DETECTION OF INTERSTELLAR PICK-UP HYDROGEN IN THE SOLAR-SYSTEM

Interstellar hydrogen ionized primarily by the solar wind has been det ected by the SWICS instrument on the Ulysses spacecraft at a distance of 4.8 astronomical units from the sun. This ''pick-up'' hydrogen is i dentified by its distinct velocity distribution function, which drops abruptly at twice the local solar wind speed. From the measured fluxes of pick-up protons and singly charged helium, the number densities of neutral hydrogen and helium in the distant regions of the solar syste m are estimated to be 0.077 +/- 0.015 and 0.013 +/- 0.003 per cubic ce ntimeter, respectively.