eration of electrons and ions in the high-latitude, high-altitude regions of Earth leads to the formation of highly structured streams of energetic charged particles that impact Earth's upper atmosphere, creating the aurora. Finally plasma technology devices such as electric propulsion and plasma contactors operate on a small size scale, establishing their own local boundary conditions and interacting with the nearby space plasma.

Knowledge of the physical processes operative in all of these examples is an important goal of space plasma physics for several reasons. First, it provides us with an understanding in quantitative terms of the variety of interrelated complex processes acting to shape and influence our terrestrial environment. Second, parts of the space plasma environment may be prototypical of the astrophysical environment. Third, space phenomena lead to fundamental scientific questions relating to the behavior of plasmas under conditions that can be very different from those created and studied in terrestrial laboratories. Finally, knowledge of the science underscores the development of technological applications operating in or based on the space plasma environment. As a consequence, investigations of natural space plasma processes extend the frontiers of human knowledge, enabling broader physical understanding of plasmas within the context of their general behavior.

Understanding Earth's plasma environment also has important practical consequences. Among these are an ability to model and predict ionospheric, magnetospheric, and interplanetary disturbances that could adversely affect ground-based communications, sensitive instrumentation in geosynchronous orbit, and the safety of astronauts participating in future interplanetary endeavors.

Status

The era of in situ exploration of space plasma physics began in 1946 with V-2 rocket "snapshots" of the terrestrial space environment and continues aggressively today. Measurement techniques include both direct sampling and space-based remote sensing. An excellent example of the latter is the global observation from space of aurora at UV and optical wavelengths, clearly delineating the dynamics of the auroral oval. The initial exploration of the terrestrial magnetosphere and ionosphere is now reasonably well complete, although there are still regions of the solar system that have not yet been explored at all (e.g., Pluto, the heliopause, the solar corona) and regions that have been seen only through brief flybys (e.g., Mercury, Uranus, Neptune). Emphasis now is shifting to the details of physical processes controlling these plasmas. The results of all modern theories and models have depended significantly on the progress of in situ observations.

Ground-based remote sensing studies of space plasma physics have played an important role by providing long-term, localized observations and understanding. Incoherent and coherent radar observations of natural ionospheric



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