This image shows the torus of material around the moon Io. Click on image for full size version (55K GIF) Image courtesy of Catalina Observatory.

A satellite which has an atmosphere, such as Jupiter's moon Io, and which also is inside a magnetosphere (unlike the Earth's moon), will leave a cloud of particles behind as it orbits the planet. This cloud of particles is called a torus.

Eventually all the particles in the cloud will be turned to ions. The presence of ions causes a number of noticable effects in a magnetosphere:

• slows down the magnetic field as it goes around the moon
• may drive currents