Feasibility of a space-borne terahertz heterodyne spectrometer for atomic oxygen and temperature in the mesosphere and lower thermosphere

Abstract. We investigate the feasibility of a satellite-borne heterodyne spectrometer for the retrieval of atomic oxygen concentration and temperature in the mesosphere and lower thermosphere. We use the vertical density and temperature profiles provided by the NRLMSIS 2.1 atmosphere model to simulate 2.1 and 4.7 THz atomic oxygen emission spectra as measured by a satellite in a near-polar circular orbit at 500 km altitude. We then apply retrieval algorithms for the atomic oxygen concentration and temperature and compare the retrieved profiles to the reference, i.e. the original NRLMSIS 2.1 profiles. The emission spectra are simulated using radiative transfer under the assumption of local thermodynamic equilibrium. By considering two separate heterodyne receivers with sensitivity of 11 000 and 25 000 K noise temperature for the 2.1 and 4.7 THz lines, respectively, and data accumulated over 177 s of measurement time, corresponding to a ground track of 1250 km, we can retrieve vertical temperature profiles from 100 km altitude to 200 km altitude within ±2 % relative uncertainties and an atomic oxygen concentration profile from 110 to 300 km within ±3 % relative uncertainties. From 100 to 110 km the uncertainty in the atomic oxygen concentration is higher but still within ±15 %.