Studies of Jupiter's auroras by scientists from the University of Leicester have challenged current theories about the processes controlling the biggest light-shows in the Solar System.

The scientists compared a series of ultraviolet images of Jupiter's auroras taken by the Hubble Space Telescope with simultaneous measurements taken by Cassini showing conditions in the solar wind as the spacecraft flew past the giant planet in December 2000 - January 2001. They found that there was a strong correlation between the strength of the solar wind and the behaviour of the auroras that occurred towards the planet's poles. Until now, scientists had believed that Jovian auroras were caused by the planet's rapid spin and a stream of material emitted from the volcanic moon Io at the rate of one tonne per second.

"The argument is certainly not cut and dried," said Dr Jonathan Nichols, who is presenting the results at the Royal Astronomical Society's National Astronomy Meeting.

"Previous work by our group has shown that Jupiter's main auroral oval is not caused by the same type of processes that cause the Northern Lights on Earth. However, this new study shows that the auroras located polewards of the main ovals are directly linked to the strength with which the solar wind is blowing, which means that Earth-like processes are causing these polar auroras. Surprisingly, we've also found that the main oval also shows a direct correlation to solar wind strength, which is completely the opposite result to the one we were expecting from our predictions."

The results indicate that substantial energy is transferred from the solar wind to the planet and this may account for the puzzle as to why Jupiter is significantly warmer than it 'should' be. The new findings may affect theories surrounding other aspects of the Jovian magnetosphere, such as the mechanism by which the plasma originating from Io is lost from the system and determining the length of Jupiter's huge comet-like magnetic tail.

In the same session, Sarah Badman will be presenting results of a study of Saturn's auroras carried out over three weeks in January 2004. This study also combined images taken by the Hubble Space Telescope with measurements of the solar wind recorded by Cassini as it approached the ringed planet. Miss Badman collated all available images of Saturn's aurora and determined, for the first time, the most common shape and position of the aurora, as well as the occurrence of more unusual features. Her findings corroborate the theory that Saturn's auroras are caused by the explosive release of solar wind energy that is built up and stored in the planet's magnetic field.

Planetary auroras

The Earth's auroras are caused by the interaction of the planet's magnetic field with the solar wind, a stream of charged particles emitted by the Sun at roughly a million miles per hour. Some of these charged particles are able to 'leak' into the magnetosphere and are channelled into the high polar atmosphere, where they interact with electrically charged air molecules, releasing light. The brightness of the Earth's auroras therefore depends on the rapidly changing conditions in the solar wind.

The magnetic field of Jupiter is five times larger than the Sun and the auroras illuminating the Jovian atmosphere are up to a hundred times brighter than those on Earth. The main oval auroras are formed by a hundred million amps of electric current flowing around the magnetic field and into the atmosphere as a result of Jupiter's attempt to keep plasma emitted from its moon, Io, rotating at the same velocity as the planet. Until now, it has been thought that the system is completely dominated by this rotation, and that the energy imparted by the solar wind is negligible by comparison.

Like Jupiter, Saturn also possesses an enormous magnetic field and exhibits bright auroral displays. Early, infrequent images taken by previous spacecraft flybys and HST showed a narrow oval of emissions with some varying brightness. The images taken in January 2004 showed several unexpected features, including spirals of aurora around Saturn's pole (compared to the oval shape at Earth), bright 'blobs' of aurora that rotate around the pole, and a unique auroral 'storm' where half the polar region was totally filled-in with very bright auroras. Work at Leicester has shown how these different auroral forms are controlled by the interaction of the solar wind with Saturn's magnetic field.

Royal Astronomical Society's National Astronomy Meeting The 2006 RAS National Astronomy Meeting is hosted by the University of Leicester. It is sponsored by the Royal Astronomical Society, the UK Particle Physics and Astronomy Research Council (PPARC), the University of Leicester and the National Space Centre, Leicester.

Space Plasma Physics at Leicester

The Radio and Space Plasma Physics Group at the University of Leicester is the largest in the UK, with approximately 40 members including academic staff, technical and support staff, research fellows, and research students. The group's research programme concerns the study of the outer plasma environment of the Earth and other planets, including the interactions of planets' ionospheres and magnetospheres with the solar wind plasma and the neutral layers of the atmosphere below.

Cassini-Huygens

Cassini-Huygens is an international collaboration between three space agencies. Seventeen nations contributed to building the spacecraft. The Cassini orbiter was built and managed by NASA's Jet Propulsion Laboratory. The Huygens probe was built by the European Space Agency. The Italian Space agency provided Cassini's high-gain communication antenna. More than 250 scientists worldwide are studying the data streaming back from Saturn on a daily basis.

Cassini-Huygens was launched in 1997 and went into orbit around Saturn on 1st July 2004. In order to reach Saturn, the spacecraft needed gravity assist boosts from the Venus, the Earth and Jupiter. From 1st October 2000 to 31st March 2001, Cassini's instruments were used to study Jupiter as the spacecraft approached the sunlit side and receded from the dark side of the planet. Cassini-Huygens reached the closest approach of the flyby on 30th December 2000 at a distance of 10 million kilometres from Jupiter.

Hubble Space Telescope

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The Space Telescope Science Institute in Baltimore conducts Hubble science operations. The Institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington.

Jupiter Millennium Flyby

From the 14th December 2000 - 21st January 2001, researchers carried out a joint campaign with Cassini and the NASA/ESA Hubble Space Telescope, gathering simultaneous data and images of Jupiter's auroras. Cassini's instruments, including the Cassini Plasma Spectrometer (CAPS), Magnetometer (MAG), Ion and Neutral Mass Spectrometer (INMS), measured the solar wind to determine the pressure exerted by these winds on Jupiter. Hubble's STIS (Space Telescope Imaging Spectrograph) instrument was used to capture detailed ultraviolet images of the aurora.