joseph (4).
(1) Department of Physics and Astronomy University College London, Gower Street, London WC1E 6BT, UK
(2) Institut d'Astrophysique Spatiale UMR-CNRS 120, Batiment 121 Universite de Paris XI, 91405 Orsay Cedex, France
(3) Space and Atmospheric Physics Imperial College, London SW7 2BZ, UK
(4) Institute for Astronomy University of Hawaii, Woodlawn Drive, Honolulu, HI 96822, USA