Vertical transport and photochemistry in the terrestrial mesosphere and lower thermosphere (50–120 km)

Abstract

The coupled effects of kinetics, solar cycle flux variations and vertical transport on the distribution of long‐lived hydrogen‐carbon‐oxygen compounds in the terrestrial mesosphere and lower thermosphere are studied using a one‐dimensional aeronomy model. The calculations account for the important chemical reactions and use rocket measurements of the solar flux at solar minimum and maximum. Photodissociation rates appropriate for the mesosphere are determined with a spherical shell atmosphere formalism; detailed corrections for the O 2 Schumann‐Runge bands and the temperature dependence of the CO 2 cross sections are used. Then an eddy diffusion profile is derived which gives agreement with the Aladdin 74 mass spectral measurements of atomic O, O 2, CO 2, and Ar in the lower thermosphere and observations of the O 3 minimum at ∼80 km. The 115 GHz CO radio emission line computed for the CO mixing ratio profile predicted with the new eddy diffusion profile compares well with recent observations of W. J. Wilson. Differences between the calculated CO mixing ratio profile and previous theoretical and observational determinations are discussed. Our derived eddy diffusion profile has a sudden decrease at 92 km which is necessary to produce the atomic O peak at 98 km that appears in the Aladdin 74 measurements. This stagnant region apparently is a recurrent or persistent feature of the upper atmosphere since an atomic O peak around 98 km has been seen by different techniques in different seasons over several years. Slow eddy diffusion in the lower thermosphere through the homopause was also the conclusion of earlier Ar/N 2 rocket measurements studies. The analytic approach of this paper could be used in the future to monitor variations in middle atmosphere dynamics, if regularly conducted simultaneous observations of various groups of species were available.