Atmos. Chem. Phys. Discuss., 2, 1635-1654, 2002
www.atmos-chem-phys-discuss.net/2/1635/2002/
doi:10.5194/acpd-2-1635-2002
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
The H Lyman-α actinic flux in the middle atmosphere
Th. Reddmann and R. Uhl
1Institute of Meteorology and Climate Research, Forschungszentrum Karlsruhe und Universität Karlsruhe, Germany

Abstract. 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.

Citation: Reddmann, Th. and Uhl, R.: The H Lyman-α actinic flux in the middle atmosphere, Atmos. Chem. Phys. Discuss., 2, 1635-1654, doi:10.5194/acpd-2-1635-2002, 2002.
 
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