Evaluating the Importance of Outflow Velocity at the MHD Inner Boundary

Including an ionospheric source of magnetospheric plasma in global magnetohydrodynamic models (MHD) is an exercise in setting inner boundary mass density and radial velocity. Recently, in order to account for the complex processes that accelerate plasmas up from ionospheric altitudes to MHD inner boundary altitudes (typically 2.5 to 3 Earth Radii), empirical and first-principles-based models have been developed to set inner boundary conditions in a dynamic and activity-dependent manner. However, such measures are not necessary to achieve outflowing fluences of the order observed by various spacecraft. Spatially and temporally constant boundary conditions, even with zero radial velocity, have been shown to produce dynamic outflow patterns and supply the bulk of magnetospheric plasma. Noteworthy of this approach is the inherent assumption that no acceleration has occurred between the ionosphere and the inner boundary, that is, the ionosphere is simply a mass reservoir. This assumption is contrary to our understanding of the magnetosphere-ionosphere system, yet the net result - outflowing heavy and light ions that populate the rest of geospace - is similar to that when a more realistic outflow specification is applied. The implication is that radial velocity matters little when supplying outflow to global MHD models. This paper investigates the importance of radial velocity at the inner boundary of MHD codes in driving ionospheric outflows into the greater domain. Multi-fluid BATS-R-US is used to simulate an idealized storm, first using zero radial velocity at the inner boundary, then non-zero constant values, and finally with spatially and temporally dynamic values driven by the Polar Wind Outflow Model (PWOM), which sets radial velocity and number density based on physics-based modeling of gap region populations. The results, in terms of total fluence, spatial outflowing flux patterns, and overall magnetospheric response, are compared to investigate how the simulation depends on this value. Magnetospheric implications are evaluated via density, temperature, and composition within the lobes, plasmasheet, and ring current regions. MHD-derived global indices, such as DST and CPCP, will also be leveraged. The results of this study are a systematic test of the importance of sub-magnetosphere acceleration of ionospheric plasma in magnetospheric dynamics.