The Io Plasma Torus

Jupiter's Galilean moon Io has multiple active volcanoes on its surface. These spew a gas of particles into space that become ionized as Io moves about its orbit, and these particles eventually diffuse into the rest of the region surrounding Jupiter. Io is probably also being eroded by collisions with those particles as it orbits through them.

Io's Ion Cloud
The above figure shows a Galileo spacecraft false-color image of Io. This image was taken through a filter that emphasizes yellow-green light. Io is being illuminated by sunlight from the left side. On the right side of Io two active volcanoes can be seen, Prometheus and Pele. The plume of Prometheus extends 100 kilometers above the surface and part of it is being illuminated by sunlight. The diffuse yellow glow around Io comes largely from sodium atoms, which have a very strong spectral line at 589 nm (yellow light). Thus the glow marks the cloud of ions such as sulfur and sodium that surrounds Io because of the volcanic activity. (Many of the points of light in the image are background stars.)

The Io Flux Tube

As Io moves around its orbit in the strong magnetic field of Jupiter and through the plasma torus, a huge electrical current is set up between Io and Jupiter in a cylinder of highly concentrated magnetic flux called the Io Flux Tube. The Flux Tube has a power output of about 2 trillion watts, comparable to the amount of all manmade power produced on Earth. It is responsible for bursts of radio frequency radiation long detected on Earth (right panel).

The Plasma Torus
The magnetic field of Jupiter rotates with the planet in about 10 hours, while Io orbits Jupiter in 1.8 days. Thus, the magnetosphere sweeps past Io at high velocity many times each revolutionary period.

As the magnetosphere sweeps around Io's orbit it accelerates the charged particles to high velocity. As a result, a doughnut shaped region around the orbit of Io becomes filled with a hot ionized gas (a plasma). This region, which is illustrated in the above figure, is called the Io Plasma Torus. This animation illustrates the Plasma Torus and the Flux Tube that is described in the adjacent box.