HELIOSPHERIC HYDROGEN BEYOND 15 AU - EVIDENCE FOR A TERMINATION SHOCK

The Voyager and Pioneer 10 spacecraft are moving upstream and downstre am into the local interstellar flow, monitoring H Lyman alpha radiatio n resonantly scattered from heliospheric hydrogen. Voyager Cruise Mane uver observations obtained between 15 and 35 AU reveal that H Lyman al pha intensities in the upstream direction fall as r-0.75+/-0.05. Beyon d 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 tr ansfer calculations using the hot H model predict that upstream intens ities should fall more rapidly as a function of heliocentric distance than downstream intensities, precisely opposite to the observed trends . The Voyager H Lyman alpha intensities also show a distinctive trend to decrease less rapidly with increasing heliocentric distance. Betwee n 15 and 20 AU, Voyager intensities fall as r-1, whereas between 30 an d 35 AU they fall as r-0.35. This flattening trend implies that the up stream H density is increasing rapidly with heliocentric distance beyo nd almost-equal-to 25 AU. A simple analysis suggests that the density distribution changes from nearly uniform between 15 and 20 AU, to r0.6 5 dependence between 30 and 35 AU. This steepening trend is significan t because similar H density gradients are predicted in models which in clude the effects of the termination shock. Taken together, the Voyage r and Pioneer 10 H Lyman alpha observations beyond 15 AU imply the exi stence of a solar wind termination shock, suggesting that it lies betw een 70 and 105 AU in the upstream direction.