ESTIMATE OF THE DISTANCE TO THE SOLAR-WIND TERMINATION SHOCK FROM GRADIENTS OF ANOMALOUS COSMIC-RAY OXYGEN

The radial gradient of anomalous cosmic ray oxygen measured instantaneously between Voyager 2 and Pioneer 10 during 1985-1988 is correlated with the inferred tilt of the heliospheric neutral sheet. This is consistent with a simple model in which the radial gradient is related to the length of the neutral sheet between the two spacecraft. With this model we show that the radial gradient and the tilt of the neutral sheet near the solar wind termination shock can be inferred from the Voyager and Pioneer observations. By comparing the time history of the inferred tilt with that derived from solar observations, we estimate that the termination shock was at 62(+5/-7) AU at solar minimum in 1987. At solar maximum the shock should be located at approximately 90 AU due to the increased pressure of the solar wind.