Observations of atomic oxygen in the MLT from a stratospheric balloon with the OSAS-B terahertz heterodyne spectrometer

The Oxygen Spectrometer for Atmospheric Science on a Balloon (OSAS-B) is a heterodyne receiver for the thermally excited ground state transition of neutral atomic oxygen at 4.75 THz [1]. It has been shown that this transition is favorable for the determination of atomic oxygen in the mesosphere and lower thermosphere (MLT) region of Earth [2]. Due to water absorption, it cannot be observed from ground. Atomic oxygen is the dominant species in the MLT, and thus plays an important role for the chemistry and energy balance in the MLT region as well as for the deceleration of low-earth orbit satellites. OSAS-B uses a combined helium/nitrogen cryostat for the detector of the instrument, a superconducting hot-electron bolometer mixer, as well as for cooling the quantum-cascade laser, which serves as the local oscillator for heterodyne detection. A turning mirror allows for measurements at different vertical inclinations and for radiometric calibration against two blackbody sources. The first flight took place as a one-day flight in September 2022 from Esrange, Sweden in the framework of the EU-funded Hemera 2020 program. During the course of the flight, several hundred spectra for different elevation angles and azimuth directions were recorded. Preliminary results show a reasonable agreement with predictions from the current MSIS model (NRL MSIS 2.0/2.1). Besides, we are able to observe the spectral signatures of shear winds in the MLT as they are predicted by the horizontal-wind model (HWM 2014).

[1] Wienold, M. et al. 48th IRMMW-THz, Montreal, Canada (2023), doi: 10.1109/IRMMW-THz57677.2023.10299165
[2] Richter, H. et al. Commun Earth Environ 2,19 (2021), doi: 10.1038/s43247-020-00084-5