HST GHRS OBSERVATIONS OF THE INTERPLANETARY MEDIUM DOWNWIND AND IN THE INNER SOLAR-SYSTEM

We present high dispersion spectra of the interplanetary medium (IPM) hydrogen Ly alpha emission along lines of sight to infinity and to Mar s, obtained with the Hubble Space Telescope (HST) and Goddard High Res olution Spectrograph (GHRS) in 1991 May, including a detailed descript ion of the data analysis procedures. The observed IPM emission at 46 d egrees from the downwind direction has a line of sight bulk velocity o f 13.6 km s(-1). The line profile is significantly broader than expect ed for the measured 8000 K temperature of the IPM, due in part to an i ncreased velocity dispersion resulting from selection effects in the n ear-solar environment. The IPM line profile is well fitted by a model of the IPM flow through the solar system, with the best fit derived fo r a H inflow velocity far from the Sun of 21 km s(-1). This derived H inflow speed in the solar system is significantly less than the 26 km s(-1) independently determined for both the local interstellar medium and the He inflow speed in the solar system. This may be due to a dece leration of the H inflow from charge exchange with solar wind protons in the heliospheric interface region, as suggested by independent mode ls. We also find that the column of emission along the line of sight f rom the Earth to Mars is roughly 1/2 of the total emission observed to infinity, suggesting a significant ''filling in'' of the solar ioniza tion cavity compared with existing models. The general method of obser ving isolated columns of interplanetary medium gas along the lines of sight to other planets is demonstrated to be a powerful technique for determining the properties of the IPM flow within the solar system and particularly in the ionization region near the Sun.