One approach scientists use to make sense of the data from instruments is to make pictures and graphs to represent the data. This is called “data visualization”. Some types of data, especially radio signals, are very similar in many ways to sound. The power of a radio signal is analogous to the volume of a sound. The radio signal also varies in terms of the frequency and wavelength of the radio waves, which is like the variation in pitch of sound waves. So scientists sometimes translate radio signals into sound to better understand the signals. This approach is called “data sonification”.
On June 27, 1996, the Galileo spacecraft made the first flyby of Jupiter’s largest moon, Ganymede. The Plasma Wave Experiment (PWS), using an electric dipole antenna, recorded the signature of a magnetosphere at Ganymede. This is the first example of a magnetosphere associated with a moon. The PWS data are represented here as both sounds and a rainbow-colored spectrogram. Approximately 45 minutes of PWS observations are transformed and compressed to 60 seconds. Time increases to the right and frequency (pitch) increases vertically. Color is used to indicate wave intensity, red corresponding to strong waves, blue corresponding to weak waves. The audio track represents the PWS data and is synchronized with the display of the rainbow-colored spectrogram. The pitch of the sound is reduced by a factor of 9 from the measured frequency and follows the location of the signal on the rainbow-colored spectrogram. The entrance into the Ganymede magnetosphere is marked by a strong burst of noise about 6-10 seconds into the recording. As the spacecraft approaches Ganymede, an irregular tone can be heard rising in frequency, reaching a peak and then declining. The pitch of this tone is a measure of the density of charged particles near Ganymede. Both the plasma wave and magnetometer data show that a strong magnetic field exists around Ganymede.
More information on the PWS instrument and other Galileo science instruments is available at http://www.jpl.nasa.gov/galileo/instruments/.
All of the planets are bathed by a hot plasma called the solar wind which boils off the sun and moves outward at speeds of a million miles per hour. The planets are a little like supersonic aircraft in Earth’s atmosphere. Should a supersonic jet fly over your house, you would hear a sonic boom caused by the jet moving faster than sound waves in the air. Since the solar wind is moving past the planets at supersonic speeds, a similar ‘sonic boom’ is created in the solar wind. The signals in this sound file were acquired as Voyager 1 was approaching the ‘sonic boom’ (or bow shock, as scientists refer to it) of Jupiter. The chirps heard at the beginning of the interval are waves generated by electrons coming from the shock and moving ‘upstream’ into the approaching solar wind. These soon die out and, except for a slight hum from one of the science instruments onboard and the firing of one of Voyager’s thrusters (making a short thud) things become quiet. Then, suddenly, the spacecraft enters the bow shock and is enveloped by the turbulence in this planetary ‘sonic boom’. The bow shock is nature’s way of slowing, deflecting, and heating the solar wind as it runs into an object, in this case the Jovian magnetosphere. In fact, the waves you are hearing are at least partly responsible for heating the solar wind as it is slowed and deflected around the magnetosphere.
These melodious tones are created at a special frequency in a plasma with a magnetic field. The frequency is set by the number of electrons in a given volume (the electron density) and the strength of the magnetic field. Hence, the frequency of these waves, called upper hybrid waves, can provide a very accurate measure of the density of the plasma; a fundamental property of the Jovian environment of interest to scientists. These emissions were acquired by Voyager 2 as it passed through the outer magnetosphere in 1979.
While somewhat difficult to hear, this whistling tone provided Voyager investigators confirmation that there was lightning in Jupiter’s atmosphere. This emission is called a ‘whistler’ because of its whistling sound. These have been studied at Earth for many decades. Whistlers are just one part of the electromagnetic spectrum of a lightning stroke which happens to propagate away from the planet, into the magnetized plasma above. An interesting thing occurs when these waves reach the plasma; the higher frequency waves travel faster along the planets magnetic field than the lower frequencies. So, a satellite detecting these signals some distance from the planet will first pickup the high frequencies, then the low ones from an individual lightning stroke, thereby generating the whistling tone. We know of no other way of producing such a distinct tone, hence, the discovery of whistlers like this one at Jupiter provides strong evidence of lightning there. At about the same time, Voyager’s cameras took time exposures of the dark side of Jupiter and saw regions of light which have been identified as clouds momentarily lit by lightning within them. So, the plasma wave instrument and cameras together provided the first definitive evidence for lightning at a planet other than Earth.