The magnetosphere-ionosphere system has been explored by a multitude of Earth-orbiting spacecrafts, but still the sparsity of the satellite fleet and the vast regions to be covered means that, at any given time, direct measurements of the magnetospheric processes are limited to only a few points in space. The European Space Agency’s Cluster mission with its four satellites flying in constellation has for the first time allowed for separation of space and time and by identification of full three-dimensional vector quantities from four point measurements (see, e.g., Escoubet et al., 2001, and other articles in the same volume). However, the limited number of measurement points still means that many quantities must be evaluated using proxy parameters derived either from point measurements in space or from ground-based observations.
For a long time, the auroral light from the polar region ionospheres was the only source that could provide two-dimensional images of the large-scale space plasma processes. Recently, the NASA IMAGE mission with its onboard neutral atom imagers demonstrated that also the charge-exchange processes with the neutral geocorona and the charged ring current particles can be strong enough to produce enough signal to monitor the plasmaspheric and ring current dynamics in the inner magnetosphere. As these are populations highly sensitive to processes occurring during space weather events, neutral atom imaging is becoming a new tool to monitor the state of the inner regions of the magnetosphere (Burch, 2000; Burch et al., 2001). However, the low signal to noise ratio and the complex inversion process from the line-of-sight measurements to spatially resolved ion distributions still limits the imaging applications. Figure 6 from Fok et al. (2003) shows an example of neutral atom imaging results during a magnetic storm. The red and green colors reflect the enhanced fluxes of energetic neutral atoms that are created by Coulomb collisions between the energetic ring current ions and cold geocoronal material. Such recreation of the ring current particle population requires detailed modeling of the neutral geocorona and the charge-exchange processes.
However, even with neutral atom imaging, much of the magnetosphere remains invisible to our eyes and instrumentation. To provide conceptual and predictive models of the magnetospheric evolution, large-scale global magnetohydrodynamic (MHD) simulations have been developed (Lyon et al., 2004; Janhunen, 1996). These models describe the solar wind-magnetosphere interaction as well as the coupling to the ionosphere in the single-fluid approximation. With limitations discussed in more detail below, these models have been successfully utilized to provide a large-scale framework for local observations as well as to infer global quantities that cannot be obtained directly from observations.
This section summarizes the most commonly used observational parameters and methods used in space weather research and gives an overview of the global MHD simulations whose results will be presented and discussed in the following Sections 5 and 6.
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