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 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 colored light is emitted.
Knowing where these particles come from and how they are accelerated is the critical step in understanding how an aurora forms.
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.
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.
Related images:
Aurora - Ultra-violet image taken with the Hubble Space Telescope Imaging Spectrograph (STIS). Credits: NASA/ESA, John Clarke (University of Michigan)
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: Professor Stan Cowley, Julia Maddock]
27-Mar-2002
