Diverse plasmas have been observed in the magnetospheres of Earth and other planets, and in interplanetary space. Recently, measurements were made by a space probe at Comet Giacobini-Zinner. In these various plasma regimes investigated during the space era, plasma characteristics vary widely: particle densities range from <0.01 to 1000 per cc, energies from <1 eV to many MeV, temperatures from <1 eV to >1000 eV, and bulk flow speeds from near stationary to >1000 km/s. A variety of plasma-wave activity and instabilities characterize these plasmas. The measurements are made with several different generic types of instruments including electrostatic analyzers and systems which combine electrostatic analysis with time-of-flight or magnetic analysis in order to identify ions and their charge states. This paper outlines the plasma regimes found in space, discusses instrument design principles and constraints imposed by limited spacecraft resources, describes a few instruments, and highlights a small selection of scientific results derived from diagnostics of space plasmas.
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