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

Abstract

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 7320 and 7330 Å 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 7320‐Å 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]. We 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 given approximately by n = (n120T120/T) exp[−z] where T = T∞ ‐ (T∞ ‐ T120)exp[−S(h ‐ h120)], z is the reduced height, and h is the altitude. 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 we report here the solar EUV flux is specified. In a separate paper we will report how the solar EUV ionization rate can be independently derived from various twilight emissions. 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, we 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‐Å emission from the ground during twilight. The results show excellent agreement with the measured [O] values which were ∼ 50% lower than the mass spectrometer incoherent scatter (MSIS‐86) model values at ∼ 300 km on that day, thereby demonstrating the value of the method for monitoring day‐to‐day variations in [O] and the temperature.