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Astron. Astrophys. 325, 745-754

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Refined analysis of interplanetary H-Ly spectra obtained
with the Hubble-Space Telescope GHRS spectrometer

Horst Scherer1, Hans J. Fahr1, and John T. Clarke2

1Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
2Space Physics Research Lab., University of Michigan, Ann Arbor, MI 48109, USA

Received 8 November 1996 / Accepted 17 March 1997


Here we present a new synoptic analysis of 5 HST-GHRS interplanetary H-Ly FORMULA spectra obtained at different lines of sight from different positions of the earth in its orbit. The spectral analysis aimed at the derivation of actual interstellar gas parameters is carried out on the basis of a newly developed Ly FORMULA radiation transport code taking into account the effects of angle-dependent partial frequency redistribution, selfabsorption by the interplanetary hydrogen, and the actual spectral profile of the solar Ly FORMULA emission line. As is shown the five different spectra, registered over a total time period of 18 months, when analyzed separately will lead to slightly different LISM parameters, however, if analyzed synoptically can also be reasonably well explained by one common set of LISM parameters. Hereby the fit achieved with the most conventional LISM parameter set (T = 8000 K; v = 26 km/s) can be improved by red-shifting the upwind spectra by -5 km/s. This shift, as we are going to argue, is best explained by a 30FORMULA-tilt of the symmetry axis of the heliospheric interface with respect to the LISM helium inflow direction, which could easily be established by LISM magnetic fields of a few µGauss tilted with respect to the helium inflow direction. Enhanced values for the solar Ly FORMULA radiation pressure ( FORMULA ) would reduce the needed redshift to less than 2.5 km/s (tilt angles much lower than FORMULA ).

Key words: radiative transfer - interplanetary medium

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© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 28, 1998