M. W. Faivre

Correlated measurements of mesospheric density and near infrared airglow

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.

Astronomical observation through the NIR atmospheric emissive layer

The emission of the upper atmosphere introduces an additional component into photometric observations of astronomical objects. In the I band for instance, the intensity of the atmospheric emission is of the order of 1 to 2 Imag20 per square arcsecond. The subtraction of this component is not easy because it varies during the night by as much as 50% and it is not homogeneous over the sky. A program aimed at understanding the main characteristics of the atmospheric emission was undertaken. A set of CCD images of the OH emission in the I band covering the sky was assembled in a panorama, it shows wide converging arches. An algorithm was developed in order to invert the perspective projection of the photographs. The result is a 2200 km wide view over Europe and Mediterranean Sea of the emission as seen from a virtual satellite. This image shows the presence of an extended wave field. A Fourier analysis allows to infer mean horizontal wavelength, mean temporal period and horizontal phase velocity. The atmospheric emission varies under the influence of atmospheric waves. A stereoscopic imaging program is currently under development to measure the amplitude and the energy of the atmospheric waves.