The red dots and blue squares in 1a are an average of nine profiles measured with SABER along the flight track between the two blue stars in 1c on 14 January 2015 between 0:22 and 0:30 am local time (Map: © Open Street Map contributors, ).Īt the atmospheric conditions of the MLT the fine structure line at 4.7448 THz is thermally Doppler broadened with a line width of ∼12 MHz for emission originating at ∼100 km where the density of atomic oxygen is largest. The dashed blue line indicates the flight trajectory of SABER. The white circles mark the positions where the spectra of Fig. Short interruptions are due to a reorientation of the telescope. The numbers along the trajectory indicate the flight time in seconds. c Trajectory of the SOFIA flight (red line) where the spectra have been acquired between 1:15 and and 4:15 am on 14 January 2015. Only with GREAT the atomic oxygen line is spectrally resolved. b Comparison of the spectra measured with the grating spectrometer of CRISTA (black line in the inset), with the balloon-borne FIRS-2 Fourier transform spectrometer (blue line) and with the GREAT heterodyne spectrometer (red line). The straight red and blue lines are calculated profiles with NRLMSISE-00 (at position B in Fig. An accurate knowledge of the global distribution of atomic oxygen and its concentration profile as well as diurnal and annual variations are therefore essential for understanding the photochemistry and the energy budget of the MLT.Ī Concentration profile of atomic oxygen and temperature measured by the SABER instrument (blue squares) at altitudes ranging from 80 to 100 km. This leads to a strong coupling between the dynamics and the photochemistry in the MLT with the energy being released significantly after and far away from the location, where the UV photon is absorbed. Because of this long lifetime it can be transported over large distances before it releases its energy and therefore it might be used as tracer for the dynamical motions, vertical transport, tides, and winds 5, 6, 7. In the MLT the lifetime of atomic oxygen in the 3P 1 state is several hours. This is the dominant cooling mechanism above ~250 km. In addition, direct radiative cooling by atomic oxygen occurs via the fine structure transition from the lowest excited state, 3P 1, into the ground state, 3P 2, at 63.2 µm (4.7448 THz) 4. Therefore, the knowledge about the distribution of atomic oxygen is crucial for retrieval of the kinetic temperature in the MLT from the 15 µm CO 2 radiance 3. In particular, quenching of CO 2 vibrational levels by collisions with atomic oxygen is important. Both molecules are excited by collisions with ground state atomic oxygen. The latter occurs mainly via emission from CO 2 at 15 µm and NO at 5.3 µm. It plays an important role for the energy balance of the mesosphere and lower thermosphere (MLT), because it participates in exothermic chemical reactions and it contributes to radiative cooling 1, 2. We suggest that this direct observation method may be more accurate than existing indirect methods that rely on photochemical models.Ītomic oxygen extends from about 80 km to above 300 km in altitude, but with more than 90% concentrated between 85 and 125 km (Fig. We find that our measurements of the concentration of atomic oxygen agree well with atmospheric models informed by satellite observations. Here we present direct measurements-independent of photochemical models-of the ground state 3P 1 → 3P 2 fine-structure transition of atomic oxygen at 4.7448 THz using the German Receiver for Astronomy at Terahertz Frequencies (GREAT) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). Current indirect methods involve photochemical models and the results are not always in agreement, particularly when obtained with different instruments. However, its concentration is extremely difficult to measure with remote sensing techniques since atomic oxygen has few optically active transitions. Atomic oxygen is a main component of the mesosphere and lower thermosphere of the Earth, where it governs photochemistry and energy balance and is a tracer for dynamical motions.