As part of the Active Magnetospheric Particle Tracer Explorers (AMPTE) mission, releases of ~1025 atoms of lithium, and later barium, were made in the solar wind and magnetosheath. Initial photoionization produced a release plasma of sufficient densities that a diamagnetic cavity was seen to form at the release center, disrupting the ambient field and flow over a localized region (approximately tens of kilometers for Li, approximately hundreds of kilometers for Ba). Since the gyroradii of both the oncoming solar wind protons and the release ions were either greater than or of the order of the scale size of this perturbation while the electron gyroradii remained small, a hybrid description, where the ions are treated as particles, and the electrons as a charge-neutralizing fluid, is appropriate. Here we present the results of one-dimensional simulations for lithium using a hybrid description in order to investigate the process of momentum transfer between the two ion species that occurs locally over the boundary layer which forms between them. We find that a well-defined field and plasma structure evolves in which the bulk of the lithium ions are moved en mass by a ''snowplough'' type process, those remaining either being accelerated to higher speeds to produce ''taillike'' structures or remaining upstream to form part of the snowplough momentum balance. Field and ion density structures found in the simulations are in good gross agreement with observations. We aslo obtain an analytical estimate of the snowplough speed which is in good agreement with that obtained computationally. When set in the context of the three-dimensional release geometry, features resolved by our one-dimensional simulation are found to have clear parallels in the AMPTE data. ¿ 1987 American Geophysical Union