Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…the inner magnetosphere [Lui and Hamilton, 1992; De Michelis et al., 1999], the magnetosheath [Phan et al., 1994], and the plasma sheet [DeCoster and Frank, 1979; Takahashi and Hones, 1988], we have extended the global MHD model BATS-R-US [Powell et al., 1999] to solve for anisotropic ion pressure.
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