Improved analysis of interplanetary HST – H Ly α spectra using time – dependent modelings

During a period of 18 months 5 Hubble–Space– Telescope GHRS interplanetary H Lyα glow spectra were obtained at different lines of sight from different positions of the earth on its orbit, but despite employment of a radiation transport model that takes into account the angle–dependent partial frequency redistribution, the self-absorption by interplanetary hydrogen, the realistic spectral profile of the solar H Lyα emission line, and a stationary hydrogen model with a heliospheric interface, no common parameter set for density, temperature and velocity of the interstellar hydrogen could be deduced (see our earlier paper Scherer et al. 1997). One possible explanation 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 the HST H Lyα spectra. As we show here the theoretical predictions can be improved by using a time– dependent hydrogen model that also takes into account heliospheric interface effects, long–term variation of the H Lyα irradiance, its influence on the radiation pressure and the long–term variation of the hydrogen ionisation rate. The attempt at finding a common LISM parameter set, fitting 3 HST spectra is improved, though there still remain some discrepancies between data and the theoretical description, mainly manifest over time scales of the order of a year. This residual could be explained by possible variations in the spectral shape of the solar H Lyα line profile adopted as constant to model the radiation pressure and the resonance intensities.