Heliospheric hydrogen beyond 15 AU: Evidence for a termination shock

The Voyager and Pioneer 10 spacecraft are moving upstream and downstream into the local interstellar flow, monitoring H Lyman α radiation resonantly scattered from heliospheric hydrogen. Voyager Cruise Maneuver observations obtained between 15 and 35 AU reveal that H Lyman α intensities in the upstream direction fall as r^-0.75+/-0.05. Beyond 15 AU downstream, Pioneer 10 intensities fall as r^-1.07+/-0.1. These trends cannot be simultaneously reproduced using a hot H distribution model that does not include termination shock structure. Radiative transfer calculations using the hot H model predict that upstream intensities should fall more rapidly as a function of heliocentric distance than downstream intensities, precisely opposite to the observed trends. The Voyager H Lyman α intensities also show a distinctive trend to decrease less rapidly with increasing heliocentric distance. Between 15 and 20 AU, Voyager intensities fall as r^-1, whereas between 30 and 35 AU they fall as r^-0.35. This flattening trend implies that the upstream H density is increasing rapidly with heliocentric distance beyond ~25 AU. A simple analysis suggests that the density distribution changes from nearly uniform between 15 and 20 AU, to r^0.65 dependence between 30 and 35 AU. This steepening trend is significant because similar H density gradients are predicted in models which include the effects of the termination shock. Taken together, the Voyager and Pioneer 10 H Lyman α observations beyond 15 AU imply the existence of a solar wind termination shock, suggesting that it lies between 70 and 105 AU in the upstream direction.