A method for the retrieval of atomic oxygen density and temperature profiles from ground-based measurements of the O+(2D-2P) 7320-angstrom twilight airglow

This paper describes a technique for the retrieval of altitude profiles of the atomic oxygen concentration (n = [O]) and temperature (T) from ground-based measurements of the +O(2D-2P) doublet at 7,320 and 7,330 angstrom in the twilight airglow. The technique is based on previously demonstrated knowledge that at solar zenith angles (SZA) characteristic of twilight conditions, the upper state of the 7,320-angstrom doublet transition is produced by photoionization and photoelectron impact ionization of atomic oxygen and lost mainly by radiative decay, thereby providing a sensitive dependence on [O]. The authors apply inverse problem theory to retrieve the exospheric temperature T∞, the atomic oxygen concentration at 120 km (n120), the temperature at 120 km (T120) and the temperature profile shape factor (S) using a Bates-Walker representation of n. The algorithm is tested and theoretically verified using synthetic data sets where random errors of measurements are characterized by Poisson noise due primarily to sky background. In the tests that they report here, the solar EUV flux is specified. By comparing retrieved with known input values, it is demonstrated that for the altitude range 200 to 500 km the atomic oxygen concentration [O] can be retrieved with relative errors ± 15% and systematic errors of about 25% if the solar EUV is given. Sensitivity of the results to noise, sample size (degrees of freedom), and absolute calibration are quantitatively evaluated. In addition, to demonstrate the validity of the technique experimentally, they utilized the Atmosphere Explorer E (AE-E) in situ measurements of the solar EUV flux and [O], with the latter taken when perigee was over Arecibo on an occasion when the observatory airglow spectrometer was simultaneously measuring the 7320-angstrom emission from the ground during twilight