Detection of Interstellar Pick-Up Hydrogen in the Solar System
G. Gloeckler 1,
A. B. Galvin 1,
F. M. Ipavich 1,
J. Geiss 2,
H. Balsiger 2,
R. von Steiger 2,
L. A. Fisk 3,
K. W. Ogilvie 4, and
B. Wilken 5
1 Department of Physics, University of Maryland, College Park, MD 20742
2 Physikalisches Institut, Universitat Bern, 3012 Bern, Switzerland
3 National Aeronautics and Space Administration, Office of Space Science and Applications, Washington, DC 20546
4 National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, MD 20770
5 Max-Planck-Institut für Aeronomie, 3411 Katlenburg-Lindau, Germany
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.
Submitted on January 19, 1993
Accepted on May 3, 1993
