HST/GHRS Observations of the Velocity Structure of Interplanetary Hydrogen

Open Access
Publisher Website
Google Scholar
Abstract
We present high-resolution spectra of the emission-line profile of inflowing interplanetary hydrogen atoms along lines of sight with the Earth orbital motion upwind (into the flow), downwind, and across the flow to Doppler-shift the line from the geocoronal emission. The line-center positions, in comparison with hot-model profiles, confirm that the inflow speed of H atoms far from the Sun (~50 AU) is in the range 18-21 km s-1, which implies a decrease in the velocity distribution of 5-8 km s-1 for hydrogen within the solar system, relative to the He flow and to the local interstellar medium. Best-fit values are derived for the speed and effective solar gravity along the three lines of sight by comparison with model profiles convolved with the instrument line-spread function. For the assumed inflow direction, the cross-flow line profile requires that the μ-value be slightly less than unity near solar minimum, and a technique is presented for determining the exact inflow direction and μ-value independently of the other parameters. The line widths indicate a broadening along the flow direction in addition to the dynamical effects near the Sun expected from two different hot models, whereas the cross-flow line width is similar to the hot-model profiles. The altered velocity distribution in the inflow direction appears likely to be related to the crossing of the interstellar/interplanetary medium interface structure, although questions remain about the cumulative effects of changing solar activity on the timescale of the H atom flow through the solar system.