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

Here we present a new synoptic analysis of 5 HSTGHRS interplanetary H-Lyα 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 Lyα 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 Lyα 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 LISMparameters. 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 30o-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 Lyα radiation pressure (μ = 1.8) would reduce the needed redshift to less than 2.5 km/s (tilt angles much lower than 30o).