New observations from the Akebono satellite <Cully et al., 2002> evaluate the magnitude and solar wind dependence of the thermal component of terrestrial ion outflow. In this paper, we address the eventual supply of <20 eV ions to the central plasma sheet (CPS) by tracing their trajectories using empirical models of the electric and magnetic fields. We show the flux of ionospheric origin ions across the surface bounding the CPS to be substantial and largely controlled by the convection electric field. We find that under times of enhanced convection, ionospheric ions preferentially supply the region where substorm activity is thought to originate, and that a reduction in the convection (as caused, for example, by a northward turning of the interplanetary magnetic field (IMF)) immediately and dramatically reduces this supply. This effect arises due to the modification of the trajectories of in-flight ions. It is postulated that this sudden reduction in CPS ion supply in response to a northward IMF turning may have consequences for substorm triggering.