INTERPRETATION OF DYNAMICS EXPLORER FAR UV IMAGES OF THE QUIET TIME THERMOSPHERE

A selected set of far ultraviolet images of the Earth have been analyzed quantitatively to establish their validity for studying thermospheric ''weather.'' The set of images chosen for study was restricted to mostly geomagnetically quiet conditions in order to obtain a baseline understanding of the relationship between the observations and thermospheric phenomenology. The images included low to moderate solar activity levels. A new model was developed to generate global dayglow images using first principles methods. The mass spectrometer/incoherent scatter (MSIS-86) model was used to predict the thermospheric concentrations. The analyses of thermospheric images observed in the 123 to 160-nm nominal passband show that the spectral composition for observations on the projected Earth disk is dominated by O I 130.4-nm radiation (85-90%), with contributions from O I 135.6-nm and N-2 Lyman-Birge-Hopfield (LBH) bands of about 5-8% each. The synthetic images reproduce the global features of the observed images rather well. Differences between the model and the data are attributed to real atmospheric effects, such as atomic oxygen depletions which are not well reproduced by the MSIS model when geomagnetic activity is elevated. The absolute values recorded were 38-54% higher than predicted. We attribute this discrepancy to low values of the solar extreme ultraviolet irradiances used in the model. Images obtained in the 136 to 165-nm nominal passband are a factor of 2.7 greater than the model. The excess signal observed is most likely due to a long wavelength tail in the instrument sensitivity which allowed Rayleigh scattered sunlight between 180 and 250 nm to be detected. The understanding of the DE 1 images gained by this study provides the basis for future work on the global response of the thermosphere to geomagnetic forcing.