1. Bates, D. R., Massey, H. S. W., and Pearse, R. W. B., Emission Spectra of the Night Sky and Aurora, 97 (The Physical Society, London, 1948).
2. Barbier, D., Roach, F. E., and Steiger, W. R., J. Res. NBS, 66D, 145 (1962).
3. McIlwain, C. E., J. Geophys. Res., 66, 3681 (1961).
4. Ionospheric Prediction Service, Series D (1964–1969).
5. Filosofo, I., Greenspan, J. A., and Groom, C. M., Appl. Opt., 4, 215 (1965).