A persistent, unresolved problem in terrestrial magnetospheric physics is determining the dominant source and associated entry mechanism for plasma in the Earth’s magnetosphere. This study uses the multispecies MHD code, Block Adaptive Tree Solar Wind Roe-Type Upwind Scheme (BATS-R-US), to investigate this issue. Two proton species, ionospheric origin and solar wind origin, are defined in the system and the evolution of each population is followed under different idealized solar wind conditions. It is found that during southward oriented interplanetary magnetic field (IMF), the dominant source is ionospheric plasma entering deep down tail through reconnecting field lines. During northward IMF, the dominant source is solar wind plasma entering through the flanks of the magnetosphere. This two-mode behavior is tested through data-model comparisons of real world simulations. Comparisons of model results against Los Alamos National Laboratory Magnetospheric Plasma Analyzer density, pressure, and inferred oxygen content support the conclusions of the idealized results.
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