Calculations of Lyman Alpha Absorption in the Mesosphere

Abstract

Detailed calculations of the absorption of solar Lyman alpha flux in the earth’s mesosphere which include wavelength and temperature dependent O2 and H2O absorption cross sections, are used to investigate the effects on the water vapour photodissociation of a) absorption by atmospheric species other than O2 b) changes in the solar Lyman alpha line profile with solar activity and c) atmospheric temperature and density variations. These calculations indicate that neglecting the absorption by N2, H2O, CO2 and CH4, in addition to that by O2, may overestimate the Lyman alpha photodissociation of H2O by 8–9% in the altitude region of maximum photodestruction. Changes in the Lyman alpha line profile from the minimum to the maximum of the 11 year solar activity cycle cause only a 2% increase in photodissociation, which can be compared with the approximately factor of two increase associated with the solar cycle variation of the integrated line flux. However, at solar maximum, using the Meier and Prinz line profile underestimates the peak photodestruction by 4%.