PIONEER-10 AND VOYAGER-1 OBSERVATIONS OF ANOMALOUS COSMIC-RAY HYDROGEN IN THE OUTER HELIOSPHERE

Neutral hydrogen is the major constituent of the interstellar medium, and energetic hydrogen nuclei are expected to be present in the anomalous cosmic-ray component within our heliosphere. Detection of anomalous hydrogen has proven difficult in part because its charge state is the same as that of Galactic cosmic-ray hydrogen. However, the 1994 data from Voyager 1 (58 AU) and Pioneer 10 (61 AU) conclusively show the presence of anomalous hydrogen with approximately the same relative distribution in total kinetic energy as that found for the anomalous helium component. With the methods of analysis developed for the 1994 data, it is also possible to identify anomalous hydrogen in the 1987 (43 AU) and 1977/78 (15 AU) Pioneer 10 data over the two previous solar minimum periods.