Ground-based radio telescopes are just as important as space missions for studying Jupiter's magnetosphere. When Jupiter's magnetic field accelerates and funnels charged particles into its atmosphere, the collisions generate dazzling auroras. But they also produce the most powerful radio signals in the solar system other than from the sun.

When Jupiter spins on its axis, so does its magnetic field--which emits radio signals. By tracking these oscillating signals, radio telescopes on Earth can measure how fast Jupiter rotates. 
Radio telescopes were also used to discover Jupiter's radiation belts. Telescopes detected a type of signal called synchrotron emission that was most prevalent in a belt-shaped region surrounding Jupiter. Since this emission is produced when electrons travel at near light speed, these belts are likely filled with extremely energetic electrons that can damage spacecraft. Juno's polar orbit will allow it to avoid this hazardous region.