Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 2 5 2002 10.5194/acpd-2-1635-2002 http://www.atmos-chem-phys-discuss.net/2/1635/2002/ http://www.atmos-chem-phys-discuss.net/2/1635/2002/acpd-2-1635-2002.html http://www.atmos-chem-phys-discuss.net/2/1635/2002/acpd-2-1635-2002.pdf 1635 1654 2002-10-28 The H Lyman-α actinic flux in the middle atmosphere Th. Reddmann R. Uhl Institute of Meteorology and Climate Research, Forschungszentrum Karlsruhe und Universität Karlsruhe, Germany The penetration of solar H Lyman-a radiation into the terrestrial middle atmosphere is studied in detail. The Lyman-a actinic flux is calculated with a Monte Carlo approach including multiple resonance scattering of Lyman-a 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 \OD\ quantum yield of the O2 dissociation by Lyman-aphotons is found to decrease from 0.58 in the lower thermosphere to 0.48 in the lower mesosphere. Parameterisations for Lyman-a actinic flux, mean O2 absorption cross section and \OD\quantum yield including temperature dependence and resonance scattering are given valid up to a O2 column density of about 1025 m-2.