File:Cassini RPWS Jupiter 010104.ogg

English: NASA's Cassini Spacecraft Detected Sound Waves - NASA's Cassini spacecraft, flying past Jupiter, detected waves in the thin gas of charged particles that fills the space between the Sun and its planets. The waves are in low radio frequencies, which have been converted to sound waves to make the patterns audible.

This audio clip and chart come from waves detected by Cassini's radio and plasma wave science instrument during a two-minute interval on Jan. 1, 2001, while Cassini was 10 million kilometers (6.2 million miles) from Jupiter. The spacecraft has passed its closest approach to Jupiter two days earlier.

The oscillations discernible in the clip were derived from an interaction between the magnetic field that surrounds Jupiter and the solar wind of particles speeding away from the Sun. They are a type called "Langmuir waves." The chart, comparable to a voice print, shows how the spectrum of wave frequencies (given in kilohertz, or kHz, on the vertical scale) changes with time. The narrow band of emissions at about 3.5 kilohertz is at a special frequency, called the "electron plasma frequency," and its existence tells scientists that the density of electrons in the solar wind where these waves were detected is about 150,000 electrons per cubic meter (about 2.5 per cubic inch). Wave bursts at somewhat higher frequencies are the result of interaction between those narrow-band waves and lower-frequency waves.

At the time these waves were detected, Cassini was in the solar wind, near the bow shock where the solar wind is diverted around Jupiter's magnetosphere, a vast bubble of charged particles controlled by the planet's magnetic field. The bow shock is similar to a sonic boom from a supersonic jet flying through Earth's atmosphere, except that the bow shock is caused by the supersonic solar wind moving past the magnetosphere. The solar wind gets heated, slowed and deflected by the magnetosphere. The bow shock is a source of energetic electrons needed to drive the waves that are presented in visible and audible formats here.

Cassini is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages Cassini for NASA's Office of Space Science, Washington, D.C.