New transient radiation belt discovered at Saturn (EPSC09/02)
September 14, 2009


Scientists using the Cassini spacecraft’s  Magnetospheric Imaging instrument (MIMI) have detected a new, temporary radiation belt  at Saturn, located around the orbit of its moon Dione at about 377 000 km from the center of the planet. The discovery will be presented at the European Planetary Science Congress in Potsdam by Dr Elias Roussos on Monday 14 September.

Radiation belts, like Earth’s Van Allen belts, have been discovered at Jupiter, Saturn, Uranus and Neptune.  However, to date, it has only been possible to observe the variability of their intensity at Earth and Jupiter. Now that Cassini has been orbiting Saturn for more than five years, it has been possible to assess for the first time changes in Saturn’s radiation belts.

An international team of astronomers made the discovery analysing data from the MIMI’s LEMMS sensor, which measures the energy and angular distribution of charged particles in the magnetic bubble that surrounds Saturn.

“The most dramatic changes have been observed as sudden increases in the intensity of high energy charged particles in the inner part of Saturn’s magnetosphere, in the vicinity of the moons Dione and Tethys”, said Dr. Roussos. “These intensifications, which could create temporary satellite atmospheres around these moons, occurred three times in 2005 as a response to an equal number of solar storms that hit Saturn’s magnetosphere and formed a new, temporary component to Saturn’s radiation belts”, he added.

The new belt, which has been named “the Dione belt”, was only detected by MIMI/LEMMS for  a few weeks after each of its three appearances. The team believe that newly formed  charged particles  in the Dione belt were gradually absorbed by Dione itself and another nearby moon, named Tethys, which lies slightly closer to Saturn at an orbit of 295 000km.

Unlike the Van Allen belts around the Earth, Saturn’s radiation belts inside the orbit of Tethys are very stable, showing negligible response to solar storm occurrences and no variability over the five years that they have been monitored by Cassini.

Interestingly, it was found that the transient Dione belt was only detected outside the orbit of Tethys. It appeared to be clearly separated from the inner belts by a permanent radiation gap all along the orbit of Tethys.

“Our observations suggest that Tethys acts as a barrier against inward transport of energetic particles and is shielding the planet’s inner radiation belts from solar wind influences.  That makes the inner, ionic radiation belts of Saturn the most isolated magnetospheric structure in our solar system“, said Dr Roussos.

The radiation belts within Tethys’s orbit probably arise from the interaction of the planet’s main rings and atmosphere  and galactic cosmic ray particles that, unlike the solar wind, have the very high energies needed to penetrate the innermost Saturnian magnetosphere. This means that the inner radiation belts will only vary if the cosmic ray intensities at the distance of Saturn change significantly.

However, as Dr. Roussos emphasised, “Outside the orbit of Tethys, the variability of Saturn’s radiation belt might be enhanced in the coming years as we start approaching the solar maximum. 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 affect significantly Saturn’s global magnetospheric dynamics.”


An image of Dione (credit: NASA/JPL/Space Science Institute) can be found at:


Caption: Radiation belt map of the ions with energies between 25-60 MeV, in Saturn’s magnetosphere, based on several years of Cassini MIMI/LEMMS data. The structure of this radiation belt is almost perfectly stable for more than 5 years of Cassini observations, despite the intense variability of the radiation belts, outside the location of Tethys. Source: Roussos, E., N. Krupp, T. P. Armstrong, C. Paranicas, D. G. Mitchell, S. M. Krimigis, G. H. Jones, K. Dialynas, N. Sergis, and D. C. Hamilton (2008), Discovery of a transient radiation belt at Saturn, Geophys. Res. Lett., 35, L22106, doi:10.1029/2008GL035767




The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA’s Cassini spacecraft has been in orbit around Saturn since July 2004.  Cassini completed its initial four-year mission to explore the Saturn System in June 2008. The Cassini Equinox mission is a two-year extension to September 2010 and is named for the Saturnian equinox, which occurred in August 2009 when the sun shone directly on the equator. Cassini will observe seasonal changes brought by the Sun as it begins to illuminate the northern hemisphere and the rings’ northern face. Saturn, the rings and moons were illuminated by the Sun from the south during the mission’s first four years.
The Magnetospheric Imaging Instrument (MIMI) is the first instrument ever designed to produce an image of a planetary magnetosphere. It detects energetic ions and electrons and fast neutral particles, in order to study the overall configuration and dynamics of the Saturnian magnetosphere and its interactions with the solar wind. The Principal Investigator is Professor Stamatis Krimigis of the JHU/APL.
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EPSC 2009 is organised by Europlanet, a Research Infrastructure funded under the European Commission’s Framework 7 Programme, in association with the European Geosciences Union. It is the major meeting in Europe for planetary scientists. The programme comprises 37 sessions and workshops covering a wide range of planetary topics.

EPSC 2009 is taking place at the Kongresshotel am Templiner See, Potsdam, Germany from Sunday 13 September to Friday 18 September 2009.
For further details, see the meeting website:


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Activities, aimed at fostering a culture of cooperation in the field of planetary sciences, Transnational Access Activities, providing European researchers with access to a range of laboratory and field site facilities tailored to the needs of planetary research, as well as on-line access to the available planetary science data, information and software tools, through the Integrated and Distributed Information Service.   These programmes are underpinned by Joint Research Activities, which are developing and improving the facilities, models, software tools and services offered by Europlanet
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