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An orange glow, reminding me of a blow-torch flame in size and intensity, shimmered inside the glass tubing in a chemistry lab. "That's a plasma," said my friend, the chemist. "But there are lots of different types of plasmas," he cautioned.
Nonetheless, having seen that small bit of created plasma helps me relate a little better to the complexity of studying natural plasmas in space.
A plasma is a collection of charged particles, usually made up of about equal numbers of ions and electrons. In some respects, plasmas act like gases; but, unlike gases, they are good electrical conductors and are affected by magnetic fields.
Galileo's plasma subsystem is designed to determine the properties of low-energy plasmas through Jupiter's magnetosphere, including plasma temperatures, densities, bulk motions, and composition. Observations by the Pioneer and Voyager spacecraft revealed that Jupiter's magnetosphere is a large and complex reservoir of charged particles. Building on the information provided by its forerunners, Galileo has the advantage of a spinning section that allows all-sky observations of fields and particles, a flexible command system, and long-term observations as the spacecraft orbits Jupiter for a least 22 months. The plasma instrument is also an advance over previous systems, with an extended sensitivity range for measuring electrons and positive ions, improved temporal resolution, and the ability to identify the composition of ions.
Jupiter's magnetosphere is of special interest for several reasons. It is the largest single object within the solar system, with an average diameter of about 15 million kilometers (9 million miles). If it could be seen from Earth--about 733 million kilometers (440 million miles) away--it would occupy 1.5 degrees of sky, compared to the Sun's 0.5 degrees of sky. In addition, Jupiter's rapid rotation creates a magnetosphere that resembles a pulsar, although much weaker.
Three ion sources have been identified for the plasma in Jupiter's magnetosphere: the Sun, the planet's ionosphere, and the large satellites. In the outer magnetosphere (beyond 60 Jupiter radii), the ions of helium, carbon, nitrogen, oxygen, neon, manganese, silicon, and iron probably originate from the solar wind. Closer to Jupiter, the ions of sulfur, sodium, and oxygen probably originate from Io and its dense plasma torus. Energetic molecular hydrogen most likely comes from the ionosphere.
Galileo's plasma instrument has two electrostatic analyzers that measure the energy per unit charge for electron and positive ion intensities both separately and simultaneously. They also measure the direction of flow of the charged particles. Each of the plasma analyzers is powered and programmed separately.
In addition, three miniature mass spectrometers measure the mass per unit charge of positive ions. Since neither Pioneer nor Voyager carried such spectrometers, Galileo will provide the first direct identification of ion species.
The overall field of view is fan-shaped, with seven aperture slits for the plasma analyzers' sensors. The fields of view for the mass spectrometers are positioned near the exit apertures of the plasma analyzers. The fan-shaped field of view is rotated about the spacecraft's spin axis during all-sky surveys to determine the directional flow and velocity of the charged particles.
The range of the plasma analyzer is 1 to 50,000 volts, compared to 100 to 4800 V for the Pioneer spacecraft and 10 to 5920 V for the Voyager spacecraft. In addition, Galileo's temporal resolution is 5 seconds, compared to 200 seconds for Pioneer's ion analyzer and Voyager's Faraday cup. These improved resolutions are important for the fast flybys of the satellites and crossings through various plasma areas.
Principal investigator for the plasma subsystem is Lou Frank of the University of Iowa.
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