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Saturn's auroras defy scientists' expectations

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Press release issued by the Particle Physics and Astronomy Research Council (PPARC)

For immediate release
Thursday 17 February 2005

Results which combine data from the joint NASA/ESA/ASI Cassini Huygens space mission and the Hubble Space Telescope, published in Nature today (17th February 2005), reveal that Saturn's auroras, long thought to be a cross between those of Earth and Jupiter, are in fact different and may even be unique to Saturn.

Over several weeks Hubble snapped ultraviolet images of Saturn's aurora, whilst Cassini's radio and plasma wave science instrument (RPWS) recorded radio emissions from the same regions and the Cassini Plasma Spectrometer (CAPS) and Magnetometer (MAG) instruments measured the solar wind. All measurements were combined to reveal the most accurate glimpse yet of Saturn's auroras and the role of the solar wind in generating them.

The observations show that Saturn's auroras vary from day to day, as they do on Earth, moving around on some days and remaining stationary on others. But compared to Earth, where dramatic brightening of the auroras lasts only 10 minutes, Saturn's can last for days.

Professor Michele Dougherty from Imperial College London is Principal Investigator for the Magnetometer instrument on Cassini and co-author on all three papers.

She explains more:

"These latest findings show that the Sun's magnetic field and the solar wind play a much greater role in Saturn's auroras than previously thought. Cassini measurements show that the auroras sometimes move along with Saturn as it spins on its axis whilst the Hubble images show that some displays remain still while the planet rotates beneath. This difference indicates that Saturn's auroras are driven in an unexpected manner by the Sun's magnetic field and the solar wind and in particular one big surprise is the fact that the magnetic field embedded in the solar wind plays a much smaller role at driving the aurora than we expected it to."

Dr Andrew Coates from the Mullard Space Science Laboratory, who heads up the team responsible for the electron spectrometer (part of CAPS) on Cassini, is a co-author on one of the papers along with his colleague, Dr Abigail Rymer.

He says: "With our new results we are re-writing the textbooks on how the solar wind controls Saturn's beautiful aurora. Unlike the Earth the solar wind pressure dominates in driving Saturn's dynamic magnetosphere. It's like pushing a balloon on one side and the balloon distorts. This seems to be the dominant effect on Saturn."

Seen from space, an aurora appears as a ring of energy circling a planet's polar region. Auroral displays are spurred when charged particles in space interact with a planet's magnetosphere and stream into the upper atmosphere. Collisions with atoms and molecules produce flashes of radiant energy in the form of light. Radio waves are generated by electrons as they fall toward the atmosphere.

The team observed that even though Saturn's auroras do share characteristics with other planets, they are fundamentally unlike those on either Earth or Jupiter. When Saturn's auroras become brighter and thus more powerful, the ring of energy encircling the pole shrinks in diameter. Saturn's auroras become brighter on the sector of the planet where night turns to day as the storms increase in intensity, unlike either of the other two planets. At certain times, Saturn's auroral ring is more like a spiral, its ends not connected as the energy storm circles the pole.

Professor Stan Cowley from University of Leicester is a Co-Investigator on the Magnetometer instrument on Cassini. His team is involved in considering the theoretical consequences of these results.

"We are particularly interested in how the plasma dynamics in Saturn's magnetosphere associated with the compression produces an auroral spiral. This work has clear implications for the interpretation of future Cassini data and will centrally inform our thinking on plasma dynamics at Saturn for some considerable time to come."

The new results do show an aspect of Saturn's aurora that matches Earth's however. Radio waves appear to be tied to the brightest auroral spots. Dr William Kurth from the University of Iowa explains, "We know that at Earth, similar radio waves come from bright auroral arcs, and the same appears to be true at Saturn. This similarity tells us that on the smallest scales, the physics, which generates these radio waves is just like what goes on at Earth, in spite of the differences in the location and behaviour of the aurora."

These findings appear in three papers published in the February 17th issue of Nature (see notes to editors for further details).

Links to releases and images
Hubble Space telescope -
Boston University -

For latest images and further information about the Cassini-Huygens mission see:-

Gill Ormrod - PPARC Press Office
Tel: 01793 442012. Mobile: 0781 8013509.

Carolina Martinez - JPL Press Office
Tel: 00 1 818 354 9382

Professor Michele Dougherty, Principal Investigator on Magnetometer, Imperial College London
Tel: 020 7594 7757

Professor Stan Cowley - Co-Investigator on Magnetometer University of Leicester
Tel: 0116 223 1331

Dr Andrew Coates - Co-Investigator on CAPS-ELS Mullard Space Science Laboratory, UCL
Tel: 01483 204145

Dr Abigail Rymer - Co-Investigator on CAPS-ELS
Tel: 01483 204142

Dr Hugo Alleyne - Co-Investigator on RPWS instrument
Tel: 0114 222 5630

Notes to Editors

1. The 3 papers appear in Nature today - 17th February 2005.

Saturn's Aurora: controlled by the solar wind but not in the same manner as Earth's. Crary, F.J., et al UK co-authors - Professor Michele Dougherty (Imperial College), P Hanlon (Imperial College), Dr Andrew Coates (Mullard Space Science Laboratory), Dr Abi Rymer (Mullard Space Science Laboratory).

Saturn's Ultraviolet Aurora Differs Morphologically from Earth's and Jupiter's. Clarke, J.T., et al UK co-authors - Professor Michele Dougherty (Imperial College), Professor Stan Cowley (University of Leicester), Dr Emma Bunce (University of Leicester)

The Relationship between Saturn's Ultraviolet Aurora and Kilometric Radio Emissions. Kurth, W.S., et al UK co-author - Professor Michele Dougherty (Imperial College London)

2. Cassini Instruments
UK scientists are playing significant roles in the mission with involvement in 6 of the 12 instruments onboard the Cassini orbiter and 2 of the 6 instruments on the Huygens probe. The UK has the lead role in the magnetometer instrument on Cassini (Imperial College) and the Science Surface Package on Huygens (Open University).

UK scientists are involved in all three of the Cassini instruments used in these studies.

Cassini Plasma Electron Spectrometer - will study how the solar wind interacts with the planet, its rings and moons, helping us to understand how planets formed in the early solar system.

Dual Technique Magnetometer - designed to determine Saturn's magnetic field, as well as the electric and magnetic interactions between the planet and its moons and rings.

Radio and Plasma Waves Instrument - investigates plasma waves, natural emissions of radio energy and dust.

The Particle Physics and Astronomy Research Council (PPARC) has funded UK scientists from Open University, Imperial College, Mullard Space Science Laboratory, Rutherford Appleton Laboratory, Oxford University, Queen Mary, University of London, Leicester University, Sheffield University and Southampton University. A number of UK industries have also contributed to the mission, namely Martin Baker Aircraft Company Ltd, Logica CMG, Irvin GQ, IGG Component Technology, Ultra Electronics.

The Cassini-Huygens mission is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C. NASA's Hubble Space Telescope is a cooperative program with the European Space Agency and is operated by the Space Telescope Science Institute on the Johns Hopkins University Homewood Campus in Baltimore, Maryland.

The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science investment agency. It funds research, education and public understanding in four broad areas of science - particle physics, astronomy, cosmology and space science. PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.