Odd oxygen measurements during the Noctilucent Cloud 93 rocket campaign

Abstract

The NLC‐93 campaign at Esrange, Sweden, addressed the chemical, dynamical and electrical environment of the summer mesopause in the presence of noctilucent clouds (NLC). A major topic was the detailed investigation of odd oxygen abundances and their structure in the vicinity of the cloud layer. Applying independent in situ measurements, we have obtained consistent profiles of atomic oxygen, ozone, and mesospheric total density. Atomic oxygen was measured on two rocket flights by means of a newly developed probe utilizing the 1304 Å resonance fluorescence / absorption technique. Detailed Monte Carlo simulations of the radiative transfer and the instrument aerodynamics are included in the data analysis. We find relatively low atomic oxygen densities with a peak of 2 × 1011 cm−3 at 96 km. An airglow radiometer recorded the O2 IR‐atmospheric band emission, allowing the retrieval of ozone densities by means of an inversion model that considers both direct and indirect sources of O2 (a1Δg). The odd oxygen measurements are compared to a steady state photochemical model. While good agreement is found in the mesosphere, the results deviate close to the cold mesopause. Significant vertical structures are found in the atomic oxygen profiles; sharp gradients and distinct minima coincide with the observed NLC layer. Possible influences of heterogeneous chemistry on the odd oxygen abundance are discussed.