Airglow Research With Astronomical Spectra

• Applicant: Dr. Stefan Noll
• Subject Area: Atmospheric Science
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 368912568

The radiation of the Earth’s atmosphere in the optical and near-infrared wavelength regime is essentially characterised by an emission line spectrum. This so-called airglow is induced by energetic solar ultraviolet radiation, which destroys air molecules, ionises atoms, and hence causes various chemical reactions and physical processes, which partly generate light emission, also at night. Most radiation originates at around 90 km in the mesopause region, where contributions of hydroxyl, oxygen, and iron oxide molecules as well as oxygen and sodium atoms are important. The compact layering makes airglow very sensitive to atmospheric dynamics. Regular studies of the mesopause region are only possible by means of ground- and satellite-based remote sensing. Most airglow instruments either produce images or spectra of a few lines. However, a comprehensive characterisation of the different emissions requires a simultaneous coverage at a sufficiently high spectral resolution. This can only be achieved with powerful echelle spectrographs, which are operated at astronomical observatories.The planned project will consider the largest set of astronomical data that has been used for airglow research, so far. It consists of more than 100,000 spectra taken with the X-shooter spectrograph at the Very Large Telescope in Chile since the year 2009. The data will reveal the strengths and variations of various emission lines and continuum components. In particular, the latter are poorly investigated. The interrelations between the different emissions will show how the airglow layer reacts to impacts such as gravity waves, solar tides, seasons, and solar activity. In this way, the study will contribute to a better understanding of the chemical and dynamical processes in the mesopause region.An important result of the project will be an airglow parametrisation which reproduces the variability on different time scales and provides realistic spectra. This will also be valuable for estimates of the natural night-sky brightness, which are crucial for the operation of astronomical observatories in terms of observation planning, instrument development, and data processing. As an astronomical application of this project, there will be an investigation of the influence of airglow brightness and variability on the detectability of molecular absorption features in atmospheres of Earth-like extrasolar planets observed with the future Extremely Large Telescope in Chile and a so-called starshade orbiting the Earth and blocking the light of the exoplanetary host star. Hence, this study will also contribute to the search for a second Earth and life outside the solar system.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Michael Bittner