IT is usually assumed that the solar system is embedded in a region of neutral interstellar hydrogen, that is, an HI region. Up to now it has not been possible to measure the density of this interstellar medium in the vicinity of the solar system. Radio astronomy methods can only measure the integrated number of hydrogen atoms within the active antenna cone of the measuring telescope, and therefore are unable to give the density of interstellar hydrogen in the local region of our galaxy. On the other hand, measurements of the background of Lyman- by spacecraft and satellites are difficult to interpret because the sources of this background radiation are not certain. If the Lyman- background has a galactic origin, or is due to scattering of the Lyman- radiation of the nearby stars by interstellar hydrogen, nothing can be deduced about the density of the interstellar hydrogen in the neighbourhood of the solar system. But there is a good chance of obtaining a density value for the surrounding interstellar medium if the measured Lyman- background can be ascribed to the scattering of the solar Lyman- radiation by the neutral hydrogen near the Sun.
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