title: Detection of interstellar pick-up hydrogen in the solar system.
abstract: Interstellar hydrogen ionized primarily by the solar wind has been detected by the SWICS instrument on the Ulysses spacecraft at a distance of 4.8 astronomical units from the sun. This "pick-up" hydrogen is identified 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 system are estimated to be 0.077 +/- 0.015 and 0.013 +/- 0.003 per cubic centimeter, respectively.
authors.forename: G
authors.initials: G
authors.lastname: Gloeckler
authors.forename: A B
authors.initials: AB
authors.lastname: Galvin
authors.forename: F M
authors.initials: FM
authors.lastname: Ipavich
authors.forename: J
authors.initials: J
authors.lastname: Geiss
authors.forename: H
authors.initials: H
authors.lastname: Balsiger
authors.forename: R
authors.initials: R
authors.lastname: von Steiger
authors.forename: L A
authors.initials: LA
authors.lastname: Fisk
authors.forename: K W
authors.initials: KW
authors.lastname: Ogilvie
authors.forename: B
authors.initials: B
authors.lastname: Wilken