Excitation of the Lyman-? in the night sky

Abstract

The experiments pertaining to the excitations of Lyman-α radiation in the night sky are reviewed. Various possible modes of excitation are considered. Scattering of solar Lα in interplanetary hydrogen is not suitable because of an insufficient amount of hydrogen and difficulty in accounting for the large night time albedo contrasted with the small daytime value. Transport by multiple scattering in the exosphere produces an appreciable excitation as well as the correct variation of intensity as function of zenith angle. However, the process fails by a factor of 4 for a solar flux of 6 ergs/cm2 sec and the 3 × 1012 atoms/cm2 geocorona consistent with absorption measurements. A direct scattering in a geocoma far from the earth overcomes the albedo and thin atmosphere problems. However, there is difficulty in accounting for the presence of the amount of hydrogen necessary beyond 15 earth radii. Also the absence of Hα excitation by Lyman-β is a problem. No explanation so far proposed appears adequate. Experiments are suggested.