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SPACE PLASMAS 105 A variety of standard observatories provide continuous measurements of magnetic fields, ionospheric conditions, and solar activity. Also, a number of facilities operate only occasionally, such as the large incoherent-scatter radar facilities. Plasma Theory and Simulations Theory must develop a framework for interpreting observations of various physical systems. With this framework as a basis, quantitative analytic and numerical descriptions of model physical systems are constructed and refined through comparison with observations. Ultimately, the model physical systems must be sufficiently similar to the natural physical systems, and the numerical and/or analytic descriptions of these model systems must be sufficiently refined to provide a high level of predictability of the observed behavior of space plasmas. (See Plate 5.) An outstanding issue involving modeling and simulations is how to properly represent multiscale phenomena numerically. For example, how can one best incorporate anomalous transport processes occurring in boundary layers (such as shocks), which occur on both temporal and spatial scales that are microscopic, into meso- or macroscale numerical codes? The ultimate test of accomplishment in theoretical and simulation research must be the degree to which closure is achieved with ground and space experiments. Theoretical investigations may be conducted with the goal of explaining physical phenomena that have been observed and measured, and theoretical studies may be designed to lead to predictions testable in space plasma physics missions. FIGURE 6.1 (page 104, above): Example of an active space-plasma experiment designed to study the deposition of energetic electron fluxes in the atmosphere. This controlled experiment, which was flown on the Space Shuttle ATLAS-1 mission in 1992, used an electron beam and an optical imager to study the deposition of high-energy electrons into the polar atmosphere. Shown in the top panel is the artificial aurora generated by the beam (upper right in the figure) and a quiet auroral arc (left). The electron beam pulse was 1 s in duration. The camera viewed downward along the magnetic field direction, and the direction of motion of the Shuttle Orbiter was to the left. The width of the image at a height of 110 km is about 80 km. The bar is linear in optical intensity. The image in the bottom panel is the same as that in the top panel but taken at a later time. It shows the artificial aurora superimposed on a large auroral arc. (Courtesy of S. Mende, Lockheed Research.)