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A new way to create Saturn's radiation belts

They might arise from a certain type of plasma waves.

Around Saturn, and other planets including the Earth, energetic charged particles are trapped in magnetic fields generated by the planets. Here the particles arrange in doughnut-shaped zones, known as radiation belts, such as the Van Allen belts around the Earth where electrons travel close to the speed of light.

A new computer model based on data collected by the NASA Cassini spacecraft, which orbited Saturn for 13 years, has now allowed deeper insight into this phenomenon to a team of international scientists lead by British Antarctic Survey (BAS), the polar research programme of Great Britain. Yuri Shprits, Head of GFZ section Magnetospheric Physics, was part of the team as well as researchers from the University of Iowa. The discovery overturns the accepted view among space scientists about the mechanisms responsible for accelerating the electrons to such extreme energies in Saturn’s radiation belts. The results are published in the journal Nature Communications.

For a long time, it had been assumed that around Saturn, electrons are accelerated to extremely high energies by a process called radial diffusion, where electrons are repeatedly nudged towards the planet, increasing their energy.

Plasma waves could be the answer

An alternative way of accelerating electrons is their interaction with a certain type of plasma waves as happens around the Earth and Jupiter. A plasma is a gas whose particles are at least partially ionized. Saturn has a plasma layer that rotates with the planet. Around Saturn, plasma waves had been dismissed as ineffective. However, the researchers discovered that in its unique environment, it is another form of plasma wave called the Z-mode wave that is critical.

The team concludes that electron acceleration by Z-mode waves is more rapid at energizing electrons in Saturn’s radiation belts than radial diffusion and both mechanisms will work together to maintain the radiation belts at Saturn.

Lead author Emma Woodfield from BAS says: “This study provides us with a better understanding of how radiation belts work across the Solar system and will help modellers like me forecast space weather more accurately at the Earth, which in turn will protect both astronauts and satellites from radiation hazards.”

Yuri Shprits: “I think it’s most critical to understand the extreme radiation environments of the outer planets. These studies provide us with a unique opportunity to evaluate the potential extremes of terrestrial space weather and to understand what space weather conditions may be around exoplanets which are located beyond our Solar system”. (ph)

Original study: Woodfield, E. E., Horne, R. B., Glauert, S. A., Menietti, J. D., Shprits, Y. Y. & Kurth, W. S., 2018. Formation of electron radiation belts at Saturn by Z-mode wave acceleration. Nature Communications. DOI: 10.1038/s41467-018-07549-4

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