The image above displays the green air glow in the atmosphere of the Earth above the curved horizon. It has been taken by astronauts on board the International Space Station. The airglow is caused by emission from an excited state of atomic oxygen (¹S -> ¹D transition, see inset), which is a main component of the Earth’s mesosphere and lower thermosphere. It extends from about 80 km to above 300 km where it governs photochemistry and energy balance and is a tracer for dynamical motions. There are several methods to measure the distribution of this important species. Atomic oxygen concentrations can be inferred indirectly from the green air glow or from observation of other molecules, which are involved in photochemical processes related to atomic oxygen. Such measurements have been performed with several satellite instruments. However, these methods are indirect and rely on photochemical models and assumptions such as quenching rates, radiative lifetimes, and reaction coefficients. The results are not always in agreement, particularly when obtained with different instruments.

We have explored an alternative approach, namely the observation of the ground state fine-structure transition of atomic oxygen at 4.7 THz (63 µm, ³P₁ -> ³P₂ transition, see inset) with high spectra resolution. Since the populations of the states involved in this transition are purely determined by temperature and quantum mechanics the derivation of of the atomic oxygen concentration from the measured profiles is rather direct. The line profile of the emission is Doppler broadened and varies between approximately 10 and 30 MHz depending on the altitude from where the emission originates. This means that the altitude information of the atomic oxygen concentration is contained in the line profile.

We have used the heterodyne spectrometer GREAT (German Receiver for Astronomy at Terahertz frequencies) on board of the Stratospheric Observatory for Infrared Astronomy, SOFIA, a Boing 747 equipped with a 2.5-m telescope. The high spectral resolution of GREAT allows spectrally resolving the 4.7-THz transition, which in turn enabled us to derive altitude profiles of the atomic oxygen concentration in Earth’s atmosphere. The data are a by-product of astronomical observations in the same frequency band during a flight along the west coast of the US. These are the first spectrally resolved measurements of this transition. We find that our measurements agree well with atmospheric models informed by satellite observations. It should be noted that Recent progress in terahertz heterodyne technology has initiated the development of balloon-borne and satellite instruments for measuring the 4.7-THz transition. In fact, the European Space Agency is funding the development of critical THz technologies for a future heterodyne space instrument dedicated to the measurement of atomic oxygen.

Richter, H., Buchbender, C., Güsten, R., Higgins, R., Klein, B., Stutzki, J., Wiesemeyer, H., Hübers, H.-W.  Direct measurements of atomic oxygen in the mesosphere and lower thermosphere using terahertz heterodyne spectroscopy. Commun Earth Environ 2, 19 (2021). https://doi.org/10.1038/s43247-020-00084-5