The H Lyman-a actinic flux in the middle atmosphere

T. Reddmann (1) , R. Uhl (1)

1 IMK - Institute for Meteorology and Climate Research

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.

Domaines: Océan, Atmosphère

T. Reddmann, R. Uhl. The H Lyman-a actinic flux in the middle atmosphere. Atmospheric Chemistry and Physics, 2003, 3 (1), pp.225-231. ⟨hal-00295234⟩