Sodium layer

Abstract: With computational techniques developed in the investigation of high-energy laser-beam-control systems, a set of concise analytic models describing the essential properties of a laser-guide-star phase-conjugation system has been assembled. With the aid of these models an optimization strategy for mating adaptive optics to a 4-m-class optical telescope is evolved, and it is shown that such a system might be expected to improve the effective atmospheric seeing conditions by nearly a factor of 10 within the isoplanatic patch of the turbulence probe. For operation at visible wavelengths, a compensation system having ~300 actuators and a closed-loop bandwidth of 20 Hz is recommended. All the key hardware components have already been built and tested, with the exception of a suitable laser source for high-repetition-rate illumination of the Earth’s sodium layer.

Abstract: A new concept is described for creating artificial beacons for atmospheric-turbulence compensation. A laser tuned to the sodium D2 line is used to excite resonant optical backscattering from the sodium layer in the upper atmosphere. The performance of such a system is compared with that expected from artificial beacons based on Rayleigh scattering in the lower atmosphere. It is found that sodium scattering has a more favorable scaling to large-aperture systems. Applications to the compensation of ground-based astronomical telescopes for atmospheric turbulence are described.

Abstract: Lidar observations of the mesospheric sodium layer, made at Sao Jose dos Campos (23°S, 46°W), show three distinct types of organized structure in the vertical distribution of sodium. Profiles averaged over several days show a smooth but asymmetrical distribution. A sequence of profiles for a given night normally shows a wavelike structure which descends through the layer with time. Very occasionally an extremely narrow peak is observed at a constant height for several hours. On one occasion a layer 2.5 km wide, with scale heights of 700 m on the bottomside and 900 m on the topside, was observed to persist at a constant height for three hours. It is concluded that such a layer could neither be produced by neutral density perturbations nor by photochemical processes and therefore indicates a source of sodium in the mesosphere.

Abstract: A global model of sodium in the mesosphere and lower thermosphere has been developed within the framework of the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model (WACCM). The standard fully interactive WACCM chemistry module has been augmented with a chemistry scheme that includes nine neutral and ionized sodium species. Meteoric ablation provides the source of sodium in the model and is represented as a combination of a meteoroid input function (MIF) and a parameterized ablation model. The MIF provides the seasonally and latitudinally varying meteoric flux which is modeled taking into consideration the astronomical origins of sporadic meteors and considers variations in particle entry angle, velocity, mass, and the differential ablation of the chemical constituents. WACCM simulations show large variations in the sodium constituents over time scales from days to months. Seasonality of sodium constituents is strongly affected by variations in the MIF and transport via the mean meridional wind. In particular, the summer to winter hemisphere flow leads to the highest sodium species concentrations and loss rates occurring over the winter pole. In the Northern Hemisphere, this winter maximum can be dramatically affected by stratospheric sudden warmings. Simulations of the January 2009 major warming event show that it caused a short-term decrease in the sodium column over the polar cap that was followed by a factor of 3 increase in the following weeks. Overall, the modeled distribution of atomic sodium in WACCM agrees well with both ground-based and satellite observations. Given the strong sensitivity of the sodium layer to dynamical motions, reproducing its variability provides a stringent test of global models and should help to constrain key atmospheric variables in this poorly sampled region of the atmosphere.