Atmospheric Research with Large Astronomical Facilities

Ground-based astronomical observations are affected by the Earth`s atmosphere. The light of an astronomical target is scattered and absorbed. Moreover, atmospheric emission and scattered light from other directions is added to the detected flux. Hence, each observation contains information on the status of the atmosphere during the exposure. In view of the large amounts of data that are produced by astronomical facilities each night, the archived data are a treasure for atmospheric sciences. In particular, the use of telescopes with main mirrors of the 10 m class allows one to study the Earth`s atmosphere at high spectral resolution, high spatial resolution, and high signal-to-noise ratio, which is not possible with the standard equipment of atmospheric scientists. For this reason, we propose to use spectroscopic data from the archive of the Very Large Telescope of the European Southern Observatory at Cerro Paranal in the Chilean Atacama desert to study properties of the Earth`s atmosphere that were difficult to investigate in the past. Spectra from the medium-to-high resolution spectrograph X-Shooter will allow us to analyse the simultaneously observed wavelength range from 0.3 to 2.5 micron. In this way, we will be able to study the intensities and variability of all prominent nocturnal airglow lines and bands, which are produced above 80 km by chemiluminescence. We will also thoroughly check the quality of mesopause temperatures derived from only a few reference airglow lines, which is the most important method for studying the climate change in the upper atmosphere. Moreover, we will investigate the intensity, variability, and origin of the airglow continuum, which is particularly in the near-IR almost unexplored due to instrumental limitations in the past. Therefore, discoveries are very probable. Finally, the Cerro Paranal is an ideal location for studying the properties of aerosol modes with long life times in the (sub)tropical troposphere and stratosphere. The low aerosol optical depths that can be measured there are hardly affected by local aerosol emission sources. Since comparable observing sites of aerosol research are lacking in the Southern Hemisphere, new findings on the variability and origin of these aerosol modes are expected. The results of the proposed atmospheric studies will also be considered for an improvement of our night-sky radiance model for Cerro Paranal, which is important for an accurate exposure time prediction for astronomical observations. We also expect that astronomical data reduction procedures can benefit from this project.

Ground-based astronomical observations are affected by the Earth's atmosphere. The light of an astronomical target is scattered, absorbed, and superimposed by atmospheric radiation. Hence, each observation contains information on the status of the atmosphere during the exposure. In view of the large amounts of data that are produced by astronomical facilities each night, the archived data are a treasure for atmospheric sciences. This is now widely accepted in the geophysics community after our presentations of the results at the international conferences. We used mainly spectroscopic data from the archive of the Very Large Telescope of the European Southern Observatory at Cerro Paranal in the Chilean Atacama desert. Spectra of the spectrograph X-shooter with medium to high resolution (wavelength range from near UV to near IR) were used. In addition, high-resolution spectrograph UVES data (red optical light) were used. In this way, we were able to study the intensity and variability of all prominent nocturnal airglow lines and bands generated in the upper Earth's atmosphere. The strongest unexpected result was the magnitude of the departure of OH line intensity ratios from those expected for a local thermodynamic equilibrium. This resulted in the need to correct temperature measurements based on these lines (which are important for climate research at about 90 km) by several degrees. We also found that these effects vary throughout the night and depend on the effective air density in the emission layer.In addition to individual lines of molecules such as OH, we also studied the faint optical pseudo-continuum by FeO emission. For the first time, this could be done in a systematic way based on several thousand spectra. Metals like Fe originate from meteoric dust particles, which are constantly delivered from space. A new collaboration even allowed a comparison of our results with those from a theoretical model, which improved our understanding of the atmospheric metal chemistry.In the final phase of the project, extensions to the northern hemisphere were started. For this purpose, a cooperation with the SDSS / APO / MaNGA Surveying Telescope Team was set up for future work.