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Saturn's radiation shield

Posted: September 14, 2009

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Newly released results from the European Planetary Science Congress (EPSC) being held in Potsdam, Germany all this week show how Saturn's moon Tethys is protecting the ringed planet from the onslaught of solar storms.

Tethys, Saturn's very own radiation shield. Image: NASA/JPL/Space Science Institute.

NASA’s Cassini spacecraft has been in orbit around Saturn now for five years, and has been studying the variability of Saturn’s magnetic environment (termed its magnetosphere) with the Low Energy Magnetospheric Measurement System (LEMMS) that is part of its Magnetospheric Imaging Instrument (MIMI). Like the other gas giants, and Earth, the ringed planet has radiation belts similar to Earth’s Van Allen belts, all the way through the 1.8 million kilometre extent of its magnetosphere. MIMI is able to measure the entire structure of the magnetosphere, and LEMMS can determine the angular motions and energy of charged particles within the radiation belts.

The new research, which is being presented today at EPSC by Dr Elias Roussos of the Max Planck Institute in Germany, reveals a new variable radiation belt between the orbits of the moons Tethys and Dione. This belt of radiation was seen to intensify three times in 2005 in response to solar storms that had strengthened the solar wind.

A schematic, courtesy of MIMI data, showing radiation belts inside the orbit of Tethys. Image: E Roussos et al.

“The most dramatic changes have been observed as sudden increases in the intensity of high-energy charged particles in the vicinity of the moons Dione and Tethys,” says Roussos. “These intensifications, which could create temporary atmospheres around these moons, formed a new and temporary component to Saturn’s radiation belts.”

This new belt, now known as the ‘Dione belt’, lasted for a few weeks after every storm before Dione itself absorbed the charged particles swept into Saturn’s magnetosphere from the solar wind. However, MIMI detected a permanent radiation gap in the magnetosphere along the orbit of 1,060-kilometre wide Tethys, which orbits Saturn at a distance of 294,660 kilometres. It seems that Tethys acts as a barrier, protecting Saturn’s inner magnetosphere from the vagaries of solar storms. Instead, the charged particles found in the magnetosphere within the orbit of Tethys come from higher-energy cosmic rays that can penetrate through.

“That makes the inner radiation belts of Saturn the most isolated magnetospheric structure in our Solar System,” says Roussos.

A schematic, courtesy of MIMI data, showing radiation belts inside the orbit of Tethys. Image: E Roussos et al.

Variability has only been observed so far in the Dione belt, and if there is any variability in the radiation belts inside the orbit of Tethys, they can only be caused by variations in the influx of cosmic rays from beyond the Solar System (which may be related to solar storms and the strength of the Sun’s magnetic ‘bubble’ – called the heliosphere – which extends out into the Kuiper Belt beyond Neptune, and the stronger the heliosphere, the less cosmic rays enter the Solar System).

“Outside the orbit of Tethys, the variability of Saturn’s radiation belt might be enhanced in the coming years as we start approaching solar maximum,” adds Roussos. At solar maximum – the peak of the Sun’s 11-year cycle – activity on the Sun increases leading to more flares, coronal mass ejections, sunspots and a stronger solar wind. “If solar storms occur frequently in the new solar cycle, the Dione belt might become a permanent, although highly variable, component of Saturn’s magnetosphere, which could significantly affect Saturn’s global magnetospheric dynamics.”

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