The instrument measures emission of highly excited molecular oxygen centered around 764 nm. The emission is produced in various processes related to the photolysis of molecular oxygen and ozone as well as chemiluminescence. Five production mechanisms have been identified which produce oxygen in the required excited state. Fig. 1 shows the emission rate of the transition to be observed in dependence of altitude between 50 and 130 km and the contribution of the five production mechanisms.
The A-band emission stems from a rotational-vibrational transition from the first excited vibrational state to the ground state. The rotational distribution within the band depends on ambient temperature. Thus, the temperature can be derived by measuring the intensity of multiple lines without an absolute calibration. This simplifies the instrument considerably and allows it to be flown on a CubeSat. Analysis shows that the measurement of six fine-structure lines, which are marked red in Fig. 2, provide most information within the trade-off of spectral resolution versus spectral bandwidth.