Correlated measurements of mesospheric density and near infrared airglow

Abstract

An observational program aimed at simultaneously measuring the mesospheric density and the evolution with time of the near-IR emission at the mesopause level was conducted in July 2000 and July 2001. The atmospheric density is measured vertically using the Rayleigh scatter lidar located at Observatoire de Haute Provence (OHP) (43°56′N, 5°43′ E). The near-IR emission, which is mainly generated by OH, is measured along a slant path from the Pic de Château-Renard (44°41′ N, 6°54′ E) (Hautes-Alpes, altitude 2989 m). The field of view of the CCD camera encompasses a region located over OHP. Rayleigh scattering by air molecules is much less efficient than fluorescence by alkaline atoms. Therefore, the lidar density data could only be retrieved with a one-hour time resolution at altitudes of 65, 70, 72.5 and 75 km. The time resolution for the airglow intensity measurements was equal to three minutes. Up to 75 km the variation of the density over the 5-hour duration of the night was opposite to the variation of the near IR airglow. In the middle of the night, the airglow shows a minimum intensity about 28% lower than its maximum value. During the first part of the night, the intensity decreases. During the second part, it increases. The increase during the second part cannot be explained by the evolution of the atmospheric chemical system. Given the opposite variation of the air density and of the observed OH emission, it is suggested that the near-IR airglow is a semi-direct tracer of the density variations at the mesopause level, the air molecules being effective quenchers of the excited OH radicals. The excitation and quenching rates will therefore be discussed.