Although radio waves are invisible, sound waves can still be picked up by Cassini. With these waves, it is possible to create an ‘audio image’ of Saturn's radio waves and their regular variation.
This 10-second movie shows 10 days of radio observations by Cassini's Radio and Plasma Wave Science instrument.
The variation in the received intensity is due to the planet's rotation and the variation in the solar wind speed near Saturn. The latter is not random, but follows a saw-tooth pattern, first building up in speed and then suddenly slowing down. This causes the apparent rotation period fluctuations.