The H Lyman-α actinic flux in the middle atmosphere

The penetration of solar H Lyman-α radiation into the terrestrial middle atmosphere is studied in detail. The Lyman-α actinic flux is calculated with a Monte Carlo approach including multiple resonance scattering of Lymanα photons within the terrestrial atmosphere and a temperature dependent absorption cross section of molecular oxygen. The dependence of the actinic flux on the temperature profile is significant for O2 column densities greater than about 1024 m−2. For column densities greater than about 5 · 1024 m−2 resonance scattering becomes important at solar zenith angles > 60. The O(1D) quantum yield of the O2 dissociation by Lyman-α photons is found to decrease from 0.58 in the lower thermosphere to 0.48 in the lower mesosphere. Parameterisations for Lyman-α actinic flux, mean O2 absorption cross section and O(1D) quantum yield including temperature dependence and resonance scattering are given valid up to a O2 column density of about 1025 m−2.