Atomic oxygen in the mesosphere and lower thermosphere of the Earth

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Atomic oxygen (O) is an important component of the Earth’s atmosphere. It extends from the mesosphere to the lower thermosphere (MLT), i.e. from approximately 80 km to beyond 500 km in altitude. O is generated through photolysis of molecular oxygen by UV radiation. It is the most abundant species in the MLT and an important component with respect to the photochemistry of the MLT. In addition, O is important for the energy budget of the MLT, because CO2 molecules are excited by collisions with O and the excited CO2 molecules radiate in the infrared and cool the MLT. This means that global climate change also affects the MLT, because the increase of the CO2 concentration in the MLT leads to a more efficient cooling and consequently shrinking of the MLT. The O concentration is affected by dynamical motions, vertical transport, tides, and winds Therefore, an accurate knowledge of the global distribution of O and its concentration profile as well as diurnal and annual variations are essential for understanding the photochemistry, the energy budget and the dynamics of the MLT. The goal of this proposal is to determine column densities and concentration profiles of O in the MLT using the fine structure transitions at 4.74 THz and 2.06 THz. The data, which will be analyzed, are measured with the GREAT/upGREAT (German REceiver for Astronomy at Terahertz frequencies) heterodyne spectrometer on board of SOFIA, the Stratospheric Observatory for Infrared Astronomy. This is a direct observation method which may yield more accurate results than existing indirect satellite-based methods which involve photochemical models in order to derive O concentration profiles. This instrument provides a data base with approximately 500,000 spectra starting in May 2014 and covering four different geolocations, namely North America, New Zealand, Europe and Tahiti/Pacific. Temporal variations as well as the influence of solar cycles, winds and gravity waves will be studied. The results will be compared with satellite data, which are available for altitudes from 80 to 100 km, and with predictions from a semi-empirical model. It should be noted that these data are the first spectrally resolved, direct measurement of O in the MLT. This is a promising alternative for the determination of the O concentration compared to indirect satellite-based methods, which rely on photochemical models.