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SPACE PLASMAS 110 plasma, and the plasma's chemical nature. This permits study of various processes occurring within ionospheric plasmas, including production, loss, and transport. It is also a way of creating unstable environments that evolve in interesting and new ways not normally found in the natural environment. The possibility of creating a large-scale ionic plasma (positive and negative ions dominating the overall composition) is both interesting and important in that it allows new processes to become dominant in plasma behavior. Such experiments are difficult, if not impossible, to perform in terrestrial laboratories. Pulsed plasma beam or contactor experiments, where a dense plasma cloud is released into the ambient medium, can give information on plasma transport. The plasma cloud expands and distorts in response to its internal diamagnetic structure as well as the external flow field. This expansion sheds light on the fundamental plasma physics of high-beta plasma clouds, such as occur in the magnetotail, as well as the nature of the transport process when the cloud is diluted. Such experiments are conceptually similar to those already under way in ground-based laboratories, with an important exception. By using pulses of sufficient density and duration, it is possible to create steady-state diamagnetic plasma regions near the source. Information about the various processes acting in such an unusual plasma configuration is an important step toward understanding a new regime of plasma physics. Particle Beam Experiments Particle-beam experiments have been conducted from many sounding rockets, satellites, and space shuttle missions. Objectives in these experiments have been (1) to map the geomagnetic field structure and parallel electric fields by observing echoes of beam electrons from the magnetic conjugate mirror point or from electrostatic structures along auroral field lines; (2) to study auroral processes, such as optical emissions and wave turbulence in auroral particle beams; (3) to stimulate electromagnetic and electrostatic wave excitation; (4) to create suprathermal electron tails; (5) to observe the interaction of the particle beam with the neutral gas in the vicinity of the source payload; and (6) to study spacecraft charging and neutralization. Wave Injection Experiments Space-based wave injection experiments make use of a number of different techniques to launch waves into the plasma. Topside sounders rely on the excitation of plasma resonances. Transmitters have been used on a number of sounding rockets. Finally, modulated electron beams have been used as ''virtual antennas" on STS-1 and the Spacelab-2 shuttle missions and on numerous sounding rocket experiments. Waves have been detected to a distance of a few kilometers. In these experiments, receivers have been located on the transmitter platform, on
