Fig. 1: Atomic oxygen observation with GREAT on SOFIA.

a Concentration profile of atomic oxygen and temperature measured by the SABER instrument (blue squares) at altitudes ranging from 80 to 100 km. The straight red and blue lines are calculated profiles with NRLMSISE-00 (at position B in Fig. 1c). 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). Only with GREAT the atomic oxygen line is spectrally resolved. 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. The numbers along the trajectory indicate the flight time in seconds. Short interruptions are due to a reorientation of the telescope. The white circles mark the positions where the spectra of Fig. 2 have been measured (A: 50.6°, 1:21 am; B: 38.3°, 3:11 am; C: 29.1°, 4:01 am). The dashed blue line indicates the flight trajectory of SABER. 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, https://www.openstreetmap.org/copyright).