HST GHRS OBSERVATIONS OF THE VELOCITY STRUCTURE OF INTERPLANETARY HYDROGEN/

We present high-resolution spectra of the emission-line profile of inf lowing interplanetary hydrogen atoms along lines of sight with the Ear th orbital motion upwind (into the flow), downwind, and across the flo w to Doppler-shift the line from the geocoronal emission. The line-cen ter positions, in comparison with hot-model profiles, confirm that the inflow speed of H atoms far from the Sun (similar to 50 AU) is in the range 18-21 km s(-1) which implies a decrease in the velocity distrib ution of 5-8 Ism s(-1) for hydrogen within the solar system, relative to the He flow and to the local interstellar medium. Best-fit values a re derived for the speed and effective solar gravity along the three l ines of sight by comparison with model profiles convolved with the ins trument Line-spread function. For the assumed inflow direction, the cr ossflow Line profile requires that the mu-value be slightly less than unity near solar minimum, and a technique is presented for determining the exact inflow direction and mu-value independently of the other pa rameters. The line widths indicate a broadening along the inflow direc tion in addition to the dynamical effects near the Sun expected from t wo different hot models, whereas the cross-flow line width is similar to the hot-model profiles. The altered velocity distribution in the in flow direction appears likely to be related to the crossing of the int erstellar/interplanetary medium interface structure, although question s remain about the cumulative effects of changing solar activity on th e timescale of the H atom flow through the solar system.