The Physical Significance of 'Shifted' Gradients and the Location of the Solar Wind Termination Shock

We use observations of anomalous cosmic-ray (ACR) oxygen from the cosmic-ray experiments on the Voyager 2 (V2) and Pioneer IO (PIO) spacecraft during 1985-1988 to investigate the physical significance of the "shifted" radial gradient The "shifted" gradient is computed by comparing fluxes measured at different times at the two spacecraft to "correct" for the propagation delay of solar modulation. We use a simple model of particle propagation in which the flow is inwards along the wavy neutral sheet to suggest that for qA < 0 near solar minimum conditions the "shifted" gradient is a measure of the gradient near the solar wind termination shock. This simple model leads to an estimate of the location of the termination shock at 55(+9/-5) AU, suggesting that the shock could be encountered within this decade