Interplanetary Gas. I. Hydrogen Radiation in the Night Sky.

Lyman-a in the light of the night sky seems to be produced through resonance scattering of solar Ly-a by H in the interplanetary medium. Arguments are given against the hypothesis that the radiation might originate in a geocorona (extended outer atmosphere of the earth). The radiative-transfer problems of diffuse reflection hy the atmosphere, of the night-sky radiation and of the solar emission, are treated. The Ly-a airglow observed between 85 and 120 km could arise entirely from scattering of the night-sky radiation if the latter originated in a geocorona. But if, as seems to he the case, interplanetary gas is responsible, the airglow can be attributed to an albedo effect only for a rather large (hut not impossible) value of the hydrogen abundance in the upper atmosphere, and it seems likely that an additional source of Ly-a radiation may be present. The intensity distribution over the night sky indicates that the H is located principally within 1 a.u. of the earth. This intensity distribution and the observed intensity ratio, Ly-a/Ha, tend to substantiate the assumption made here that the interplanetary gas is optically thin in Ly-a. The H density implied by the night-sky intensity is at least 0.2 atom/cm3 in the neighborhood of the earth. The position of H in the solar system and the optical thickness may be investigated further by measurements of Ly-a polarization, computed here for an optically thin model. The data available so far do not provide a determination of the temperature of interplanetary H, but it could be derived from accurate measurements of the solar emission profile, the daytime albedo, and the variation with rocket altitude of the night-sky intensity in various directions.