Radio waves accelerate electrons within Jupiter's magnetic field in the same way as they do on Earth, according to new research published in Nature Physics this week. The discovery overturns a theory that has held sway for more than a generation and has important implications for protecting Earth-orbiting satellites.
Using data collected at Jupiter by the Galileo spacecraft, Dr Richard Horne of British Antarctic Survey (BAS) and colleagues from the University of California, Los Angeles, and the University of Iowa found that a special type of very low frequency radio wave is strong enough to accelerate electrons up to very high energies inside Jupiter's magnetic field.
According to lead author, Dr Richard Horne, "We've shown before that very low frequency radio waves can accelerate electrons in the Earth's magnetic field, but we have now shown that exactly the same theory works on Jupiter, where the magnetic field is 20,000 times stronger than the Earth's and the composition of the atmosphere is very different. This is the ultimate test of our theory."
"On Jupiter, the waves are powered by energy from volcanoes on the moon Io, combined with the planet's rapid rotation -- once every 10 hours. Volcanic gasses are ionized and flung out away from the planet by centrifugal force. This material is replaced by an inward flow of particles that excite the waves that in turn accelerate the electrons."
Understanding how electrons are accelerated will help scientists make better predictions of when satellites are at risk of damage by high-energy charged particles. These particles encircle the Earth in the Van Allen radiation belts and can damage satellites by causing malfunctions and degrading electronic components. However, the number of particles in the radiation belts can change dramatically within a few minutes, which is why more accurate forecasting is needed.
The discovery also has other scientific implications for Jupiter -- it overturns a theory that has held sway for more than 30 years. According to Dr Horne,
"For more than 30 years it was thought that the electrons are accelerated as a result of transport towards Jupiter, but now we show that gyro-resonant wave acceleration is a very important step that acts in concert. Once the electrons are accelerated, they are transported closer to the planet and emit intense synchrotron radiation out into interplanetary space. Our theory provides the missing step to explain this high intensity radiation from Jupiter, which was first detected on Earth more than 50 years ago."
The paper 'Gyro-resonant electron acceleration at Jupiter', is published in Nature Physics on 9 March 2008.
The waves accelerate the electrons by a process known as gyro-resonance -- a process where the electrons spiral around the Earth's magnetic field at the same rate as the waves rotate around the magnetic field -- around 3,000 times a second, or 3 kiloHertz. The waves were found to be particularly strong outside the orbit of Jupiter's moon Io and are known as whistler mode chorus waves. The same waves are found in the Earth's magnetic field and are measured by BAS scientists in the Antarctic.
Van Allen radiation belts
Jupiter, like the Earth, has belts of energetic charged particles (electrons and ions) that surround the planet like a ring doughnut. They are named after James Van Allen who discovered the Earth's radiation belts 50 years ago using the first US satellite, Explorer I. Jupiter's belts were also discovered by Van Allen when Pioneer 10 flew past the planet in 1973 on its way to the outer planets. Jupiter's magnetic field constrains the particles to move in a spiral along the magnetic field. The particles 'bounce' between mirror points in the northern and southern hemispheres, and drift around the planet.
Launched by the space shuttle in October 1989, the NASA spacecraft Galileo was sent to explore Jupiter and its moons, reaching the planet in December 1995. Unfortunately the high-gain antenna failed to deploy properly and the spacecraft could only send a trickle of information back to Earth via its low-gain antenna. Nevertheless, several important new discoveries were made about Jupiter, its moons, magnetic field and particles. As a result, NASA is planning another mission, called Juno, which will study Jupiter's atmosphere, auroral light emissions, and magnetic field. Juno is due for launch in 2011.
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