During a period of 18 months 5 Hubble-Space-Telescope GHRS interplanetary H
-Ly alpha: glow spectra were obtained at different lines of sight from diff
erent positions of the earth on its orbit, but despite employment of a radi
ation trans port model that takes into account the angle-dependent partial
frequency redistribution, the self-absorption by interplanetary hydrogen, t
he realistic spectral profile of the solar H-Ly alpha emission line, and a
stationary hydrogen model with a heliospheric interface, no common paramete
r set for density, temperature and velocity of the interstellar hydrogen co
uld be deduced (see our earlier paper Scherer et al. 1997). One possible ex
planation is the uncertainty in the interstellar hydrogen inflow direction,
but even this would not completely dissolve some discrepancies between the
theoretical predictions of spectral Doppler shifts and those observed in t
he HST H-Ly alpha spectra. As we show here the theoretical predictions can
be improved by using a time-dependent hydrogen model that also takes into a
ccount heliospheric interface effects, long-term variation of the H-Ly alph
a irradiance, its influence on the radiation pressure and the long-term var
iation of the hydrogen ionisation rate. The attempt at finding a common LIS
M parameter set, fitting 3 HST spectra is improved, though there still rema
in some discrepancies between data and the theoretical description, mainly
manifest over time scales of the order of a year. This residual could be ex
plained by possible variations in the spectral shape of the solar H Ly-alph
a line profile adopted as constant to model the radiation pressure and the
resonance intensities.