Plasma Science: From Fundamental Research to Technological Applications (1995)
Chapter: Plasma Confinement and Transport
elevate their energy selectively from a thermal bath. Space plasmas are an accessible microcosm for examining some acceleration processes. Indeed, the observational origins of space plasma physics trace to cosmic-ray physics. Acceleration processes can be either systematic or random. An example of the former is the function of electric double layers that form at low altitudes along terrestrial magnetic field lines as a result of collective processes. Double layers have been created in laboratory devices and replicated in numerical simulations, but owing to their spatial extent, like so many phenomena they are difficult to identify unambiguously from space data. Much more widely invoked, because it is a process capable of raising particles to very high energies, is stochastic acceleration by random and repeated encounters with electromagnetic fluctuations. Outstanding success has been achieved in analyzing this mechanism in connection with shock-induced fluctuations. The treatment has been kinetic and self-consistent in the sense that the accelerated particles contribute to the energizing wave spectrum. An important next step is to assess the relative importance of such accelerated particles to the shock structure itself.
Plasma Confinement and Transport
Plasma confinement and transport are widely inclusive concepts. At the time of the Colgate study,3 the primary reference was to the longevity of energetic, magnetically trapped particles, such as those populating the Earth's Van Allen belts. Like the solar wind, these are enduring features of the Earth's space environment, and analogous structures have been discovered in the environs of all the magnetized planets. The slow loss of Van Allen particles to Earth's atmosphere due to Coulomb collisions at very low altitudes and wave-particle scattering at higher altitudes is well understood. Of a much more speculative nature is the process that replenishes the belts so that they maintain their long-term existence. The issue is especially important at Jupiter, where MeV electrons are continuously losing energy due to synchrotron radiation. Present conjecture is that global-scale, low-frequency electromagnetic fluctuations, perhaps induced by solar wind buffeting of the magnetosphere or time-dependent atmospheric dynamo processes, allow particles to randomly traverse magnetic field lines to close-in distances, gaining energy in the process. Because of the global nature of the physics, it is difficult to verify this process observationally. What has been done is to create diffusion models using representative fluctuation spectra and to compare output particle distributions with observations—and this has been carried out with reasonable success. However, confinement and transport issues during the next decade will undoubtedly be far more expansive. To what extent are the outer reaches of the Earth's magnetosphere and the Sun's atmo-
