University of Leicester eBulletin

University of Leicester Theory Runs Rings Around Jupiter

March 2002

No 70

Jupiter’s electric aurora

The planet Jupiter has spectacular rings of auroras around each pole but until now scientists have not been able to explain how they form. All auroras are caused by energetic charged particles crashing into the top of the atmosphere and making it glow. In the Earth’s auroras, these particles come from the Sun in a flow of charged particles known as the solar wind. But this can’t account for Jupiter’s auroras because the solar wind does not reach to the region where the brightest are found. Space physicists from the University of Leicester have now proposed a new theory of how Jupiter’s auroras are formed.

An enormous disk of plasma gas rotates around Jupiter, flowing outwards from the moon Io. They believe that a large-scale electric current system (stream of charged particles) flows between the planet’s upper atmosphere and this disk of gas. They have also calculated that in order for such large currents to flow between the atmosphere and the disk, electrons must be strongly accelerated between these regions, causing the bright ring of auroras around each pole when they hit the top of the atmosphere and make it glow.

Professor Stan Cowley, of the University of Leicester said: “The force associated with this electric current causes the plasma gas to spin at the same rate as the planet as it flows outwards. Our calculations suggest that the total current in this giant circuit is 100 million amps.  The power transferred from the atmosphere to the plasma disk is about a thousand million megawatts or about 20,000 times the peak electricity demand in the UK!”

The brightness of the aurora depends upon the intensity of the electron beams that hit the top of the atmosphere.  Scientists had previously developed a number of theories about how the auroras are formed, but they underestimated this brightness by factors of between a hundred and a thousand compared to the measurements taken! 

Images available from PPARC website: or by contacting Julia Maddock at PPARC (contact details below).  

Jupiter and Io – taken by the Cassini imager
during the flyby in December 2000 and January 2001. Credits: NASA/JPL/University of Arizona.

Aurora – Ultra-violet image taken with the Hubble Space Telescope Imaging Spectrograph (STIS). Credits:  NASA/ESA, John Clarke (University of Michigan)

North and South Auroras – Composite images taken with STIS. In this colour representation, the planet's reflected sunlight appears brown, while the auroral emissions appear white or shades of blue or red. Credits: NASA, John Clarke (University of Michigan)

Contact details:
Professor Stan Cowley, Department of Physics & Astronomy, University of Leicester, email, telephone 0116 2231331
Julia Maddock, PPARC Press Office, email, telephone 01793 442094

Notes for Editors:

All auroras are formed by energetic charged particles such as electrons crashing into the top of the atmosphere and making it glow.  The same effect is used to create pictures on a television screen – when electrons collide with various chemicals on the back of the screen, different coloured light is emitted.  Knowing where these particles come from and how they are accelerated is the critical step in understanding how an aurora forms.

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 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 Laboratory for Particle Physics (CERN), and the European Space Agency. 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, which includes the Lovell Telescope at Jodrell Bank observatory.

PPARC's Public Understanding of Science and Technology Awards Scheme funds both small local projects and national initiatives aimed at improving public understanding of its areas of science.

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