Measurements of the interstellar gas

Abstract

Two types of interstellar gas observations are considered; namely those made in the local interstellar medium on paths to nearby stars, and those obtained from the study of solar system gas. Optical measurements, and in particular Hubble Space Telescope spectra of the star Capella, have recently confirmed detection of the Local Interstellar Cloud (LIC) surrounding the Sun. The heliocentric velocity and temperature of the cloud are 26 km s^-1 and 7000K. The agreement with direct measurements of neutral helium made by the Ulysses spacecraft within the solar system is excellent /1/. These results provide a firm reference for measuring perturbations of hydrogen (or other species) at the heliospheric interface. In particular, H cell observations suggest that neutral H is decelerated by a few km s^-1 when entering the heliosphere. Recent observations of the Lyman-alpha glow made by the Voyager spacecraft at 30 AU and the glow line profile recorded by the Hubble UV spectrograph GHRS are discussed, as are future interstellar gas measurements which could provide some constraints on the values that can be placed on the heliospheric interface parameters.