Planets within our solar system are under constant bombardment from a number of charged particles. Those that boil off the surface of the Sun and flow outward, carrying its magnetic field are known as the solar wind. Those coming in from outside the region dominated by the Sun (the Heliosphere) and its wind are known as Galactic Cosmic rays. Solar Cosmic Rays may also be produced by high energy events on the solar surface, producing small amounts of unusually high energy particles.
This means that the solar magnetic field dominates interplanetary space. However, close to the planets themselves, their own individual magnetic fields can be dominant (in the case of nonmagnetic planets, there may be smaller magnetic fields generated by turbulence in the solar wind as it encounters the planet). The region over which a planet is magnetically dominant is that planet’s magnetosphere. Charged particles inside the magnetosphere may be due to particles boiling off the top of the planet’s atmosphere, solar wind particles penetrating the magnetic field or cosmic rays doing the same.
In the case of Saturn’s magnetosphere, it has been found that solar wind particles directly entering the magnetic field tend to enter a belt between the orbits of the satellites Tethys and Dione. Known as the Dione belt, this is a temporary structure that vanishes away as the solar storm abates and Dione absorbs the particles. Tethys clears up anything that thinks of surviving this, meaning there is a gap in radiation belts between those populated by high energy Cosmic rays and the orbit of Tethys.
The study was carried out by Dr Elias Roussos of the Max Planck Institute in Germany and other using the Low Energy Magnetospheric Measurement System (LEMMS), part of the Magnetospheric Imaging Instrument (MIMI) of the Cassini space probe.