From grade school on, we have been taught that space is a black, empty void, interrupted by occasional planetary systems or stars. This is an unfortunate misconception. True, space is a vacuum. But it is not totally empty.

Low-density, ionized gases called "plasmas" emanate from the Sun, the planets themselves, and some of the satellites. Composed entirely of atoms that are broken apart into electrons and charged positive ions, a plasma is a good electrical conductor with properties that are strongly affected by electric and magnetic fields. The solar wind is a plasma of charged particles travelling outward from the Sun at supersonic speeds, sometimes accelerating by solar flares.

Individual ions and electrons in a plasma interact with the rest of the plasma by both emission and absorption of waves. These low-frequency oscillations originate in instabilities in the interplanetary plasma. Plasma waves are of two types: electrostatic oscillations similar to sound waves, and electromagnetic waves.

Localized interactions between waves and particles strongly control the dynamics of the entire plasma medium. Plasma waves generally cannot be studied far from their sources, so we must rely on our spacecraft observations to study plasma waves near the planets and in interplanetary space.

Galileo will carry instrumentation designed to study plasma waves. An electric dipole antenna will study the electric fields of plasmas, while two search coil magnetic antennas will study the magnetic fields. The electric dipole antenna consists of two 10-meter (33-foot) graphite epoxy antennas mounted at the tip of the 10-meter-long magnetometer boom. The search coil magnetic antennas are mounted on the high-gain antenna feed. Nearly simultaneous measurements of the electric and magnetic field spectrum will allow electrostatic waves to be distinguished from electromagnetic waves.

As with all of the Galileo fields and particles instruments, the plasma wave subsystem is mounted on the spinning section of the spacecraft. Scientists will thus be able to use radio direction-finding techniques to determine the source of certain types of plasma waves.

By virtue of its extended tour of Jupiter's system, Galileo will afford long-term observations not obtainable with previous flyby missions.

The Voyager missions detected several types of plasma waves and radio emissions near Jupiter. Galileo will study the mechanisms by which these waves are produced. For example, electrostatic electron cyclotron waves occur near the equatorial plane in and around the plasma torus that surrounds Io. Galileo will allow three-dimensional measurements of this area to aid in understanding why these types of waves occur in the torus. At the inner edge of the Io Torus, auroral hiss emissions were detected, similar to emissions associated with low-energy electron beams and field-aligned currents in Earth's auroral regions. Further study of these emissions at Jupiter should unveil much about energy transfer between the Io Torus and Jupiter's ionosphere, and about polar auroras on Jupiter that occur along Jovian magnetic field lines that pass through Io.

Jupiter is the most intense radio source in the sky. Voyager detected several types of radio emissions from the inner region of Jupiter's magnetosphere. These include decametric and kilometric radiation (the names relate to the length of the radio waves), and Galileo will search for the source of these radio waves.

In addition to studying locally generated plasma waves, Galileo's plasma wave subsystem will be used as a remote-sensing tool to monitor changes in the inner magnetosphere during the long periods that the spacecraft is near apojove (the highest point above Jupiter in each orbit). Large changes occur in Jupiter's radio emissions on time scales ranging from a few hours to several months, but the reasons for this are not presently known.

During the orbital tour, the spacecraft will pass many times through the turbulent boundary between the magnetosheath and the quieter inner regions of the magnetotail.

The final orbit of Galileo's Jovian tour will take the spacecraft down the planet's magnetotail--the windsock-shaped region of the Jovian magnetosphere that trails away from the Sun as the solar wind blows past the planet. Scientists expect to gain important information on the structure and dynamic processes in the magnetotail. At Earth, important plasma energization processes occur in the terrestrial magnetotail.

All of Jupiter's four largest satellites--Io, Europa, Ganymede, and Callisto--lie well within the planet's magnetosphere. Consequently, there are interesting interactions between these large orbiting bodies and the rapidly moving plasma that is being swept around in space with Jupiter's rotation. Wakes are formed as the plasma is swept past slower-moving satellites. Many things influence the interactions, including the size and magnetic moment of the satellite, the flow velocity and magnetic field strength of the corotating plasma, the surface properties of the satellite, and the satellite atmospheres, if such exist. We know that Io, for example, has a very strong interaction, accounting for a power input of 10¹² watts into the Jovian magnetosphere. At Saturn, the atmosphere of the satellite Titan may actually be being swept away by the planet's rapidly rotating the magnetosphere. Studies of plasma physics at Jupiter promise to be intriguing.

Principal investigator for the plasma wave observations is Don Gurnett of the University of Iowa, Iowa City. There are four coinvestigators.