The audio clip comes from waves that were detected by Cassini on Dec. 8, at a distance of about 23 million kilometers (14 million miles) from Jupiter. They are likely to have derived from an interaction of the magnetic field that surrounds Jupiter and the solar wind of particles speeding away from the Sun, said Cassini science team member Dr. William Kurth of the University of Iowa, Iowa City.

Cassini, a cooperative mission of NASA, the European Space Agency and the Italian Space Agency, will pass Jupiter on Dec. 30 for a gravity boost to reach its ultimate destination, Saturn. The Jet Propulsion Laboratory, Pasadena, Calif., manages Cassini for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena.

"Sounds" Near Jupiter Bow Shock

NASA's Cassini spacecraft, approaching Jupiter, is detecting 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 can be converted to sound waves to make the patterns audible.

The waves presented here were detected by Cassini's radio wave and plasma science instrument (RPWS) on Dec. 8, 2000, at a distance of about 23 million kilometers (14 million miles) from Jupiter. They are likely to have derived from an interaction of the magnetic field that surrounds Jupiter and the solar wind of particles speeding away from the Sun.

The oscillations discernible in the graph and in the audio file are from ion-acoustic waves, which result from electrons moving in non-random patterns driven by a flow of energy. In this case, the energy flow probably comes from the heat of Jupiter's bow shock. 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 being diverted around Jupiter's magnetic field. The shock is a place where the solar wind is heated, slowed and deflected by the magnetic field surrounding Jupiter. Cassini has not reached the bow shock, but the shock is probably the source of energy driving the waves that are reaching the spacecraft.

The period represented in the graph and audio file lasted 30 seconds. In the process of presenting as sound waves what were originally electric waves, the frequency has been sped up and a few short gaps have been spliced out, resulting in a 10-second audio clip.

This color display is a kind of "voice print" which shows the variation in strength of radio emissions in both frequency (like a set of stations on a radio dial) and time. Strength is represented by color with red being strongest and blue weakest. The radio frequency increases along the vertical axis of the display from bottom to top and time increases along the horizontal axis from left to right. It is called Spacecraft Event Time (SCET for short).

The numbers below the plot provide information on the position of Cassini relative to Jupiter. Rj tells us the distance from Jupiter to Cassini in units of the radius of Jupiter (71,492 km). LonIII is the longitude of Cassini measured in Jupiter's system III coordinate system, which is believed to rotate with the deep interior of the planet. Mlat is the latitude of the spacecraft relative to the magnetic equator; this plane is tilted about 10 degrees from the rotational equator. LT is the local time of the spacecraft; at 12 hours local time, Cassini is directly between the sun and Jupiter and at 18 hours, it is over the dusk terminator (the sunset line separating day and night on the planet). Io Phase tells us the position of Io in its orbit about Jupiter as viewed by Cassini. One of the first things discovered about Io was that its orbital position strongly influences the intensity of Jovian radio emissions viewed by an observer.

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

For higher resolution image, click here.

Contact Information:
Prof. Donald A. Gurnett, donald-gurnett@uiowa.edu
or Dr. William Kurth, william-kurth@uiowa.edu
Department of Physics and Astronomy
University of Iowa
Van Allen Hall
Iowa City, IA 52242
Website: http://www-pw.physics.uiowa.edu/plasma-wave/cassini/home.html

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