archive: SETI [ASTRO] Jupiter's Supersonic Winds

SETI [ASTRO] Jupiter's Supersonic Winds

Larry Klaes ( )
Thu, 13 May 1999 13:40:35 -0400

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>Date: Wed, 12 May 1999 20:33:08 GMT
>From: Ron Baalke <>
>Subject: [ASTRO] Jupiter's Supersonic Winds
>Reply-To: Ron Baalke <>
>Jupiter's supersonic winds
>May 12, 1999
>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,
>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
>"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