Each of the planets are imbedded in the interplanetary media which is often referred to as the solar wind. The solar wind is the upper atmosphere of the sun which is accelerated off the sun to speeds of around 400 km/s or about one million miles per hour. This wind is moving supersonically with respect to the planets. Much in the same way as a sonic boom is created by a supersonic aircraft moving through the Earth's atmosphere, a bow shock is created in the solar wind in front of each of the planets.

The shock (or sonic boom) is the surface in space where the supersonic flow is first affected by the obstacle. In the case of a planet like Venus with no magnetic field, the planet (or its upper atmosphere/ionosphere) is the obstacle to the solar wind flow. In the case of a planet like Earth or Jupiter with a substantial magnetic field, the planet's magnetic field is the obstacle to the flow. Since the magnetic field at Jupiter extends as much as 80 planetary radii or more from the planet, the magnetospheric obstacle is very large. In fact, Jupiter's magnetosphere is the largest object in the solar system and, if visible, could be easily seen from Earth without magnification.

The bow shock of a planet serves the function of slowing the solar wind flow past the planet and deflecting it around the obstacle. In the process, the energy in the bulk motion of the solar wind is converted into thermal energy at the shock. That is, the solar wind observed downstream (planetward) of the shock is found to be much hotter than the unperturbed supersonic solar wind. In many respects, these same processes occur in the atmosphere around a supersonic aircraft. To be sure, there are several complications associated with the fact that the solar wind is a plasma and not simply a neutral gas, but the analogy is still good in many qualitative ways.

The bow shock is an interesting region within which to study plasma waves. Recall that any perturbation to a plasma may result in the generation of plasma waves. In some cases it may be that the plasma waves act as one mechanism for heating the plasma, turning the energy of bulk flow of the supersonic plasma into the more chaotic downstream flow of a heated plasma. In other cases, the waves generated at the shock may simply be a result of the heating process and be of more interest for their diagnostic utility than for playing a major role in the thermodynamics of the shock.

Voyager 1 made a remarkable set of observations of the Jovian bow shock as it crossed the boundary on 28 February 1979 some 72 Jovian radii from the planet. The schematic view of the Jovian magnetosphere above shows the relative position of the shock and the magnetosphere and the trajectory of Voyager 1. The wave measurements are most often studied in the form of frequency-time spectrograms such as the one depicted here.

This spectrogram can be thought of as a voice print or sonogram as it shows the variation of wave intensity and frequency with time as Voyager 1 passed through the shock. The ordinate, or vertical axis depicts the frequency of waves detected by the Voyager plasma wave instrument. The horizontal axis (abscissa) represents time and the entire time for this spectrogram is 144 seconds (2.4 minutes). The color scheme is employed in order to convey information about the intensity of the waves. We use red for the most intense waves and blue for the least intense waves.

The spectrogram shows a narrowband emission near 6 kHz during the early part of the time interval. This emission is seen when the spacecraft is still in the supersonic solar wind upstream of the shock. These emissions are called electron plasma oscillations or Langmuir waves and represent the oscillatory motion of electrons about their equilibrium positions. From the frequency of this band of emissions we can say that the plasma density of the solar wind just upstream of the bow shock is about 0.44 electrons per cubic centimeter. The basic generation mechanism for these waves is called the two-stream instability. The shock is a source of energetic electrons and these electrons can stream away from the shock towards the sun. These electrons are moving in the opposite direction of the bulk of the solar wind, hence, we have one stream of electrons moving from the shock towards the sun and another, the solar wind electrons, moving from the sun towards the shock. This situation is not an equilibrium state and, in fact, the two streams represent a source of free energy in the plasma upstream of the shock. The natural response of the plasma to such a situation is to generate an instability known as electron plasma oscillations or Langmuir waves which will tend to scatter the shock-associated electrons and thermalize them, incorporating them back into the normal solar wind population of electrons.

The band of electron plasma oscillations fades out and is replaced by a lower frequency but broader band of emission. This low-frequency, broadband emission is directly associated with the bow shock and, in our analogy with sonic booms, is very similar to the sound waves one hears from a sonic boom. There are a few fairly clearly defined regions within this shock-related noise over the minute or so during which these waves are detected; the waves ramp up in both intensity and bandwidth to an abrupt maximum at about 90 seconds into the interval and subsequently decay over the next minute or so. Since the spacecraft is moving some 20 km/sec with respect to Jupiter, it covers some 1200 km (750 miles) during the shock noise signature, and if we were sure that the shock was not moving with respect to Jupiter during this time, we could identify this distance as the thickness of the shock. Unfortunately, the shock is almost always moving in and out from the planet in response to ever-present variations in the pressure of the solar wind. When the solar wind pressure (density and or speed) decreases, the pressure on the magnetosphere decreases and the magnetosphere expands. When the magnetosphere expands, so does the bow shock distance. The opposite occurs when the solar wind pressure increases.

The observations which are presented in the spectrogram represent the wave data in the frequency domain. However, the original observations are a simple time series of measurements of the voltage across the antenna on Voyager. The instrument collects these waveforms in a frequency bandwidth extending from about 40 Hz (cycles per second) to 12 kHz (12,000 cycles per second). This frequency range is well within the audio frequency range, hence, if we could convert the electrical energy into sound waves, we could listen to these waves. In fact, the computer can take the electrical signals detected by Voyager and use them to generate sound waves by sending the electrical signals through the coil of the speaker in your computer. In some ways, this is similar to the way a CD player converts digital samples of the music on your CD into sound. Since the frequency range is already well within hearing range, no shift in frequency is required. If your Web client has audio capabilities, you may be able to listen to the signals detected by Voyager as it passed through the bow shock. The waveform instrument has a 92.6% duty cycle and we have further reduced the total duration of the audio by retaining only one-third of the available time slices, compressing 2.4 minutes of data into 44 seconds (without frequency shift). It is useful to refer to the spectrogram above as you listen to the audio.

8000 samples/second (344Kb *.au file)

28800 samples/second (1238Kb *.wav file)

At the beginning of the audio, the electron plasma oscillations are heard as a series of chirping noises. As the shock itself is approached, these chirps fade out. A boom and a short tone are heard during this relatively quiet period. The boom is likely a thruster on the spacecraft firing to maintain proper attitude -- a normal operation, and the tone is electrical interference from one of the other instruments on the spacecraft. Shortly thereafter, the broadband wave turbulence can be heard, rapidly building into a crescendo enveloping the spacecraft and then slowly fading over the following minute or so.