A self-consistent time-dependent model is used to study the modification of the heliospheric termination shock in the upwind direction under the influence of three suprathermal particle populations, namely pickup, anomalous and galactic cosmic ray protons. By ensuring that the resulting modulated cosmic ray proton spectra are consistent with those observed by the Voyager 2 and Pioneer 10 spacecraft during the solar activity minimum in 1987, two alternative modifications of the shock can be identified. While the first is characterized by a low injection efficiency of pickup protons into the process of diffusive shock acceleration and mainly determined by those particles, the second, resulting from a high injection efficiency, is clearly dominated by anomalous cosmic rays.¿ 1997 American Geophysical Union