Detection of Interstellar Pick-Up Hydrogen in the Solar System
Balsiger, H.; Geiss, J.; Galvin, A. B.; Ipavich, F. M.; Gloeckler, G.; Fisk, L. A.; Wilken, B.; von Steiger, R.; Ogilvie, K. W.
United States, Switzerland, Germany
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.