Local Hydrogen Gas and the Background Lyman-alpha Pattern

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Abstract
The trapping of resonant solar photons in the local hydrogen cloud contributes substantially to the Lyman- α background. The contribution, commonly ignored as ‘multiply-scattered’ photons, is described here as ‘cavity radiation’ (the Sun being within a cavity as it moves through the gas). Two types of cavity are described and the backscattered radiation intensity calculated—a physical cavity produced by destructive processes, appropriate to hot gas; and a ‘Doppler cavity’ due to gravitational acceleration of nearby hydrogen atoms and relevant to cool gas. The OGO 5 Lyman- α intensities are re-interpreted as the sum of near-isotropic cavity radiation plus the direct backscatter from asymmetrically distributed gas inside the cavity. The gas densities derived are in excess of of 0.1 H-atoms cm −3 and quite possibly as high as the 0.2–0.3 cm −3 of independent local derivations. If the latter is the case, the Sun's speed through the gas is around 15 km s −1 . In all cases the cavity radiation is substantial at around 200 R. It explains the relatively steady, isotropic part of the background, and dispenses with the assumption of a large ‘galactic’ contribution. Whether the gas is hot (6–10 000 K) or cool (40–80 K) is not easy to determine. Proton elastic collisions provide effective heating and would eliminate any secondary maximum due to gravitational focusing of cool gas. The variation with solar cycle, as revealed through changes in the radiation pressure, is perhaps the simplest way to distinguish between hot and cool gas. The Mariner 9 observation (2 yr after OGO 5) of a sharp reduction in the intensity minimum, appears to exclude the hot gas possibility.

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