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Jupiter's magnetosphere is the region within which the planet's magnetic field and charged-particle population is confined by the flowing solar wind. Energetic particle charges are so intense in this region that careful design is required to protect the spacecraft against radiation damage, a particular concern for Galileo which must operate in this environment for its 22-month prime mission.
An immense and dynamic reservoir of energetic particles must be continually replenished to replace those particles that escape into interplanetary space. The processes responsible for this remarkable replenishment are unknown. Coupled with the study of magnetospheric dynamics, the identification of these processes represents the primary focus of Galileo's Energetic Particles Detector (EPD). The EPD measures the composition, intensity, energy, and angular distribution of charged particles (with energies greater than approximately 20 kiloelectron volts) within the Jovian magnetosphere.
Voyager's observations have led to the identification of three sources for Jupiter's energetic particles: the Sun, the Jovian ionosphere, and the Jovian moons.
The Sun (solar wind and energetic particles) is the most likely candidate for the helium, carbon, nitrogen, oxygen, neon, magnesium, silicon, and iron seen in the outer magnetosphere. Closer to the planet, the high abundances of sulphur, sodium, and oxygen provide strong evidence that these particles originate from Io and its plasma torus (a region containing about equal numbers of positively and negatively charged particles). Of the molecular ions observed in Jupiter's magnetosphere, hydrogen (H2) may come from both the ionosphere and the moons, whereas H3 is most likely of ionospheric origin. Jupiter's intense proton population probably comes from both the Sun and the ionosphere.
A comparison of Voyager 1 and 2 data strongly suggests that the relative contribution of Jovian and solar sources varies considerably with time. Thus, obtaining a long history of the Jovian energetic particle population is crucial to beginning a study of the dynamics of the Jovian magnetosphere.
The EPD uses two silicon solid-state detector systems: the Low Energy Magnetic Measurement System (LEMMS) and the Composition Measurement Subsystem (CMS). The magnetically focused LEMMS separately measures ions and electrons. The CMS uses a multiparameter detection technique to measure ions ranging from protons to iron (an energy range from 80 to 10,000 kiloelectron volts per atomic mass unit). The CMS also determines the velocity of these ions by measuring the time it takes to pass between the front and the back detectors, a distance of 7.5 centimeters (3 inches). This added capability allows a separate check on data validity, which is particularly helpful for particles at high incoming rates.
The detector assemblies use magnetic deflection, absorber materials to differentiate between incoming particle types, and varying aperture sizes to allow operation over a wide dynamic rate range. Radioactive calibration sources are mounted on a vertical shield that is observed by the detectors every 140 seconds.
The primary new thrust to be gained from Galileo will be an understanding of the Jovian magnetosphere and its dynamics. Unlike Pioneer and Voyager, Galileo will be placed into orbit around Jupiter and will obtain (for the first time) continuous coverage of the Jovian magnetospheric particle and field environment. Thus, it will be possible to determine characteristic time variations of the Jovian magnetosphere.
The Galileo mission also affords a much larger coverage of the Jovian magnetosphere, including the important midnight meridian region of the Jovian magnetic tail. The extended coverage will detect how and how much of Jupiter's particle population is lost to interplanetary space.
In addition, the Galileo spacecraft will be maneuvered to perform a total of 10 flybys of Jupiter's four largest satellites, ranging in altitude from a few hundred to a few thousand kilometers. From these close flybys, scientists will determine how the satellites interact with Jupiter's magnetospheric plasma and how this affected the evolution of these bodies.
Finally, the EPD will create a three-dimensional "map" of the energetic particle distribution by rotating through 225 degrees over seven Galileo spin periods (140 seconds). These data will yield information on particle energization and transport, the magnetic field configuration, and particle output from the satellites. Because the ranges of several instruments overlap, Galileo will provide the first continuous spectral observations of the overall Jovian charged particle distribution.
Jim Willett, the EPD science coordinator at JPL, emphasizes that "each of the fields and particles (F&P) instruments senses only a portion of the whole picture. By combining the results of all the F&P instruments, we will be able to more exactly shape our model of the Jovian system." The EPD science team also includes The Johns Hopkins University, the Max-Planck-Institut fur Aeronomie, the University of Alaska, the University of Kansas, and Bell Laboratories. The principal investigator for the EPD is D. J. Williams of The Johns Hopkins University.
The new dimensions of the Galileo mission may solve the many mysteries raised by the Pioneer and Voyager flyby missions. What are the intrinsic time variations of the Jovian magnetosphere? How do the charged particles escape? What physical processes maintain the intense particle populations in this vast but porous energetic particle reservoir? Are such powerful energization processes universally common? Do these processes sustain a Jovian magnetospheric wind of charged particles flowing away from the planet? Does the interaction of the Galilean satellites with the Jovian magnetosphere affect or guide their evolution? The Galileo mission gives us our best (and for the foreseeable future, the only) opportunity to answer these and other basic questions on the behavior of plasmas in the solar system.
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