close association with substorms. We use collisionless test particle motions in magnetospheric fields from a magnetohydrodynamic simulation. The simulation is used to generate bulk properties and detailed velocity distributions at key loca- tions, for comparison with observations. Particles are initiated in regions repre- sentative of the solar wind proton source upstream of the bow shock, the polar wind proton source, and the auroral zone enhanced outflows of O + , which we term "auroral wind". Results reflect steady growth phase conditions after 45 minutes of southward interplanetary field. Solar wind protons enter the ring cur- rent principally via the dawn flank, bypassing the midnight plasma sheet, while polar wind protons and auroral wind O + enter the ring current through the mid- night plasma sheet. Thus, solar wind and ionospheric plasmas take very different transport paths to the ring current region. Accordingly, they are expected to re- spond differently to substorm dynamics of the magnetotail, as observed recently by remote neutral atom imaging from the IMAGE mission. Polar wind protons make a minor contribution to ring current pressure under steady conditions, but auroral wind O + has the potential to dominate the ring current, when outflow is strongly enhanced during periods of enhanced solar wind dynamic pressure fluc- tuations.