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Detection of Interstellar Pick-Up Hydrogen in the Solar System

G. Gloeckler, J. Geiss, H. Balsiger, L. A. Fisk, A. B. Galvin, F. M. Ipavich, K. W. Ogilvie, R. von Steiger and B. Wilken
Science
New Series, Vol. 261, No. 5117 (Jul. 2, 1993), pp. 70-73
Stable URL: http://www.jstor.org/stable/2881466
Page Count: 4
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Detection of Interstellar Pick-Up Hydrogen in the Solar System
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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.

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