UNDERSTANDING COMPLEX, COUPLED SYSTEMS

Solar system plasmas are complex systems. Complexity arises from nonlinear couplings, both within a single system and between two or more different systems. Both types of coupling occur in solar and space plasmas. Beyond 10 to 15 AU, for example, the dominant constituent of the heliosphere by mass is neutral interstellar hydrogen. Charge exchange reactions couple this neutral hydrogen population and the solar wind, yielding a highly nonequilibrated, nonlinear system in which the characteristics of both populations are strongly modified. In addition to couplings between multiple constituents, solar system plasmas are characterized by couplings across a multiplicity of spatial and temporal scales; the nonlinear, dynamical, self-consistent feedback and coupling of all scales determine the evolution of the systems through the creation of large- and small-scale structures.16 Examples of such cross-scale coupling are reconnection and turbulence, which involve the nonlinear interaction of large-scale, slow magnetohydrodynamic behavior and small-scale, fast kinetic processes. Finally, distinct plasma regions and regimes are coupled across boundaries in a highly nonlinear, dynamical fashion. Such cross-system coupling is exemplified by the coupling that occurs between the solar wind and Earth’s magnetosphere as a result of the merging of interplanetary and geomagnetic field lines and by the electromagnetic coupling of the magnetosphere and the ionosphere.

The complex, nonlinear, coupled character of solar system plasmas presents significant challenges to both our observational capabilities and our theoretical understanding. The research initiatives recommended by the committee and presented in the following chapter will enable solar and space physicists to address those challenges and thereby to achieve new and deeper understanding of solar system plasmas and of the fundamental physical processes that govern them.

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Conversely, research and technology that are explicitly directed toward practical ends can make substantial contributions to “pure” scientific inquiry and the acquisition of fundamental knowledge. A classic example from the early days of the space age is the important role played by the Vela satellites in the exploration of the magnetosphere and the nearby interplanetary medium. Launched during the 1960s and in early 1970, the Velas were part of a joint program of the Department of Defense and the Atomic Energy Commission to monitor nuclear tests from space. (Besides their role in magnetospheric research, the Velas also made a major contribution to astrophysics, through their discovery of gamma ray bursts.) Operational satellites—those of the Defense Meteorological Satellite Program, the geosynchronous



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