The Origin of Radio Fade-Outs and the Absorption Coefficient of Gases for Light of Wave-Length 1215.7A

It has been suggested that the ionization in the $D$ region which causes radio fade-outs is caused by radiation of the first line of the Lyman series of hydrogen, at 1215.7A, from solar eruptions. The actual absorption coefficients for this line, previously only roughly known, have been measured with the following results: oxygen 0.28, nitrogen $<~0.005$, carbon dioxide 2.01, water vapor 390 (all values reduced to NTP). Although it can probably penetrate to sufficiently low altitudes, it appears unlikely that this line can produce ionization and hence cause fade-outs. The possibility is considered that these may be due to higher Lyman series members, or to x-rays of around 2A wave-length.