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Issue A&A
Volume 491, Number 1, November III 2008
Diagnostics of interstellar hydrogen in the heliosphere
Page(s) 7 - 19
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20078810
Published online 01 October 2008



A&A 491, 7-19 (2008)
DOI: 10.1051/0004-6361:20078810

Density of neutral interstellar hydrogen at the termination shock from Ulysses pickup ion observations

M. Bzowski1, E. Möbius2, S. Tarnopolski1, V. Izmodenov3, and G. Gloeckler4

1  Space Research Centre, Polish Academy of Sciences, Bartycka 18A, 00-716 Warsaw, Poland
    e-mail: bzowski@cbk.waw.pl
2  Space Science Center and Department of Physics, University of New Hampshire, USA
3  Moscow State University and Space Research Institute RAS, Russia
4  Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, USA

Received 8 October 2008 / Accepted 25 September 2008

Abstract
Aims. By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements near 5 AU close to the ecliptic right after the previous solar minimum, this paper presents a determination of the neutral interstellar H density at the solar wind termination shock and implications for the density and ionization degree of hydrogen in the local interstellar cloud.
Methods. The density of neutral interstellar hydrogen at the termination shock was determined from the local pickup ion production rate as obtained close to the cut-off in the distribution function at aphelion of Ulysses. As shown in an analytical treatment for the upwind axis and through kinetic modeling of the pickup ion production rate at the observer location, with variations in the ionization rate, radiation pressure, and the modeling of the particle behavior, this analysis turns out to be very robust against uncertainties in these parameters and the modeling.
Results. Analysis using current heliospheric parameters yields the H density at the termination shock equal to 0.087$\pm$0.022 cm-3, including observational and modeling uncertainties.


Key words: interplanetary medium -- Sun: solar wind -- ISM: clouds -- ISM: atoms -- Sun: UV radiation



© ESO 2008


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