Improved analysis of interplanetary HST-H-Ly alpha spectra using time-dependent modelings

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.