ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-3-225-2003

The H Lyman-a actinic flux in the middle atmosphere

Reddmann

¹ Uhl

Institute of Meteorology and Climate Research, Forschungszentrum Karlsruhe und Universität Karlsruhe, Germany

20 02 2003

3 1 225 231

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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 O(1D) quantum yield of the O2 dissociation by Lyman-a photons 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 O(1D) quantum yield including temperature dependence and resonance scattering are given valid up to a O2 column density of about 1025 m-2.