REFINED ANALYSIS OF INTERPLANETARY H-LY-ALPHA SPECTRA OBTAINED WITH THE HUBBLE-SPACE-TELESCOPE GHRS SPECTROMETER

Here we present a new synoptic analysis of 5 HST-GHRS interplanetary H -Ly alpha spectra obtained at different lines of sight from different positions of the earth in its orbit. The spectral analysis aimed at th e derivation of actual interstellar gas parameters is carried out on t he basis of a newly developed Ly alpha radiation transport code taking into account the effects of angle-dependent partial frequency redistr ibution, selfabsorption by the interplanetary hydrogen, and the actual spectral profile of the solar Ly alpha emission line. As is shown the five different spectra, registered over a total time period of 18 mon ths, when analyzed separately will lead to slightly different LISM par ameters, however, if analyzed synoptically can also be reasonably well explained by one common set of LISM parameters. Hereby the fit achiev ed with the most conventional LISM parameter set (T = 8000 K; v = 26 k m/s) can be improved by red-shifting the upwind spectra by -5 km/s. Th is shift, as we are going to argue, is best explained by a 30 degrees- tilt of the symmetry axis of the heliospheric interface with respect t o the LISM helium inflow direction, which could easily be established by LISM magnetic fields of a few mu Gauss tilted with respect to the h elium inflow direction. Enhanced values for the solar Ly alpha radiati on pressure (mu = 1.8) would reduce the needed redshift to less than 2 .5 km/s (tilt angles much lower than 30 degrees).